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7
Topic
Chemical Reactions and Energy
Part A Unit-based exercise
7
Unit 29 Energy changes in
chemical reactions
The standard enthalpy change of
formation
of a substance is the enthalpy change when one
mole of the substance is formed from its
The amount of heat required to raise the
8
states.
standard
The standard enthalpy change of
combustion
temperature of 1 g of a substance by 1 K
of a substance is the enthalpy change when
is the
one mole of the substance is
of the
specific heat capacity
bur nt in
substance.
completely
under
oxygen
Unit 29
1
Part A
in their
Fill in the blanks
elements
standard
conditions.
2
The amount of heat required to raise
the
temperature of a substance by 1 K is
the
of the substance.
heat capacity
9
The standard enthalpy change of
neutralization
is the enthalpy change when an acid reacts
with an alkali to form one mole of
3
The internal energy of a system has two
components:
and
kinetic energy
standard
conditions.
potential
10 The standard enthalpy change of
.
energy
under
water
solution
of
a substance is the enthalpy change when one
4
The heat released or taken in by a system kept
mole of the substance dissolves in an infinite
at constant pressure is called
volume of
enthalpy
a) In an
reaction, heat is
exothermic
released to the surroundings.
b) The total enthalpy of the products is
than that of the reactants.
less
c) The enthalpy change of the reaction is
a
6
negative
a) In an
quantity.
endothermic
reaction, heat is taken
in from the surroundings.
b) The total enthalpy of the products is
greater
than that of the reactants.
c) The enthalpy change of the reaction is
a
under
standard
conditions.
change.
5
solvent
positive
True or false
Decide whether each of the following statements is
true or false.
11 Energy is the capacity to provide heat or
to do work.
T
12 The universe is composed of the
surroundings and the system.
T
13 The heat released or taken in by a system
kept at constant volume is called enthalpy
change.
F
quantity.
1
Part A
Unit 29
14 In an exothermic reaction, the total
enthalpy of the products is less than that
of the reactants.
T
15 In an endothermic reaction, heat is
transferred from the system to the
surroundings.
F
16 The sign for ΔH of an endothermic
reaction is positive.
T
17 In an exothermic reaction, the amount of
energy released in the bond-forming step
is less than the amount of energy used
in the bond-breaking step.
F
18 The standard enthalpy change of
formation of diamond is zero.
F
19 The standard enthalpy change of
combustion of hydrogen equals the
standard enthalpy change of formation
of water.
T
20 The standard enthalpy change of
neutralization between hydrochloric acid
and sodium hydroxide solution is the
same as that between ethanoic acid and
sodium hydroxide solution.
F
2
12.4
28.9
33.2
39.4
°C
°C
°C
°C
D
23 How much heat is required to heat 5.00 g of
Pyrex glass from 15.0 °C to 65.0 °C?
A
B
C
D
62.8 kJ
209 J
272 J
299 J
B
What is the specific heat capacity of zinc?
A
B
C
D
0.250
0.284
0.352
0.400
J
J
J
J
g–1
g–1
g–1
g–1
K–1
K–1
K–1
K–1
D
Directions: Q u e s t i o n s 2 5 a n d 2 6 re f e r t o t h e
following experiment.
In an experiment, 110.0 g of alcohol X at 18.2 °C
were mixed with 60.0 g of water at 50.3 °C. The
final temperature of the mixture was 36.5 °C.
(Specific heat capacity of water = 4.18 J g–1 K–1)
(Specific heat capacity of aluminium
= 0.900 J g–1 K–1)
kJ
kJ
kJ
kJ
A
B
C
D
24 3 1 9 J o f h e a t a re re q u i re d t o r a i s e t h e
temperature of 93.8 g of zinc from 28.5 °C to
37.0 °C.
21 A 525 g aluminium pizza pan cools from
175 °C to 27 °C. How many joules of heat
does it lose?
12.8
41.4
69.9
86.3
(Specific heat capacity of thallium
= 0.139 J g–1 K–1)
(Specific heat capacity of Pyrex glass
= 0.837 J g–1 K–1)
Multiple choice questions
A
B
C
D
22 517 J of heat were added to a 0.300 kg
sample of thallium at 27.0 °C. What would be
the final temperature of the sample?
25 The heat lost by the water was
C
A
B
C
D
3.46
8.61
11.9
26.2
kJ.
kJ.
kJ.
kJ.
A
26 What is the specific heat capacity of alcohol X?
A
B
C
D
–1
0.653 J g
–1
1.72 J g
–1
3.55 J g
–1
5.76 J g
–1
K
–1
K
–1
K
–1
K
B
A
B
C
D
17.5
23.8
30.5
37.5
°C
°C
°C
°C
D
28 One beaker contains 156 g of water at 22.0 °C.
Another beaker contains 85.2 g of water at
95.0 °C. The water in the two beakers is
mixed. What is the final water temperature?
(Specific heat capacity of water = 4.18 J g–1 K–1)
A
B
C
D
47.8
58.5
69.2
83.0
°C
°C
°C
°C
A
29 A 25.0 g sample of an alloy was heated to
100.0 °C and dropped into a beaker containing
90.0 g of water at 25.3 °C. The temperature
of the water rose to a final value of 27.2 °C.
What is the specific heat capacity of the alloy?
(Specific heat capacity of water = 4.18 J g–1 K–1)
A
B
C
D
0.0303 J g–1 K–1
0.0393 J g–1 K–1
0.303 J g–1 K–1
0.393 J g–1 K–1
480.0 g
100.0 g
23.0 °C
52.0 g
61.6 °C
24.6 °C
(Specific heat capacities: water = 4.18 J g–1 K–1,
aluminium = 0.900 J g–1 K–1)
What is the amount of heat lost by silicon in
the experiment?
A
B
C
D
670 J
1.36 kJ
2.26 kJ
3.82 kJ
Unit 29
(Specific heat capacities: gold = 0.130 J g–1 K–1,
water = 4.18 J g–1 K–1)
Mass of aluminium calorimeter
Mass of calorimeter water
Initial temperature of calorimeter
and water
Mass of silicon
Initial temperature of silicon
Final temperature of calorimeter,
water and silicon
Part A
27 A 177 g sample of gold at some temperature
was added to 22.1 g of water. The initial
water temperature was 25.0 °C and the final
temperature was 27.5 °C. What was the initial
temperature of the gold sample?
30 A student performed a calorimetry experiment
and recorded the data below.
B
31 Which of the following is an endothermic
process?
A
B
C
D
Photosynthesis
Cellular respiration
Formation of ethanol
Combustion of propane
A
32 Commercially available hot packs contain an
inner pouch of a solid ionic compound within
an outer pouch containing water. When the
inner pouch is broken, the solid dissolves in the
water of the outer pouch.
PVUFSQPVDI
JOOFSQPVDI
D
3
A hot pack may contain
A calcium chloride, which undergoes
exothermic dissolving process.
B calcium chloride, which undergoes
endothermic dissolving process.
C ammonium nitrate, which undergoes
exothermic dissolving process.
D ammonium nitrate, which undergoes
endothermic dissolving process.
an
an
an
an
A
Part A
33 A basketball player comes out of the shower,
still damp, feeling cooler than he did when
he entered the locker room. The water on the
player’s skin undergoes an
Unit 29
A
B
C
D
endothermic physical change.
endothermic chemical change.
exothermic physical change.
exothermic chemical change.
A
Inside the can, in separate compartments, are
calcium oxide and water. When a button is
pressed, the two substances react.
The reaction is
A endothermic, releases heat and has a
positive ΔH value.
B endothermic, takes in heat and has a
negative ΔH value.
C exothermic, releases heat and has a negative
ΔH value.
D exothermic, takes in heat and has a positive
ΔH value.
C
35 Hydrogen iodide (HI) is formed from the
reaction of hydrogen and iodine.
2HI(g)
ΔH = +52 kJ mol–1
When two moles of HI decompose,
A
B
C
D
52 kJ of heat are released.
52 kJ of heat are taken in.
104 kJ of heat are released.
104 kJ of heat are taken in.
What is the ΔH value for the following process?
6CO2(g) + 8H2O(l)
A
B
C
D
2C3H7OH(g) + 9O2(g)
+3 948 kJ
+987 kJ
–987 kJ
–3 948 kJ
A
37 Consider the following thermochemical equation:
N2(g) + 3F2(g)
A
A
B
C
D
2NF3(g)
ΔHO = –264 kJ
+67.6
+33.8
–33.8
–67.6
kJ
kJ
kJ
kJ
C
Directions: Q u e s t i o n s 3 8 a n d 3 9 re f e r t o t h e
following information.
Commercial drain cleansers typically contain sodium
hydroxide and aluminium. When the solid cleanser
is poured down the drain and water is added, the
reaction that occurs is represented by the equation:
2NaOH(s) + 2Al(s) + 2H2O(l)
2NaAlO2(aq) + 3H2(g)
ΔHO = –850.0 kJ
(Relative atomic masses: H = 1.0, O = 16.0, Na =
23.0)
38 In this reaction, the oxidation number of
aluminium changes from
A
B
C
D
0 to +1.
0 to +3.
+2 to +6.
+3 to +6.
B
39 What is the amount of heat released when
24.0 g of NaOH(s) have reacted?
A
B
C
D
4
3CO2(g) + 4H2O(l)
ΔH = –1 974 kJ mol–1
What is the enthalpy change that occurs when
0.256 mole of NF3(g) is formed from N2(g) and
F2(g) under standard conditions?
34 Cans of ‘self-heating’ coffee were not available
until recently.
H2(g) + I2(g)
36 Given the following data:
9
C3H7OH(g) +
O2(g)
2
128 kJ
255 kJ
510 kJ
1 020 kJ
B
40 When 5.40 g of butanal (relative molecular
mass = 72.0) were burnt, 201 kJ of heat were
released.
What is the enthalpy change of combustion of
butanal?
A
B
C
D
+6.70 kJ mol–1
–6.70 kJ mol–1
+2 680 kJ mol–1
–2 680 kJ mol–1
44 T h e t a b l e b e l o w s h o w s t h e s t a n d a r d
enthalpy changes of combustion of the four
hydrocarbons: ethyne, propyne, propene and
propane.
103
103
106
106
kJ
kJ
kJ
kJ
(Specific heat capacity of water = 4.18 J g–1 K–1)
684 kJ mol–1
–1
848 kJ mol
–1
1 030 kJ mol
1 380 kJ mol–1
C
43 The enthalpy change of combustion of methane
is –890 kJ mol–1. What is the minimum mass
of methane that must be burnt to warm 4.00
dm3 of water from 22.4 to 94.8 °C?
(Relative atomic masses: H = 1.0, C = 12.0;
density of water = 1.00 g dm–3; specific heat
capacity of water = 4.18 J g–1 K–1)
A
B
C
D
1.36
2.18
13.6
21.8
g
g
g
g
ethyne
26.0
–1 301
HC CCH3
propyne
40.0
–1 940
H2C=CHCH3
propene
42.0
–2 060
CH3CH2CH3
propane
44.0
–2 219
This hydrocarbon is
A
B
C
D
C
42 In an experiment, a student heated 500.0 g
of water from 25.0 °C to 91.0 °C using 0.134
mole of ethanol. What is the enthalpy
change of combustion of ethanol under the
experimental conditions?
A
B
C
D
HC CH
Unit 29
x
x
x
x
ΔHOc
–1
(kJ mol )
Complete combustion of 2.00 g of one of the
above hydrocarbons releases exactly 100 kJ of
heat.
What is the amount of heat produced by the
complete combustion of 45.0 kg of octane?
2.16
4.32
2.16
4.32
Relative
molecular
mass
Part A
16CO2(g) + 18H2O(g)
ΔH = –10 940 kJ
(Relative atomic masses: H = 1.0, C = 12.0)
A
B
C
D
Name
D
41 The combustion of octane can be represented
by the following thermochemical equation:
2C8H18(g) + 25O2(g)
Compound
ethyne.
propyne.
propene.
propane.
A
Directions: Q u e s t i o n s 4 5 a n d 4 6 re f e r t o t h e
following information of four fuels.
Fuel Formula
Name
Enthalpy change Molar
mass
of combustion
–1
(g mol–1)
(kJ mol )
A
CH3OH
methanol
–726
32.0
B
C2H5OH
ethanol
–1 368
46.0
C
C4H10
butane
–2 877
58.0
D
C8H18
octane
–5 470
114.0
45 Which fuel, A, B, C or D, produces the greatest
amount of energy per gram on complete
combustion?
A
B
C
D
CH3OH
C2H5OH
C4H10
C8H18
C
D
5
46 Scientists give governments advice on technical
issues. What information would scientists use
when advising governments on the choice of
one of these fuels, if the aim was to minimize
carbon dioxide production?
A
B
C
D
Mass of
Mass of
Number
Number
carbon per gram of fuel
carbon per kilojoule produced
of kilojoules produced per gram
of kilojoules produced per mole B
Part A
47 Given the following data:
C2H6(g)
Unit 29
2C(s) + 3H2(g) ΔH = +85 kJ
7
2CO2(g) + 3H2O(l)
C2H6(g) +
O2(g)
2
ΔH = +1 560 kJ
3
3H2(g) +
O2(g)
3H2O(l) ΔH = –858 kJ
2
2C(s) + 2O2(g)
2CO2(g)
ΔH = –788 kJ
Which of the following statements is correct?
A 286 kJ of heat are released when 1 mole
of steam is formed from its elements.
B The enthalpy change of combustion of
hydrogen is more exothermic than that of
carbon.
C The enthalpy change of formation of ethane
from its elements is endothermic.
D The enthalpy change of combustion of
ethane is more exothermic than its enthalpy
change of formation.
D
49 Which of the following pairs would give
the same value of enthalpy change of
neutralization?
A 50.0 cm3 of 1.00 mol dm–3 sulphuric acid
and 50.0 cm3 of 1.00 mol dm–3 aqueous
ammonia
B 50.0 cm3 of 1.00 mol dm–3 nitric acid and
50.0 cm 3 of 1.00 mol dm –3 potassium
hydroxide solution
C 50.0 cm3 of 1.00 mol dm–3 ethanoic acid
and 50.0 cm 3 of 1.00 mol dm –3 sodium
hydroxide solution
D 50.0 cm3 of 1.00 mol dm–3 ethanoic acid
and 50.0 cm3 of 1.00 mol dm–3 aqueous
ammonia
B
50 If 100.0 cm3 of 1 mol dm–3 hydrochloric acid
were added to 100.0 cm 3 of 1 mol dm –3
sodium hydroxide solution, the temperature rise
would be
1
T °C.
2
B T °C.
3
C
T °C.
2
D 2T °C.
A
51 The reaction between hydrochloric acid and
sodium hydroxide solution can be represented
by the following equation.
HCl(aq) + NaOH(aq)
Directions: Questions 48 – 50 refer to the following
experiment.
3
–3
50.0 cm of 1.00 mol dm hydrochloric acid were
added to 50.0 cm 3 of 1.00 mol dm –3 sodium
hydroxide solution. A temperature rise of T °C was
observed.
(Specific heat capacity of water = 4.18 J g–1 K–1)
B
C
6
D
50.0 x 4.18 x T
1 000
100.0 x 20.0 x T
1 000 x 4.18
100.0 x 20.0 x 4.18
1 000 x T
100.0 x 20.0 x 4.18 x T
1 000
3
NaCl(aq) + H2O(l)
ΔHOn = –57.1 kJ mol–1
–3
60.0 cm of 2.00 mol dm HCl(aq), at
28.0 °C, are mixed with 40.0 cm3 of 2.00 mol
dm –3 NaOH(aq), also at 28.0 °C, in a wellinsulated calorimeter. The heat capacity of the
calorimeter and contents is 420 J K–1.
What is the final temperature of the solution
mixture?
48 What is the enthalpy change of neutralization (in
kJ mol–1)?
A
B
D
A
B
C
D
11.7
17.1
38.9
44.3
°C
°C
°C
°C
C
52 The following table shows some information on
mixing acids with sodium hydroxide solution:
Mixture
Temperature
rise (°C)
w
3
–3
25 cm of 1 mol dm CH3COOH(aq)
3
–3
+ 25 cm of 1 mol dm NaOH(aq)
x
50 cm3 of 1 mol dm–3 HCl(aq)
3
–3
+ 50 cm of 1 mol dm NaOH(aq)
y
3
A
B
C
D
z
>
=
=
=
x
y
x
y
=
>
>
<
y
x
y
x
>
=
=
=
z
z
z
z
B
Mixture
Temperature
rise (°C)
3
–3
25 cm of 0.5 mol dm H2SO4(aq)
3
–3
+ 25 cm of 1 mol dm NaOH(aq)
w
3
–3
50 cm of 0.5 mol dm H2SO4(aq)
3
–3
+ 50 cm of 1 mol dm NaOH(aq)
x
3
y
3
–3
50 cm of 1 mol dm H2SO4(aq)
3
–3
+ 50 cm of 2 mol dm NaOH(aq)
z
x
x
y
x
<
=
<
<
y
y
x
y
<
<
=
=
z
z
z
z
(Relative atomic masses: H = 1.0, O = 16.0, K =
39.1)
A
B
C
D
+57.8
+1.03
–1.03
–57.8
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
D
56 10.0 g urea (NH 2 CONH 2 ) are dissolved in
150.0 g of water in a simple calorimeter. A
temperature fall of 3.7 °C is observed.
(Relative atomic masses: H = 1.0, C = 12.0,
N = 14.0, O = 16.0; specific heat capacity of
water = 4.18 J g–1 K–1)
Which of the following concerning the values
of temperature rise is correct?
<
<
=
=
55 In an experiment to find the enthalpy change
of solution of potassium hydroxide, 3.60 g of
solid potassium hydroxide were added to water.
3.71 kJ of heat were produced.
What is the enthalpy change of solution of
urea?
–3
25 cm of 1 mol dm H2SO4(aq)
3
–3
+ 25 cm of 2 mol dm NaOH(aq)
w
w
w
w
A
What is the enthalpy change of solution of
potassium hydroxide?
53 The following table shows some information
on mixing sulphuric acid with sodium hydroxide
solution:
A
B
C
D
kJ
kJ
MJ
MJ
Unit 29
Which of the following concerning the values
of temperature rise is correct?
w
w
w
w
27.9
71.4
1.12
1.78
–3
50 cm of 1 mol dm CH3COOH(aq)
3
–3
+ 50 cm of 1 mol dm NaOH(aq)
A
B
C
D
(Relative atomic masses: H = 1.0, O = 16.0, Na
= 23.0)
Part A
3
–3
25 cm of 1 mol dm HCl(aq)
3
–3
+ 25 cm of 1 mol dm NaOH(aq)
54 The enthalpy change of solution of NaOH(s) is
–44.6 kJ mol–1. 25.0 g of NaOH(s) are dissolved
in water in a calorimeter. What is the amount
of heat released?
A
B
C
D
–13.9
–38.7
+13.9
+38.7
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
C
4
D
57 4.34 x 10 J of heat are required to vaporize
100.0 g of benzene (C 6 H 6 ). What is the
enthalpy change of vaporization of benzene?
(Relative atomic masses: H = 1.0, C = 12.0)
A
B
C
D
–3.39
–33.9
+33.9
+3.39
x 104 kJ mol–1
kJ mol–1
kJ mol–1
x 104 kJ mol–1
C
7
58 Which of the following is / are endothermic
process(es)?
(1) Dilution of concentrated sulphuric acid
(2) Evaporation of water
(3) Formation of methane from its elements
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
(3) 2H2O(l) + O2(g)
B
Part A
59 Which of the following processes are exothermic?
Unit 29
(1) Zn(s) + Cu2+(aq)
(2) 2C4H10(g) + 13O2(g)
(3) NH4NO3(s) + H2O(l)
A
B
C
D
61 For which of the following reactions does
the value of ΔH O represent both a standard
enthalpy change of combustion and a standard
enthalpy change of formation?
1
(1) H2(g) +
O2(g)
H2O(l)
2
(2) 2C(s) + O2(g)
2CO(g)
Zn2+(aq) + Cu(s)
8CO2(g) + 10H2O(l)
NH4NO3(aq)
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
A
B
C
D
Enthalpy (kJ mol–1)
7
O (g)
2 2
A
B
C
D
2CO2(g) + 3H2O(l)
Which of the following terms could be used
for the enthalpy change shown above?
(1) ΔHOc
(2) ΔHOf
(3) ΔHOr
A
B
C
D
8
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
C
A
(1) Iron(III) oxide is not readily decomposed
by heat.
(2) The enthalpy change of the above
reaction represents the enthalpy change
of formation of iron(III) oxide.
(3) 820 kJ are released when 1 mole of
iron(III) oxide is decomposed into its
elements.
60 Consider the following enthalpy level diagram:
O
–1
ΔH = –1 560 kJ mol
only
only
and (3) only
and (3) only
62 The reaction between iron and oxygen can be
represented as follows:
3
2Fe(s) +
O2(g)
Fe2O3(s) ΔH = –820 kJ
2
Which of the following statements are correct?
A
C2H6(g) +
(1)
(2)
(1)
(2)
2H2O2(aq)
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
A
Directions :
A
B
C
D
Each question (Questions 63 – 67) consists of two separate statements. Decide whether each
of the two statements is true or false; if both are true, then decide whether or not the second
statement is a correct explanation of the first statement. Then select one option from A to D
according to the following table :
Both statements are true and the 2nd statement is a correct explanation of the 1st statement.
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st statement.
The 1st statement is false but the 2nd statement is true.
Both statements are false.
Part A
1st statement
2nd statement
In an exothermic reaction, the total enthalpy
of the products is greater than that of the
reactants.
D
64 An endothermic reaction occurs when quicklime
is added to water.
When quicklime is added to water, the
temperature of the water rises.
C
65 The combustion of ethanol is an endothermic
reaction.
Heat is released when ethanol burns.
C
66 The standard enthalpy change of combustion
of carbon (graphite) equals the standard
enthalpy change of formation of carbon
monoxide.
Both enthalpy changes refer to the enthalpy
change of the reaction between carbon
(graphite) and oxygen to form carbon
monoxide under standard conditions.
D
67 Iron powder can be used to make warm-packs
for keeping users warm.
When iron powder reacts with oxygen, an
exothermic reaction occurs.
A
Unit 29
63 In an exothermic reaction, heat is taken in from
the surroundings.
9
Unit 30 Hess’s Law and its
applications
7
If a reaction can be written as the sum
of several steps, the enthalpy change for
the overall reaction equals the sum of
the enthalpy changes of the steps.
T
8
The enthalpy change of formation of
magnesium carbonate can be obtained
by applying Hess’s Law.
T
9
The standard enthalpy change of formation F
of ethyne (C2H2) is the sum of the standard
enthalpy changes of formation of carbon
(graphite) and hydrogen.
Fill in the blanks
1
Law states that the enthalpy
Hess’s
change of a reaction depends on the
and
2
final
initial
states of the reaction.
The following information is required to calculate
the enthalpy change of formation of magnesium
oxide:
Part A
a) enthalpy change for the reaction between
magnesium and
hydrochloric acid
;
Unit 30
b) enthalpy change for the reaction between
and
magnesium oxide
acid
hydrochloric
water
compound = sum of standard enthalpy changes
of combustion of
–
constituent elements
standard enthalpy change of combustion
4
compound
Standard enthalpy change of a reaction = sum
of standard enthalpy changes of formation
of
products
Multiple choice questions
.
Standard enthalpy change of formation of a
of
F
;
c) e n t h a l p y c h a n g e o f f o r m a t i o n o f
3
10 The standard enthalpy change for the
decomposition of calcium carbonate is
the sum of the standard enthalpy changes
of formation of calcium oxide and carbon
dioxide.
– sum of standard enthalpy
changes of formation of
reactants
11 Which of the following values CANNOT be
determined directly by an experiment?
A Enthalpy change of formation of water
B Enthalpy change of combustion of butane
C Enthalpy change of formation of carbon
monoxide
D Enthalpy change of solution of ammonium
nitrate
C
Directions: Q u e s t i o n s 1 2 a n d 1 3 re f e r t o t h e
following information.
A + B
C + D
ΔHO = –10.0 kJ mol–1
C + D
E
ΔHO = +15.0 kJ mol–1
True or false
Decide whether each of the following statements is
true or false.
5
6
10
The enthalpy change of formation of
solid sodium hydroxide can be determined
directly from an experiment.
The enthalpy change of combustion of
butane can be determined directly from
an experiment.
F
T
12 What is the standard enthalpy change of the
following reaction?
2C + 2D
A
B
C
D
+20.0
+10.0
–10.0
–20.0
2A + 2B
kJ
kJ
kJ
kJ
A
13 What is the standard enthalpy change of the
following reaction?
E
A
B
C
D
+25.0 kJ
+5.0 kJ
–5.0 kJ
–25.0 kJ
B
N2(g) + 2O2(g)
2NO2(g)
ΔH = +88 kJ
N2(g) + 2O2(g)
N2O4(g)
ΔH = +10 kJ
2N2O4(g)
kJ
kJ
kJ
kJ
W(s) + 3Br2(l)
A
B
C
D
ΔH = –312 kJ
Cu2O(s) ΔH = –170 kJ
2Cu(s) + O2(g)
ΔH = –170
Cu2O(s) +
1
O (g)
2 2
ΔH = –312
2CuO(s)
WBr4(s)
ΔHO = –147 kJ mol–1
WBr6(s)
ΔHO = –184 kJ mol–1
What is the standard enthalpy change of the
following reaction?
Br2(l) + WBr4(s)
2CuO(s)
C
15 The standard enthalpy changes of formation of
two wolfram bromides are given below.
W(s) + 2Br2(l)
2Cu(s) + O2(g)
1
2Cu(s) +
O2(g)
2
Unit 30
+196
+156
–156
–196
A
Part A
What is the enthalpy change of the following
reaction?
A
B
C
D
kJ
kJ
kJ
kJ
17 The energy level diagram shown below relates
to the following two reactions:
14 Given the following data:
4NO2(g)
–200
–295
–690
–790
Enthalpy (kJ)
A + B
A
B
C
D
WBr6(s)
–37 kJ mol–1
–331 kJ mol–1
+37 kJ mol–1
+331 kJ mol–1
What is the value of ΔH of the following
reaction?
4CuO(s)
A
B
C
D
–284
–142
+142
+284
2Cu2O(s) + O2(g)
kJ
kJ
kJ
kJ
18 Given the following data:
C(s) + O2(g)
2C(s) + O2(g)
A
D
CO2(g)
2CO(g)
ΔHO = –394 kJ
ΔHO = –222 kJ
How much heat is released in the complete
combustion of 56.0 g of CO(g)?
(Relative atomic masses: C = 12.0, O = 16.0)
16 Given the following data:
S(s) + O2(g)
SO2(g)
ΔH = –295 kJ
and
3
O2(g)
SO3(g)
ΔH = –395 kJ
2
What is the enthalpy change of the following
reaction under the same conditions?
S(s) +
2SO2(g) + O2(g)
A
B
C
D
172 kJ
566 kJ
616 kJ
1 010 kJ
B
2SO3(g)
11
19 Many insects and small animals have unique
defence systems. Bombardier beetles fight
off predators with a hot chemical spray. This
spray consists of solutions of hydroquinone
(C6H4(OH)2(aq)), hydrogen peroxide (H2O2(aq))
and enzymes.
The following equations relate to the equation
for spray formation:
21 Consider the enthalpy changes of the following
reactions:
1
N2O(g)
N2(g) +
O2(g)
ΔH = p
2
1
NO(g) +
O2(g)
NO2(g)
ΔH = q
2
N2(g) + O2(g)
2NO(g)
ΔH = r
N2O(g) + NO2(g)
ΔH = s
I
2H2O(l) + O2(g)
What is the relationship between the enthalpy
changes p, q, r and s?
II
H2O(l)
A
B
C
D
2H2O2(aq)
ΔH = +189 kJ
1
H2(g) +
O2(g)
2
ΔH = +286 kJ
Part A
III C6H4(OH)2(aq)
C6H4O2(aq) + H2(g)
ΔH = +177 kJ
Unit 30
A chemical reaction that occurs in order
to produce the hot chemical spray can be
represented by the equation
C6H4(OH)2(aq) + H2O2(aq)
hydroquinone
A
B
C
D
+274
–145
–204
–298
C6H4O2(aq) + 2H2O(l)
quinone
kJ
kJ
kJ
kJ
C
ΔH = –210 kJ mol–1
A
B
C
D
ΔH = –98 kJ mol–1
Y
What is the enthalpy change of the reaction Z
to Y?
A
B
C
D
p
p
p
p
+
–
+
–
q
q
q
q
+ r
– r
– r
+ r
D
Substance
ΔHOc (kJ mol–1)
C(graphite)
–394
H2(g)
–286
C3H6(g)
cyclopropane
–2 090
+42 kJ mol–1
–42 kJ mol–1
+378 kJ mol–1
–378 kJ mol–1
–1 410 kJ mol–1
–1
–50 kJ mol
–1
+50 kJ mol
–1
+1 410 kJ mol
C
23 The table below lists the standard enthalpy
changes of combustion of three substances.
Z
ΔH = –70 kJ mol–1
X
=
=
=
=
What is the standard enthalpy change of
formation of cyclopropane?
20 Consider the enthalpy change cycle shown
below.
W
s
s
s
s
22 The table below lists the standard enthalpy
changes of combustion of three substances.
What is the enthalpy change of reaction for
the production of the hot chemical spray?
12
3NO(g)
A
Substance
ΔHOc (kJ mol–1)
C(graphite)
–394
H2(g)
–286
CH3C CH(g)
propyne
–1 938
What is the standard enthalpy change of
formation of propyne?
A
B
C
D
–1 328 kJ mol–1
–184 kJ mol–1
+184 kJ mol–1
+1 328 kJ mol–1
C
24 The table below lists the standard enthalpy
changes of combustion of three substances.
Substance
ΔHOc (kJ mol–1)
C(graphite)
–394
H2(g)
–286
CH3COOH(l)
–870
–1
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
B
Standard enthalpy change of formation of
methanoic acid = –x kJ mol–1
Standard enthalpy change of combustion of
methanoic acid = –y kJ mol–1
Standard enthalpy change of combustion of
hydrogen = –z kJ mol–1
What is the standard enthalpy change of
formation of carbon dioxide?
A
B
C
D
x
z
x
z
+
–
–
–
y
x
y
x
–
–
+
+
z
y
z
y
B
26 The standard enthalpy changes of formation of
ethene, water and ethanol are +52.2 kJ mol–1,
–286 kJ mol–1 and –278 kJ mol–1 respectively.
What is the standard enthalpy change of the
following reaction?
C2H4(g) + H2O(l)
A
B
C
D
+60.2 kJ mol–1
–44.2 kJ mol–1
–60.2 kJ mol–1
–512 kJ mol–1
C2H5OH(l)
28 The standard enthalpy changes of formation of
cyclopropane and propene are +55.2 kJ mol–1
and +20.4 kJ mol–1 respectively.
Unit 30
25 Given the following data:
What is the standard enthalpy change of the
following reaction?
1
2FeO(s) +
O2(g)
Fe2O3(s)
2
A +140 kJ
B –140 kJ
C +280 kJ
D –280 kJ
D
Part A
–654
–490
+490
+654
–1
ΔHOf [FeO(s)] = –270 kJ mol
ΔHOf [Fe2O3(s)] = –820 kJ mol
What is the standard enthalpy change of
formation of ethanoic acid?
A
B
C
D
27 Given the following data:
What is the standard enthalpy change of the
following reaction?
CH2
CH2
A
B
C
D
CH2 (g)
+75.6
+34.8
–34.8
–75.6
kJ
kJ
kJ
kJ
CH3CH=CH2(g)
mol–1
mol–1
mol–1
mol–1
C
29 C o n s i d e r t h e f o l l o w i n g t h e r m o c h e m i c a l
equations:
1
H2(g) +
O2(g)
H2O(l)
2
ΔHO = –286 kJ mol–1
H2O(l)
H2O(g)
ΔHO = +44 kJ mol–1
What is the standard enthalpy change of the
following reaction?
2H2(g) + O2(g)
A
B
C
D
–484
–242
+242
+484
kJ
kJ
kJ
kJ
2H2O(g)
A
B
13
30 Given the following data:
32 If solid glucose is completely burnt in the flame
of a Bunsen burner, the enthalpy change is
Compound
ΔHOf (kJ mol–1)
HCOOH(l)
–409
CO(g)
–110
H2O(l)
–286
What is the standard enthalpy change of the
following reaction?
HCOOH(l)
Part A
A
B
C
D
CO(g) + H2O(l)
+13 kJ mol–1
–1
–585 kJ mol
–1
–695 kJ mol
–1
–805 kJ mol
A
Unit 30
Directions: Q u e s t i o n s 3 1 a n d 3 2 re f e r t o t h e
following information.
Glucose is a biological fuel used by cells to satisfy
the energy needs of plants and animals. The
overall reaction for the metabolism of glucose is
represented by the following equation:
C6H12O6(s) + 6O2(g)
6CO2(g) + 6H2O(l)
ΔH (kJ mol )
C6H12O6(s)
glucose
–1 274
CO2(g)
–394
H2O(l)
–286
NH4NO3(s)
–39 kJ mol–1
–41 kJ mol–1
+43 kJ mol–1
+203 kJ mol–1
+5
+2
–2
–5
354
806
806
354
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
A
34 Given the following data:
Substance
ΔHOf (kJ mol–1)
NH3(g)
–46
F2(g)
0
NF3(g)
–114
NH4F(s)
–467
31 What is the standard enthalpy change of the
reaction for the metabolism of glucose?
A
B
C
D
N2O(g) + 2H2O(g)
The standard enthalpy changes of formation of
NH4NO3(s), N2O(g) and H2O(g) are –365 kJ mol–1,
+80 kJ mol–1 and –242 kJ mol–1 respectively.
A
B
C
D
–1
Compound
33 Ammonium nitrate decomposes according to
the following equation:
What is the standard enthalpy change of the
decomposition of ammonium nitrate?
Given the following data:
O
f
A greater than it is during cellular respiration
because the production of H2O(g) releases
more energy than does the production of
H2O(l).
B less than it is during cellular respiration
because the production of H2O(g) releases
less energy than does the production of
H2O(l).
C the same as it is in the body because the
enthalpy change is independent of the state
of the products.
D the same as it is in cellular respiration
because they are identical processes.
B
C
What is the standard enthalpy change of the
following reaction?
4NH3(g) + 3F2(g)
A
B
C
D
14
–5.35
–1.10
–1.33
–1.87
x
x
x
x
2
10
103
103
103
kJ
kJ
kJ
kJ
NF3(g) + 3NH4F(s)
–1
mol
mol–1
mol–1
mol–1
C
35 H 2S(g) burns in oxygen to make H 2O(g) and
SO2(g). Given the following standard enthalpy
changes of formation:
H2(g) + S(s)
1
H2(g) +
O2(g)
2
S(s) + O2(g)
Given the following data:
Compound
ΔHOf (kJ mol–1)
UO2(s)
–1 130
UF4(s)
–1 914
UF6(s)
–2 113
HF(g)
–271
H2O(g)
–242
–1
H2S(g) ΔHOf = –40 kJ mol
H2O(g)
ΔHOf = –242 kJ mol–1
–1
O
f
SO2(g) ΔH = –297 kJ mol
What is the standard enthalpy change of
combustion of H2S(g)?
What is the amount of heat involved in
producing 2.00 x 106 g of UF6(s) from natural
uranium ore, UO2(s)?
–579 kJ mol–1
–499 kJ mol–1
–1
–95 kJ mol
–55 kJ mol–1
B
(Relative atomic masses: F = 19.0, U = 238.0)
A
B
C
D
2.18
2.76
5.46
7.65
x
x
x
x
106
106
106
106
kJ
kJ
kJ
kJ
Unit 30
36 The Thermite Process involves the reaction
between aluminium and iron(III) oxide to
produce iron and aluminium oxide.
Part A
A
B
C
D
A
Given the following data:
Compound
ΔHOf (kJ mol–1)
Al2O3(s)
–1 676
Fe2O3(s)
–825
38 The standard enthalpy changes of formation of
some vanadium compounds are listed below.
V(s) + Cl2(g)
ΔHOf = –452 kJ mol–1
What is the standard enthalpy change of the
following reaction?
2Al(s) + Fe2O3(s)
A
B
C
D
Al2O3(s) + 2Fe(s)
+2 501 kJ mol–1
+851 kJ mol–1
–851 kJ mol–1
–2 501 kJ mol–1
Equation II
UF4(s) + F2(g)
V(s) +
3
Cl2(g)
2
V(s) + 2Cl2(g)
VCl3(s)
ΔHOf = –581 kJ mol–1
VCl4(l)
ΔHOf = –569 kJ mol–1
What is the amount of heat involved when 0.400
mole of VCl 4(l) decomposes to form VCl 2(g)
and Cl2(g)?
C
37 The uranium-235 isotope is used as a fuel
in some nuclear power plants. This isotope
is used to enrich natural uranium ore. Prior
to the enrichment process, the uranium ore,
UO2(s), is converted to UF6(s). This conversion is
represented by the following equations.
Equation I
UO2(s) + 4HF(g)
VCl2(s)
UF4(s) + 2H2O(g)
UF6(s)
A
B
C
D
+176 kJ
+46.8 kJ
–46.8 kJ
–176 kJ
B
39 Given the following data:
C3H6(g) +
9
O2(g)
2
C(s) + O2(g)
2H2(g) + O2(g)
3CO2(g) + 3H2O(l)
ΔHO = –1 959 kJ
CO2(g)
ΔHO = –394 kJ
2H2O(l) ΔHO = –572 kJ
15
What is the standard enthalpy change of
formation of cyclopropane (C3H6)?
A
B
C
D
–1
+993 kJ mol
–1
+81 kJ mol
–1
–81 kJ mol
–1
–993 kJ mol
C
40 Methyl tert-butyl ether (MTBE) is prepared by
the reaction of methanol with 2-methylpropene
according to the following equation:
42 What is the standard enthalpy change of
combustion of octane?
A
B
C
D
–1
–421 kJ mol
–1
–5 470 kJ mol
–1
+421 kJ mol
–1
+5 470 kJ mol
43 Given the following data:
Compound
ΔHOf (kJ mol–1)
CO2(g)
–394
H2O(l)
–286
(CH3)2C=CH2 + CH3OH
CH3
Part A
CH3
C
O
CH3
CH3
Unit 30
MTBE
ΔHO = –57.8 kJ mol–1
Standard enthalpy changes of formation of
MTBE and methanol are –314 kJ mol –1 and
–239 kJ mol–1 respectively.
What is the standard enthalpy change of
formation of 2-methylpropene?
A
B
C
D
–133 kJ mol–1
–17.2 kJ mol–1
+17.2 kJ mol–1
+133 kJ mol–1
B
41 The standard enthalpy changes of combustion
of graphite and carbon monoxide are –394 kJ
mol–1 and –283 kJ mol–1 respectively. What is
the standard enthalpy change of formation of
carbon monoxide?
A
B
C
D
+677
+111
–111
–677
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
C
Complete combustion of 1.00 g of octane liberates
48.0 kJ at 298 K under atmospheric pressure.
16
What is the standard enthalpy change of
formation of octane?
A
B
C
D
–256
–479
+256
+479
kJ
kJ
kJ
kJ
mol–1
–1
mol
–1
mol
–1
mol
A
Directions: Questions 44 – 46 refer to the following
information.
The enthalpy change of formation of MgO(s) can
be determined by Hess’s Law.
1
Mg(s) +
O2(g)
MgO(s)
2
Mg(s) and MgO(s) were added to separate 50.0 cm3
portions of HCl(aq). The acid was in excess in both
cases.
The following results were obtained with MgO(s):
Mass of MgO(s) used = 1.10 g
Temperature change = +16.5 °C
44 Which of the following statements concerning
the reaction of MgO(s) with HCl(aq) is correct?
Directions: Q u e s t i o n s 4 2 a n d 4 3 re f e r t o t h e
combustion of octane (C8H18).
(Relative atomic masses: H = 1.0, C = 12.0)
B
(Density of HCl(aq) = 1.00 g cm–3; specific heat
capacity of HCl(aq) = 4.18 J g–1 K–1)
A An exothermic process releasing 3 450 J of
heat
B An exothermic process absorbing 3 450 J of
heat
C An endothermic process releasing 3 450 J
of heat
D An endothermic process absorbing 3 450 J
of heat
A
45 What is the enthalpy change when 1 mole of
MgO(s) reacts completely with HCl(aq)?
–1
(Molar mass of MgO = 40.3 g mol )
A
B
C
D
+94.2 kJ mol–1
+126 kJ mol–1
–94.2 kJ mol–1
–126 kJ mol–1
(NH4)2Cr2O7(s)
D
(1) ΔHOf [(NH4)2Cr2O7(s)]
(2) ΔHOc [H2(g)]
(3) ΔHOf [Cr2O3(s)]
A
B
C
D
Ca(OH)2(s)
only
only
and (3) only
and (3) only
D
48 What information are needed to determine
the standard enthalpy change of formation of
methane?
Unit 30
C
CaO(s) + H2O(l)
What information are needed to calculate
the enthalpy change of the decomposition of
Ca(OH)2(s)?
(1) E n t h a l p y c h a n g e o f t h e r e a c t i o n
between calcium hydroxide and dilute
hydrochloric acid
(2) Enthalpy change of the reaction between
calcium oxide and dilute hydrochloric
acid
(3) Enthalpy change of formation of water
(1) Enthalpy change of formation of propane
(2) Enthalpy change of combustion of
hydrogen
(3) Enthalpy change of solution of calcium
chloride
(1)
(2)
(1)
(2)
D
50 Calcium hydroxide decomposes on strong
heating to form calcium oxide and water.
47 W h i c h o f t h e f o l l o w i n g v a l u e s c a n b e
determined directly by an experiment?
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
Part A
What is the enthalpy change of formation of
MgO(s)?
+1 224 kJ mol–1
+612 kJ mol–1
–612 kJ mol–1
–1 224 kJ mol–1
N2(g) + 4H2O(l) + Cr2O3(s)
What information are needed to calculate
the enthalpy change of the decomposition of
ammonium dichromate?
46 The enthalpy change when 1 mole of Mg(s)
reacts completely with HCl(aq) is –452 kJ mol–1
and the enthalpy change of formation of water
is –286 kJ mol–1.
A
B
C
D
49 A decomposition of ammonium dichromate is
shown by the following equation:
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
A
(1) Enthalpy change of combustion of
methane
(2) Enthalpy change of combustion of
carbon (graphite)
(3) Enthalpy change of combustion of
hydrogen
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
D
17
Directions :
A
B
C
D
Each question (Questions 51 – 55) consists of two separate statements. Decide whether each
of the two statements is true or false; if both are true, then decide whether or not the second
statement is a correct explanation of the first statement. Then select one option from A to D
according to the following table :
Both statements are true and the 2nd statement is a correct explanation of the 1st statement.
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st statement.
The 1st statement is false but the 2nd statement is true.
Both statements are false.
1st statement
Part A
Unit 30
18
2nd statement
51 The standard enthalpy change of combustion
of methane is the sum of the standard enthalpy
change of combustion of carbon and that of
hydrogen.
The complete combustion of methane gives
carbon monoxide and water.
D
52 The standard enthalpy change of formation of
ethanol can be determined directly from an
experiment.
Carbon and hydrogen would give ethanol
when burnt in oxygen.
D
53 The standard enthalpy change of thermal
decomposition of potassium hydrogencarbonate
can be obtained by applying Hess’s Law.
Potassium hydrogencarbonate decomposes to
give potassium carbonate, carbon dioxide and
water when heated.
B
54 The enthalpy change of hydration of CuSO4(s)
can be determined directly from an experiment.
Both CuSO4(s) and CuSO4•5H2O(s) are
soluble in water.
C
55 The enthalpy change of the reaction between
calcium and water can be determined directly
from an experiment.
The standard enthalpy change of formation
of calcium is zero.
B
Part B
Topic-based exercise
Multiple choice questions
1
The decomposition of hydrogen peroxide
can be represented by the thermochemical
equation:
1
H2O2(l)
H2O(l) +
O2(g)
2
ΔH = –98.2 kJ mol–1
4
From this equation, it can be concluded that
the formation of two moles of hydrogen
peroxide from water and oxygen is an
2
A
B
C
D
5
exothermic, and carbon is reduced.
exothermic, and carbon is oxidized.
endothermic, and carbon is reduced.
endothermic, and carbon is oxidized.
B
For a system, ΔE is measured at constant
volume while ΔH is measured at constant
pressure.
2C(s) + O2(g)
2CO(g)
ΔH < 0
How do ΔE and ΔH compare for the system
during this reaction?
A
B
C
D
B
In an experiment, liquid sulphur is cooled and
converted to solid state as represented by the
following equation:
S(l)
0.209 J g–1 K–1
0.369 J g–1 K–1
0.473 J g–1 K–1
1.32 J g–1 K–1
Consider the following reaction:
The combustion of propane and cellular
respiration are similar processes. The reactions
that occur in both processes are
A
B
C
D
3
(Specific heat capacity of water = 4.18 J g–1 K–1)
Part B
A exothermic process releasing 98.2 kJ of heat.
B exothermic process releasing 196 kJ of heat.
C endothermic process absorbing 98.2 kJ of
heat.
D endothermic process absorbing 196 kJ of
heat.
D
A 77.5 g sample of brass is heated to 98.7 °C
in a boiling water bath. The brass is quickly
transferred to a calorimeter containing 103 g
of water at 18.5 °C. The final temperature of
the water and the brass is 23.5 °C. What is
the specific heat capacity of the brass?
S(s)
This conversion is
A endothermic, releases heat and has a
positive ΔH value.
B exothermic, releases heat and has a negative
ΔH value.
C exothermic, takes in heat and has a
negative ΔH value.
D endothermic, takes in heat and has a
positive ΔH value.
B
6
ΔE < ΔH
ΔE > ΔH
ΔE = ΔH
Impossible to tell from this information
A
C 2H 2(g) undergoes complete combustion as
represented by the following equation:
2C2H2(g) + 5O2(g)
4CO2(g) + 2H2O(l)
ΔH = –2 602 kJ
What is the amount of heat released by the
complete combustion of 100.0 g of C2H2(g)?
(Relative atomic masses: H = 1.0, C = 12.0)
A
B
C
D
2.50
5.00
7.50
10.0
x
x
x
x
103
103
103
103
kJ
kJ
kJ
kJ
B
19
7
When 2.42 g of ethanal (CH3CHO(l)) are burnt
in a calorimeter to produce H2O(l) and CO2(g),
65.5 kJ of heat are produced. What is the
enthalpy change of combustion of ethanal
under the experimental conditions?
(Relative atomic masses: H = 1.0, C = 12.0,
O = 16.0)
A
B
C
D
8
+1 520 kJ mol–1
–76.6 kJ mol–1
–165 kJ mol–1
–1 190 kJ mol–1
D
Part B
A student uses an aluminium calorimeter to
determine the enthalpy change of solution
of ammonium nitrate. The student assumes
that the heat capacity of the calorimeter
is negligible. The data were collected and
recorded in the following table.
Mass of aluminium calorimeter
25.45 g
Mass of aluminium calorimeter and
contents
175.45 g
Mass of ammonium nitrate
1.68 g
Initial temperature of calorimeter
and contents
22.3 °C
Final temperature of calorimeter and
contents
21.0 °C
What is the enthalpy change of solution of
ammonium nitrate?
+81.5
+38.8
–38.8
–81.5
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
A laboratory technician adds 43.1 cm3 of 11.6
mol dm–3 hydrochloric acid to water to form
500.0 cm3 of dilute solution. A temperature rise
of 2.60 °C is observed. What is the enthalpy
change of dilution of hydrochloric acid?
(Density of dilute solution = 1.00 g cm –3 ;
specific heat capacity of dilute solution = 4.18
J g–1 K–1)
A
B
C
D
B
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
2CH3OH(l) + 3O2(g)
B
2CO2(g) + 4H2O(g)
ΔHO = –1 276 kJ
What is the amount of methanol that must be
burnt to raise the temperature of 250.0 g of
water from 20.0 °C to 35.0 °C?
(Specific heat capacity of water = 4.18 J g–1 K–1)
6.12
1.23
2.46
2.46
x
x
x
x
10–3
10–2
10–2
10–1
mol
mol
mol
mol
C
11 25.0 cm3 of 2.00 mol dm–3 hydrochloric acid
were mixed with 50.0 cm3 of 1.00 mol dm–3
sodium hydroxide solution. Both solutions were
initially at 18.0 °C.
After mixing, the temperature of the final
solution was 26.5 °C.
What is the enthalpy change of neutralization
between hydrochloric acid and sodium
hydroxide solution?
(Density of final solution = 1.00 g cm–3; specific
heat capacity of final solution = 4.18 J g–1 K–1)
A
B
C
D
20
–5.45
–10.9
+5.45
+10.9
10 Given the following data:
A
B
C
D
(Molar mass of NH4NO3 = 80.0 g mol–1; density
of final solution = 1.00 g cm–3; specific heat
capacity of final solution = 4.18 J g–1 K–1)
A
B
C
D
9
+53.2
+35.6
–35.6
–53.2
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
D
12 50.0 cm3 of 1.50 mol dm–3 sodium hydroxide
solution were put in a polystyrene cup. 60.0 cm3
of dilute sulphuric acid in 5.0 cm 3 portions
were added to the alkali. The temperature
of the reaction mixture was taken after each
addition.
(Relative atomic masses: H = 1.0, N = 14.0,
O = 16.0, S = 32.1; specific heat capacity of
water = 4.18 J g–1 K–1)
5FNQFSBUVSFž$
A
B
C
D
0.107 mol dm–3
0.214 mol dm–3
1.07 mol dm–3
2.14 mol dm–3
1
N2(g) + O2(g)
2
1
H2(g) +
O2(g)
2
N2(g) + 2H2(g)
C
Enthalpy
Heat
change of
change
neutralization
(kJ)
–1
(kJ mol )
Alkali
1
50 cm3 of
50 cm3 of
–3
–3
1 mol dm
1 mol dm
HCl(aq)
NaOH(aq)
q1
2
50 cm3 of
50 cm3 of
–3
–3
2 mol dm
2 mol dm
HCl(aq)
NaOH(aq)
q2
A
B
C
D
q1
q1
q1
q1
<
=
<
=
q2
q2
q2
q2
ΔHn2
Enthalpy change
of neutralization
ΔHn1
ΔHn1
ΔHn1
ΔHn1
<
=
=
<
ΔHn2
ΔHn2
ΔHn2
ΔHn2
D
3O2(g) + N2H4(g)
ΔH = p
NO2(g)
ΔH = q
H2O(g)
ΔH = r
N2H4(g)
ΔH = s
What is the relationship between the enthalpy
changes p, q, r and s?
A
B
C
D
p
p
p
p
=
=
=
=
2q + 2r – s
q + r – 2s
s – q + r
s – 2q – 2r
D
16 Given the following data:
C(s) + H2O(g)
CO(g) + H2(g)
ΔHO = +131 kJ mol–1
CO2(g) + H2(g)
CO(g) + H2O(g)
ΔHO = +41 kJ mol–1
ΔHn1
Which of the following combinations is correct?
Heat change
mol–1
mol–1
mol–1
–1
mol
2NO2(g) + 2H2O(g)
13 The following experimental results were
obtained when dilute hydrochloric acid reacted
with dilute sodium hydroxide solution.
Acid
kJ
kJ
kJ
kJ
Part B
What is the molarity of the sulphuric acid?
Expt.
–13.1
–1.22
+1.22
+13.1
15 Consider the enthalpy changes of the following
reactions:
7PMVNFPGTVMQIVSJDBDJEBEEFEDN
A
B
C
D
14 I n a n e x p e r i m e n t , 1 7 . 2 g o f a m m o n i u m
sulphate were dissolved in 140.0 cm 3 of
water in a simple calorimeter. A temperature
change from 25.2 °C to 22.3 °C was observed.
What is the enthalpy change of solution of
ammonium sulphate under the experimental
conditions?
What is the standard enthalpy change of the
following reaction?
2CO(g)
A
B
C
D
+172 kJ
+90 kJ
–90 kJ
–172 kJ
C(s) + CO2(g)
D
C
21
17 Given the following data:
1
1
I2(s) +
Cl2(g)
2
2
I2(s)
20 Given the following standard enthalpy changes
of reaction:
ICl(g)
O
–1
ΔH = +18 kJ mol
ΔHO = +62 kJ mol–1
I2(g)
What is the standard enthalpy change of the
following reaction?
I2(g) + Cl2(g)
A
B
C
D
–80
–26
+26
+80
2ICl(g)
kJ
kJ
kJ
kJ
B
Part B
N2(g) + 2O2(g)
2NO2(g) ΔHO = +67.6 kJ
N2(g) + 2O2(g)
N2O4(g)
ΔHO = +9.66 kJ
What is the standard enthalpy change of
dimerization of NO2(g)?
–57.9
–77.3
+57.9
+77.3
H2(g) +
A
19 The table below lists the standard enthalpy
change of formation of four compounds:
Compound
ΔHOf (kJ mol–1)
H2O(l)
–286
HCl(g)
–92
SiO2(s)
–910
SiCl4(l)
–640
22
–76
–66
+66
+76
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
ΔH = –394 kJ
H2O(l)
CH4(g) + 2O2(g)
A
B
C
D
ΔHO = –286 kJ
CO2(g) + 2H2O(l)
ΔHO = –890 kJ
+1 068 kJ mol–1
+782 kJ mol–1
–76 kJ mol–1
–1 570 kJ mol–1
C
Directions: Q u e s t i o n s 2 1 a n d 2 2 re f e r t o t h e
following information.
One component of acid rain can be formed in the
atmosphere by the following reaction:
H2SO4(aq)
ΔHO = –227.8 kJ mol–1
21 As SO3(g) dissolves, acid rain
A
B
C
D
increases in pH and decreases in temperature.
increases in pH and increases in temperature.
decreases in pH and increases in temperature.
decreases in pH and decreases in temperature.
C
22 The table below lists the standard enthalpy
changes of formation of two compounds.
Compound
ΔHOf (kJ mol–1)
SO3(g)
–395.7
H2O(l)
–286
What is the standard enthalpy change of the
hydrolysis of SiCl4(l)?
A
B
C
D
1
O2(g)
2
SO3(g) + H2O(l)
kJ
kJ
kJ
kJ
SiCl4(l) + 2H2O(l)
O
CO2(g)
What is the standard enthalpy change of
formation of CH4(g)?
18 Given the following data:
A
B
C
D
C(s) + O2(g)
What is the standard enthalpy change of
formation of H2SO4(aq) in the atmosphere?
SiO2(s) + 4HCl(g)
B
A
B
C
D
–454
–587
–814
–910
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
D
23 The table below lists the standard enthalpy
changes of combustion of three substances.
26 Ethanoic acid is prepared by the reaction of
ethanol with oxygen.
CH3CH2OH(l) + O2(g)
ΔHOc (kJ mol–1)
C(graphite)
–394
H2(g)
–286
Compound
ΔHOf (kJ mol )
CH3OH(l)
–726
CH3CH2OH(l)
–278
CH3COOH(l)
–485
H2O(l)
–286
Given the following data:
What is the standard enthalpy change of
formation of methanol?
A
B
C
D
–548
–240
+240
+548
kJ
kJ
kJ
kJ
mol
mol–1
mol–1
mol–1
What is the standard enthalpy change of the
above reaction of ethanol with oxygen?
B
ΔH (kJ mol )
C(graphite)
–394
H2(g)
–286
C4H6(g)
buta-1,3-diene
–2 542
C
Directions: Q u e s t i o n s 2 7 a n d 2 8 re f e r t o t h e
following information.
Disposable lighters contain butane gas which
undergoes combustion, as represented by the
following equation:
–1
2C4H10(g) + 13O2(g)
A
8CO2(g) + 10H2O(g)
The table below lists the standard enthalpy changes
of formation of three substances.
B
25 Given that the enthalpy changes of combustion
of carbon, hydrogen and propane are –x, –y
and –z kJ mol–1 respectively, then the enthalpy
change of formation of propane is given by
–3x – 4y + z
3x + 4y – z
–3x + 4y + z
–3x – 8y – z
–79 kJ mol–1
–477 kJ mol–1
–493 kJ mol–1
+79 kJ mol–1
Part B
Substance
+1 862 kJ mol
+108 kJ mol–1
–108 kJ mol–1
–1 862 kJ mol–1
A
B
C
D
–1
O
c
What is the enthalpy change of formation of
buta-1,3-diene?
A
B
C
D
–1
–1
24 Given the following data:
A
B
C
D
CH3COOH(l) + H2O(l)
Substance
Substance
ΔHOf (kJ mol–1)
H2O(g)
–242
CO2(g)
–394
C4H10(g)
–125
27 What is the enthalpy change for the combustion
of 1 mole of butane gas, as represented by the
above equation?
A
B
C
D
–2
–2
–5
–5
660
910
320
820
kJ
kJ
kJ
kJ
A
23
28 How much heat is produced when 1.00 g of
butane gas in a disposable lighter is completely
bur nt to form carbon dioxide and water
vapour?
(Relative atomic masses: H = 1.0, C = 12.0)
A
B
C
D
25.0 kJ
45.9 kJ
91.5 kJ
100 kJ
29 Given the following data:
–46
H2O(l)
–286
Part B
What is the standard enthalpy change of the
following reaction of ammonia?
4NH3(g) + 3O2(g)
A
B
C
D
2N2(g) + 6H2O(l)
A
2Z + 2W
ΔHO = –562.0 kJ
Given the following data:
Substance
ΔHOf (kJ mol–1)
X
–22.5
Y
+78.3
Z
–54.8
24
kJ
kJ
kJ
kJ
kJ
kJ
kJ
kJ
mol–1
mol–1
mol–1
mol–1
D
32 ΔHOf [Ca(OH)2(s)] refers to the enthalpy change
of the following process:
Ca(OH)2(s)
mol–1
mol–1
mol–1
mol–1
What is the enthalpy change of formation of
calcium hydroxide?
A
B
C
D
–981
–695
+695
+981
kJ
kJ
kJ
kJ
mol–1
–1
mol
–1
mol
–1
mol
A
33 W h i c h o f t h e f o l l o w i n g r e a c t i o n s a r e
endothermic?
What is the standard enthalpy change of
formation of substance W?
+442
–120
–240
–451
+409
+102
–102
–409
The standard enthalpy change of formation of
water is –286 kJ mol–1.
30 Consider the following reaction:
A
B
C
D
A
B
C
D
Ca(s) + O2(g) + H2(g)
–1 532 kJ
–868 kJ
+868 kJ
+1 532 kJ
X + 3Y
(Relative atomic mass: Ca = 40.1; specific heat
capacity of water = 4.18 J g–1 K–1)
31 What is the enthalpy change of the reaction
between calcium and water?
–1
ΔHOf (kJ mol )
NH3(g)
When calcium is placed in water, calcium hydroxide
and hydrogen are formed.
In an experiment, 1.00 g of calcium was placed in
200.0 g of water and the temperature of water
increased by 12.2 °C after the reaction completed.
B
Compound
Directions: Q u e s t i o n s 3 1 a n d 3 2 re f e r t o t h e
following experiment.
B
(1) C9H20(l)
C2H6(g) + C3H6(g) + C4H8(g)
(2) C(graphite) + O2(g)
CO2(g)
(3) CaCO3(s)
CaO(s) + CO2(g)
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
B
34 Consider the following calorimetry experimental
data:
(1) Mass change of ethanol
(2) Mass of aluminium calorimeter
(3) Mass of aluminium calorimeter and
water
(4) Initial temperature of aluminium
calorimeter
(5) Maximum temperature change of ethanol
(6) Maximum temperature change of
aluminium calorimeter and water
Which of the calorimetry experimental data are
required to determine the enthalpy change of
combustion of ethanol?
(1),
(3),
(1),
(2),
(3) and (4) only
(4) and (5) only
(2), (3) and (6) only
(4), (5) and (6) only
C
35 For which of the following reactions does
the value of ΔHO represent both the standard
enthalpy change of combustion of an element
and the standard enthalpy change of formation
of a compound?
1
O2(g)
2
1
(2) Ca(s) +
O2(g)
2
(1) C(s) +
(3) H2(g) + O2(g)
A
B
C
D
(1)
(2)
(1)
(2)
only
only
and (3) only
and (3) only
2NaHCO3(s)
What other information is required to calculate
the standard enthalpy change of formation of
Na2CO3(s)?
(1) ΔHOf [NaHCO3(s)]
(2) ΔHOf [H2O(g)]
(3) ΔHOf [CO2(g)]
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
D
38 The enthalpy change of formation of Mg(OH)2(s)
refers to the enthalpy change of the following
process:
Mg(s) + O2(g) + H2(g)
Mg(OH)2(s)
What information is needed to estimate the
enthalpy change of formation of Mg(OH)2(s)?
(1) Enthalpy change of the reaction between
magnesium and dilute hydrochloric acid
(2) E n t h a l p y c h a n g e o f t h e r e a c t i o n
between magnesium hydroxide and
dilute hydrochloric acid
(3) Enthalpy change of formation of water
CO(g)
CaO(s)
H2O2(l)
Na2CO3(s) + H2O(g) + CO2(g)
ΔHO = +128 kJ
Part B
A
B
C
D
37 S o d i u m h y d ro g e n c a r b o n a t e d e c o m p o s e s
according to the following equation:
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
D
B
36 The enthalpy changes of which of the following
reactions can ONLY be obtained by applying
Hess’s Law?
(1) The conversion of hydrated magnesium
sulphate to anhydrous magnesium
sulphate
(2) The formation of ethanol from its
elements
(3) The combustion of sucrose
A
B
C
D
(1) and (2) only
(1) and (3) only
(2) and (3) only
(1), (2) and (3)
A
25
Directions :
A
B
C
D
Each question (Questions 39 – 44) consists of two separate statements. Decide whether each
of the two statements is true or false; if both are true, then decide whether or not the second
statement is a correct explanation of the first statement. Then select one option from A to D
according to the following table :
Both statements are true and the 2nd statement is a correct explanation of the 1st statement.
Both statements are true but the 2nd statement is NOT a correct explanation of the 1st statement.
The 1st statement is false but the 2nd statement is true.
Both statements are false.
1st statement
Part B
26
2nd statement
39 The enthalpy change of an endothermic reaction
is a negative value.
In an endothermic reaction, heat is transferred
from the surroundings to the system.
C
40 ΔH rO values for reactions involving carbon should
relate to diamond rather than graphite.
The enthalpy of diamond is lower than that
of graphite.
D
41 The combustion of methanol is an exothermic
reaction.
During the combustion of methanol, the
A
amount of energy released in the bond-forming
step is greater than that used in the
bond-breaking step.
42 The standard enthalpy change of neutralization
between hydrochloric acid and sodium hydroxide
solution is the same as that between hydrochloric
acid and aqueous ammonia.
During the neutralization between a strong acid C
and a strong alkali, the only chemical change is
the combination of H+(aq) ions and OH–(aq)
ions to form water.
43 The standard enthalpy change of formation of
magnesium oxide can be determined directly
from an experiment.
Magnesium reacts with oxygen to give
magnesium oxide.
C
44 The standard enthalpy change of solution of
ammonium nitrate can only be determined by
applying Hess’s Law.
Hess’s Law states the enthalpy change of a
process depends on the route by which the
process occurs.
D
Short questions
45 Classify each of the following changes as exothermic or endothermic.
a) Discharge of a flashlight
(1 mark)
exothermic
(1)
b) Melting of ice
(1 mark)
endothermic
(1)
c) Evaporation of rubbing alcohol
(1 mark)
endothermic
(1)
d) Reaction of sodium with water
(1 mark)
exothermic
(1)
(1 mark)
exothermic
Part B
e) A lightning discharge
(1)
46 How much heat is required to heat 168 g of zinc from –12.2 °C to +25.6 °C?
(1 mark)
(Specific heat capacity of zinc = 0.400 J g–1 K–1)
Amount of heat required = 168 g x 0.400 J g–1 K–1 x (25.6 + 12.2) K
= 2 540 J
(1)
47 A 44.9 g sample of copper at 99.8 °C is dropped into a beaker containing 152 g of water at 18.5 °C.
What is the final temperature of the copper?
(3 marks)
(Specific heat capacities: copper = 0.390 J g–1 K–1, water = 4.18 J g–1 K–1)
Let Tf °C be the final temperature.
–1
Amount of heat released by copper = 44.9 g x 0.390 J g
–1
Amount of heat taken in by water = 152 g x 4.18 J g
44.9 g x 0.390 J g
–1
K–1 x (99.8 – Tf) K
K–1 x (Tf – 18.5) K
(1)
(1)
K–1 x (99.8 – Tf) K = 152 g x 4.18 J g–1 K–1 x (Tf – 18.5) K
Tf = 20.7 °C
(1)
∴ the final temperature is 20.7 °C.
27
48 How much heat is released or taken in the reaction of 2.51 g of Fe2O3(s) with enough carbon monoxide
to produce iron metal?
Fe2O3(s) + 3CO(g)
2Fe(s) + 3CO2(g)
ΔHOr = –24.8 kJ
(2 marks)
(Relative atomic masses: O = 16.0, Fe = 55.8)
Number of moles of Fe2O3 reacted =
2.51 g
159.6 g mol–1
= 0.0157 mol
(1)
Amount of heat released = 24.8 kJ x 0.0157
= 0.389 kJ
(1)
∴ 0.389 kJ of heat is released.
Part B
49 State a term which can describe the standard enthalpy change of each of the following reactions:
a) H2(g) + O2(g)
H2O2(l)
O
–1
ΔH = –188 kJ mol
(1 mark)
standard enthalpy change of formation of H2O2(l)
b) C2H5OH(l) + 3O2(g)
(1)
O
2CO2(g) + 3H2O(l)
–1
ΔH = –1 368 kJ mol
standard enthalpy change of combustion of C2H5OH(l)
c) HCl(aq) + NaOH(aq)
NaCl(aq) + H2O(l)
(1)
O
CuSO4•5H2O(s)
standard enthalpy change of hydration of CuSO4(s)
28
–1
ΔH = –57.1 kJ mol
standard enthalpy change of neutralization of HCl(aq) and NaOH(aq)
d) CuSO4(s) + 5H2O(l)
(1 mark)
ΔHO = –79.0 kJ mol–1
(1 mark)
(1)
(1 mark)
(1)
Structured questions
50 The standard enthalpy changes of combustion (ΔHcO ) of cyclohexa-1,3-diene (C6H8), cyclohexane (C6H12) and
hydrogen are as follows:
ΔHOc [C6H8(l)] = –3 584 kJ mol–1
ΔHOc [C6H12(l)] = –3 924 kJ mol–1
ΔHOc [H2(g)] = –286 kJ mol–1
a) In ΔHOc , what are the conditions indicated by the symbol O?
(3 marks)
•
At a temperature of 25 °C (298 K).
(1)
•
At a pressure of 1 atmosphere (1 atm).
(1)
•
All substances involved are in their standard states, i.e. each in its most stable physical state at 25 °C and 1 atm. (1)
C6H8(l) + 2H2(g)
C6H12(l)
Part B
b) Write a chemical equation to represent the complete hydrogenation of cyclohexa-1,3-diene.
(1 mark)
(1)
c) Use the above enthalpy changes to construct an enthalpy change cycle and use it to calculate the
standard enthalpy change of hydrogenation of cyclohexa-1,3-diene.
(4 marks)
ΔH
C6H8(l) + 2H2(g) + 9O2(g)
O
O
C6H12(l) + 9O2(g)
O
O
ΔHc [C6H8(l)] + 2 x ΔHc [H2(g)]
ΔHc [C6H12(l)]
6CO2(s) + 6H2O(l)
(1)
ΔHO + ΔHOc [C6H12(l)] = ΔHOc [C6H8(l)] + 2 x ΔHOc [H2(g)]
ΔHO = ΔHOc [C6H8(l)] + 2 x ΔHOc [H2(g)] – ΔHOc [C6H12(l)]
–1
= [(–3 584) + 2(–286) – (–3 924)] kJ mol
= –232 kJ mol
–1
(1)
(1)
(1)
–1
∴ the standard enthalpy change of hydrogenation of cyclohexa-1,3-diene is –232 kJ mol .
29
51 A student used the set-up shown below to determine the enthalpy change of combustion of propan-1-ol.
UIFSNPNFUFS
CFBLFS
XBUFS
TQJSJUMBNQ
QSPQBOPM
Results:
Mass of water in beaker = 200.0 g
Part B
Weighings
Spirit lamp + propan-1-ol before combustion = 184.78 g
Spirit lamp + propan-1-ol after combustion = 183.58 g
Temperatures
Water before heating = 28.0 °C
Water after heating = 64.1 °C
Observations:
• When the spirit lamp was being weighed, its mass was continually falling.
• A black substance formed on the bottom of the beaker as the propan-1-ol burnt.
a) Calculate the enthalpy change of combustion of propan-1-ol from the experimental results.
(2 marks)
(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0; specific heat capacity of water = 4.18 J g–1 K–1;
you may assume the container has negligible heat capacity.)
Amount of heat released during combustion = 200.0 g x 4.18 J g–1 K–1 x (64.1 – 28.0) K
= 30 200 J
= 30.2 kJ
(184.78 – 183.58) g
Number of moles of propan-1-ol burnt =
60.0 g mol–1
(1)
= 0.0200 mol
–30.2 kJ
Enthalpy change of combustion of propan-1-ol =
0.0200 mol
= –1 510 kJ mol–1
–1
∴ the enthalpy change of combustion of propan-1-ol is –1 510 kJ mol .
30
(1)
b) i) Give a reason why the mass of the spirit lamp fell as it was being weighed.
Evaporation of propan-1-ol
(1 mark)
(1)
ii) Suggest the identity of the black substance that forms on the beaker. State the effect on the value
of the enthalpy of combustion obtained.
(2 marks)
Carbon
(1)
Less exothermic
(1)
c) Use the following data to construct an enthalpy change cycle and use it to calculate the standard
enthalpy change of combustion of propan-1-ol.
(4 marks)
Substance
ΔHOf (kJ mol–1)
C3H7OH(l)
–303
H2O(l)
–286
CO2(g)
–394
Part B
C3H7OH(l) +
O
ΔHc [C3H7OH(l)]
9
O (g)
2 2
O
3CO2(g) + 4H2O(l)
O
ΔHf [C3H7OH(l)]
O
3 x ΔHf [CO2(g)] + 4 x ΔHf [H2O(l)]
3C(graphite) + 4H2(g) + 5O2(g)
(1)
ΔHOc [C3H7OH(l)] + ΔHOf [C3H7OH(l)] = 3 x ΔHOf [CO2(g)] + 4 x ΔHOf [H2O(l)]
ΔHOc [C3H7OH(l)] = 3 x ΔHOf [CO2(g)] + 4 x ΔHOf [H2O(l)] – ΔHOf [C3H7OH(l)]
–1
= [3(–394) + 4(–286) – (–303)] kJ mol
–1
= –2 020 kJ mol
(1)
(1)
(1)
–1
∴ the standard enthalpy change of combustion of propan-1-ol is –2 020 kJ mol .
31
52 Complete combustion of 3.60 g of glucose (C6H12O6) liberates 56.0 kJ of heat at 298 K under atmospheric
pressure.
a) i) Write a chemical equation for the complete combustion of glucose.
C6H12O6(s) + 6O2(g)
6CO2(g) + 6H2O(l)
(1 mark)
(1)
ii) What name is given to the above energy release process when it occurs in a living species? (1 mark)
Respiration
(1)
b) Calculate the standard enthalpy change of combustion of glucose.
(2 marks)
(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0)
3.60 g
180.0 g mol–1
Number of moles of glucose burnt =
= 0.0200 mol
(1)
Standard enthalpy change of combustion of glucose =
–56.0 kJ
0.0200 mol
Part B
= –2 800 kJ mol–1
(1)
–1
∴ the standard enthalpy change of combustion of glucose is –2 800 kJ mol .
c) The table below lists two standard enthalpy changes of formation.
Compound
ΔHOf (kJ mol–1)
CO2(g)
–394
H2O(l)
–286
Use the above enthalpy changes to construct an enthalpy change cycle and use it to calculate the
standard enthalpy change of formation of glucose.
(4 marks)
ΔHfO [C6H12O6(s)] refers to the standard enthalpy change of the following process:
6C(graphite) + 6H2(g) + 3O2(g)
C6H12O6(s)
O
6C(graphite) + 6H2(g) + 3O2(g) + 6O2(g)
O
ΔHf [C6H12O6(s)]
O
O
6 x ΔHf [CO2(g)] + 6 x ΔHf [H2O(l)]
ΔHc [C6H12O6(s)]
6CO2(g) + 6H2O(l)
32
C6H12O6(s) + 6O2(g)
(1)
ΔHOf [C6H12O6(s)] + ΔHOc [C6H12O6(s)] = 6 x ΔHOf [CO2(g)] + 6 x ΔHOf [H2O(l)]
ΔHOf [C6H12O6(s)] = 6 x ΔHOf [CO2(g)] + 6 x ΔHOf [H2O(l)] – ΔHOc [C6H12O6(s)]
(1)
–1
ΔHOf [C6H12O6(s)] = [6(–394) + 6(–286) – (–2 800)] kJ mol
(1)
= –1 280 kJ mol
–1
(1)
–1
∴ the standard enthalpy change of formation of glucose is –1 280 kJ mol .
Part B
d) Draw an enthalpy level diagram to relate the enthalpy changes depicted in the enthalpy change cycle
in (c).
(2 marks)
Enthalpy (kJ)
6C(graphite) + 6H2(g) + 9O2(g)
–1 280 kJ
C6H12O6(s) + 6O2(g)
[6(–394) + 6(–286)] kJ
–2 800 kJ
6CO2(g) + 6H2O(l)
(1 mark for correct enthalpy levels; 1 mark for correct labels)
(2)
33
53 a) In an experiment to determine the enthalpy change of combustion of pentan-1-ol, a calorimeter
containing 250.0 g of water was used. Burning 1.32 g of pentan-1-ol caused the temperature of the
water in the calorimeter to rise by 40.5 °C.
i) Draw a labelled diagram of the set-up used in the experiment.
(3 marks)
ESBVHIUTDSFFO
UIFSNPNFUFS
TUJSSFS
MJE
DBMPSJNFUFSDPOUBJOJOH
XBUFS
DMBNQ
XBUFS
TQJSJUMBNQ
QFOUBOPM
Part B
(1 mark for basic set-up; 2 marks for correct labels; award 0 mark if the set-up is not workable)
(3)
ii) Assuming that the heat capacity of the calorimeter is negligible, calculate the enthalpy change of
combustion of pentan-1-ol under the conditions of the experiment.
(2 marks)
(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0; specific heat capacity of water = 4.18 J g–1 K–1)
Amount of heat released = 250.0 g x 4.18 J g–1 K–1 x 40.5 K
= 42 300 J
= 42.3 kJ
Number of moles of pentan-1-ol =
(1)
1.32 g
88.0 g mol–1
= 0.0150 mol
Enthalpy change of combustion of pentan-1-ol =
–42.3 kJ
0.0150 mol
= –2 820 kJ mol–1
(1)
–1
∴ the enthalpy change of combustion of pentan-1-ol is –2 820 kJ mol .
b) Enthalpy changes of formation can be used, with Hess’s Law, to calculate the enthalpy change of
combustion of pentan-1-ol.
i) State Hess’s Law.
34
(2 marks)
The enthalpy change of a reaction depends on the initial and final states of the reaction
(1)
and is independent of the route by which the reaction may occur.
(1)
ii) Calculate the standard enthalpy change of combustion of pentan-1-ol.
The following table gives some standard enthalpy changes of formation.
–1
ΔHOf (kJ mol )
(3 marks)
C5H11OH(l)
O2(g)
CO2(g)
H2O(l)
–354
0
–394
–286
ΔHOc [C5H11OH(l)] refers to the standard enthalpy change of the following process:
15
C5H11OH(l) +
O2(g)
5CO2(g) + 6H2O(l)
2
ΔHOc = ∑ ΔHOf [products] – ∑ ΔHOf [reactants]
ΔHOc = 5 x ΔHOf [CO2(g)] + 6 x ΔHOf [H2O(l)] – ΔHOf [C5H11OH(l)]
–1
= [5(–394) + 6(–286) – (–354)] kJ mol
–1
(1)
(1)
(1)
= –3 330 kJ mol
Part B
–1
∴ the standard enthalpy change of combustion of pentan-1-ol is –3 330 kJ mol .
iii) State how you would expect the value obtained in part (ii) to differ if steam, rather than liquid
water, is formed.
(1 mark)
Less negative
(1)
c) Suggest why the experimental value of the enthalpy change of combustion obtained in (a) is less
reliable than the value obtained in (b).
(1 mark)
Any one of the following:
•
Heat loss occurs in (a) / no heat loss in (b)
(1)
•
Incomplete combustion of pentan-1-ol in (a)
(1)
54 A rocket propellant consists of a fuel and an oxidizer. Rockets can be launched using liquid oxygen as the
oxidizer and kerosene as fuel.
a) Based on the standard enthalpy changes of formation given below, calculate the standard enthalpy
change of combustion of decane (C10H22).
(3 marks)
Compound
ΔHOf (kJ mol–1)
C10H22(l)
–301
CO2(g)
–394
H2O(l)
–286
The standard enthalpy change of combustion of decane refers to the enthalpy change of the following process:
31
O2(g)
C10H22(l) +
10CO2(g) + 11H2O(l)
2
35
ΔHOc = 10 x ΔHOf [CO2(g)] + 11 x ΔHOf [H2O(l)] – ΔHOf [C10H22(l)]
(1)
= [10(–394) + 11(–286) – (–301)] kJ mol–1
(1)
–1
(1)
= –6 790 kJ mol
b) An important property of a fuel is its energy density. This is the energy produced per kilogram of fuel.
Assuming that the kerosene used in the rockets contains only decane, calculate the energy density of
the fuel.
(1 mark)
(Relative molecular mass: C10H22 = 142.0)
Energy density of fuel =
–6 790 x 1 000
kJ kg–1
142.0
= –47 800 kJ kg–1
(1)
Part B
c) In the rockets, 108 000 kJ of energy are produced for every kilogram of liquid oxygen used to oxidize
the decane.
Calculate the mass of decane that would produce 108 000 kJ of energy.
(1 mark)
108 000 kJ
Mass of decane =
47 800 kJ kg–1
= 2.26 kg
(1)
d) The theoretical mass of decane that could be burnt for every kilogram of liquid oxygen used is 3.48 kg.
Suggest ONE reason why the rockets are designed to use less than the theoretical mass of decane per
kilogram of oxygen.
(1 mark)
To ensure complete combustion.
(1)
55 a) The table below lists the standard enthalpy changes of formation of three substances.
Substance
ΔHOf (kJ mol–1)
Water, H2O(l)
–286
Carbon dioxide, CO2(g)
–394
Octane, C8H18(l)
–250
i) With the help of a chemical equation, state the meaning of the term ‘standard enthalpy change of
combustion of octane’.
(2 marks)
The standard enthalpy change of combustion of octane is the enthalpy change when one mole of octane is
completely burnt in oxygen under standard conditions.
25
O2(g)
8CO2(g) + 9H2O(l)
C8H18(l) +
2
36
(1)
(1)
ii) Calculate the standard enthalpy change of combustion of octane.
(3 marks)
ΔHOc [C8H18(l)] = ∑ ΔHOf [products] – ∑ ΔHOf [reactants]
= 8 x ΔHOf [CO2(g)] + 9 x ΔHOf [H2O(l)] – ΔHOf [C8H18(l)]
–1
(1)
= [8(–394) + 9(–286) – (–250)] kJ mol
(1)
= –5 480 kJ mol–1
(1)
–1
∴ the standard enthalpy change of combustion of octane is –5 480 kJ mol .
Part B
b) To travel 100 km, a small petrol car requires 3.60 x 104 kJ. Assume that petrol contains only octane. If
the engine efficiency is 20.0%, what is the volume of petrol used for this journey?
(4 marks)
(Relative molecular mass of C8H18 = 114.0; density of C8H18 = 0.660 g cm–3)
At 100.0% efficiency, number of moles of C8H18 used =
3.60 x 104 kJ
5 480 kJ mol–1
= 6.57 mol
(1)
Since engine efficiency is 20.0%
∴ number of moles of C8H18 used = 6.57 mol x
= 32.9 mol
100.0
20.0
(1)
–1
Mass of C8H18 used = 32.9 mol x 114.0 g mol
Volume of C8H18
= 3 750 g
3 750 g
used =
0.660 g cm–3
(1)
= 5 680 cm3
= 5.68 dm3
(1)
3
∴ 5.68 dm of petrol are used for the journey.
37
c) The standard enthalpy change of combustion of ethanol is –1 368 kJ mol–1. Compare the enthalpy
changes of combustion, in kJ g–1, of petrol and an alternative fuel containing petrol and 10% ethanol
by mass.
(4 marks)
(Relative molecular masses: C8H18 = 114.0, C2H5OH = 46.0)
Enthalpy change of combustion of C8H18 =
–5 480 kJ mol–1
114.0 g mol–1
= –48.1 kJ g–1
(1)
–1
Enthalpy change of combustion of C2H5OH = –1 368 kJ mol
46.0 g mol–1
= –29.7 kJ g–1
(1)
–1
Enthalpy change of combustion of alternative fuel = [0.9(–48.1) + 0.1(–29.7)] kJ g
= –46.3 kJ g–1
(1)
The alternative fuel has a lower enthalpy change of combustion.
(1)
Part B
d) Besides cutting petroleum consumption, suggest ONE additional advantage of using the alternative fuel
over using gasoline.
(1 mark)
Any one of the following:
•
Ethanol is an oxygen-containing compound. This makes it easier for the alternative fuel to undergo complete
combustion / less CO is produced / less particulates are formed.
(1)
•
Ethanol is a renewable energy source. It can be obtained from crops.
(1)
•
The cost for the production of ethanol is low in agricultural countries.
(1)
56 Lauric acid, which is found in some animal fats, has the structure shown below.
CH3(CH2)10COOH
Complete combustion of 1.00 g of lauric acid liberates 36.9 kJ of heat at 298 K under atmospheric
pressure.
a) Write a chemical equation for the complete combustion of lauric acid.
CH3(CH2)10COOH(s) + 17O2(g)
12CO2(g) + 12H2O(l)
b) Calculate the standard enthalpy change of combustion of lauric acid.
(1 mark)
(1)
(2 marks)
(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0)
Number of moles of lauric acid burnt =
1.00 g
200.0 g mol–1
= 5.00 x 10–3 mol
38
(1)
Standard enthalpy change of combustion of lauric acid =
–36.9 kJ
5.00 x 10–3 mol
–1
(1)
= –7 380 kJ mol
∴ the standard enthalpy change of combustion of lauric acid is –7 380 kJ mol–1.
c) Define the term ‘standard enthalpy change of formation’.
(2 marks)
The standard enthalpy change of formation of a substance is the enthalpy change when one mole of the substance
(1)
is formed from its elements in their standard states.
(1)
Part B
d) Use the relevant enthalpy changes to construct an enthalpy change cycle and use it to calculate the
standard enthalpy change of formation of lauric acid.
(4 marks)
(Standard enthalpy changes of formation of CO2(g) and H2O(l) are –394 kJ mol–1 and –286 kJ mol–1
respectively.)
ΔHfO [CH3(CH2)10COOH(s)] refers to the standard enthalpy change of the following process:
12C(graphite) + 12H2(g) + O2(g)
CH3(CH2)10COOH(s)
O
12C(graphite) + 12H2(g) + O2(g) + 17O2(g)
O
ΔHf [CH3(CH2)10COOH(s)]
O
CH3(CH2)10COOH(s) + 17O2(g)
O
12 x ΔHf [CO2(g)] + 12 x ΔHf [H2O(l)]
ΔHc [CH3(CH2)10COOH(s)]
12CO2(g) + 12H2O(l)
(1)
ΔHOf [CH3(CH2)10COOH(s)] + ΔHOc [CH3(CH2)10COOH(s)] = 12 x ΔHOf [CO2(g)] + 12 x ΔHOf [H2O(l)]
ΔHOf [CH3(CH2)10COOH(s)] = [12 x ΔHOf [CO2(g)] + 12 x ΔHOf [H2O(l)] – ΔHOc [CH3(CH2)10COOH(s)]
(1)
–1
ΔHOf [CH3(CH2)10COOH(s)] = [12(–394) + 12(–286) – (–7 380)] kJ mol
(1)
–1
= –780 kJ mol
(1)
–1
∴ the standard enthalpy change of formation of lauric acid is –780 kJ mol .
39
57 Ethanol is a common fuel burnt in some lightweight, compact stoves suitable for use when hiking and
camping. A diagram of such a stove is given below.
DPPLJOHQPU
GVFM
TUPWF
Consider the following information:
• The standard enthalpy change of combustion of ethanol is –1 368 kJ mol–1.
• The cooking pot is made from aluminium and has a mass of 150.0 g.
Part B
• The specific heat capacity of aluminium is 0.900 J g–1 K–1.
• The specific heat capacity of water is 4.18 J g–1 K–1.
(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0)
a) Calculate the amount of heat required to heat 750.0 g of water and the pot from 28.5 °C to
100.0 °C.
(2 marks)
Amount of heat required
–1
= 750.0 g x 4.18 J g
K–1 x (100.0 – 28.5) K + 150.0 g x 0.900 J g–1 K–1 x (100.0 – 28.5) K
(1)
= 234 000 J
= 234 kJ
(1)
b) Calculate the mass of ethanol needed to be completely burnt to provide this heat.
Number of moles of C2H5OH needed =
(2 marks)
234 kJ
1 368 kJ mol–1
= 0.171 mol
(1)
–1
Mass of C2H5OH needed = 0.171 mol x 46.0 g mol
= 7.87 g
(1)
c) Only 38.0% of the heat released by the combustion of ethanol is transferred to the cooking pot and
contents. Calculate the mass of ethanol that needs to be burnt in practice to heat the water and the
pot from 28.5 °C to 100.0 °C.
(1 mark)
Mass of C2H5OH needed in practice =
7.87 g
38.0%
= 20.7 g
40
(1)
d) Other alcohols could potentially be used as fuels. The following graph shows the relationship between
the number of carbon atoms in straight-chain alcohols and their standard enthalpy changes of
combustion.
4UBOEBSEFOUIBMQZDIBOHF
PGDPNCVTUJPOL+NPMm
m
m
m
m
m
Part B
/VNCFSPGDBSCPOBUPNT
Describe and explain the pattern shown by the graph.
(4 marks)
The greater the number of carbon atoms in a molecule of a straight-chain alcohol, the greater the standard enthalpy
change of combustion is.
(1)
The standard enthalpy change of combustion of each successive alcohol differs by the same amount.
(1)
This is because the structure of each successive alcohol differs by a –CH2– unit.
(1)
As shown in the equations below, there is a constant difference in the number of bonds broken (2 C–H bonds, 1 C–C
3 =
O O bonds) and bonds formed (2 C=O bonds and 2 O–H bonds) involved in the combustion of the
bond and
2
alcohols.
CH3OH(l) +
(1)
3
O2(g)
2
CH3CH2OH(l) + 3O2(g)
9
CH3CH2CH2OH(l) +
O2(g)
2
CO2(g) + 2H2O(l)
2CO2(g) + 3H2O(l)
3CO2(g) + 4H2O(l)
41
58 Consider the following reaction:
4CO(g) + 2NO2(g)
4CO2(g) + N2(g)
Using the following data, calculate the standard enthalpy change of the above reaction.
(4 marks)
ΔHOf [NO(g)] = +90.2 kJ mol–1
ΔHOf [CO2(g)] = –394 kJ mol–1
2NO(g) + O2(g)
2NO2(g)
ΔHO = –114 kJ
2CO(g) + O2(g)
2CO2(g)
ΔHO = –566 kJ
The following data is given:
1
1
O2(g)
(1)
N2(g) +
2
2
(2) C(s) + O2(g)
NO(g)
CO2(g)
ΔHf [NO(g)] = +90.2 kJ mol–1
O
ΔHOf [CO2(g)] = –394 kJ mol–1
Part B
(3) 2NO(g) + O2(g)
2NO2(g)
ΔHO = –114 kJ
(4) 2CO(g) + O2(g)
2CO2(g)
ΔHO = –566 kJ
Looking at the target equation, we need 4 moles of CO(g) as the reactant. So, multiply equation (4) by 2, giving equation (4)’.
(4)’ 4CO(g) + 2O2(g)
4CO2(g)
ΔHO = –1 132 kJ
We need 2 moles of NO2(g) as the reactant. Equation (3) has 2 moles of NO2(g), but it is on the product side. So, reverse
the equation, giving equation (3)’.
(3)’ 2NO2(g)
2NO(g) + O2(g)
ΔHO = +114 kJ
We need 1 mole of N2(g) as the product. Equation (1) has
1
mole of N2(g), but it is on the reactant side. So, reverse the
2
equation and multiply it by 2, giving equation (1)’.
(1)’ 2NO(g)
N2(g) + O2(g)
ΔHO = –180.4 kJ
By combining the three equations, followed by collecting like terms, we can obtain the target equation.
(1)’ 2NO(g)
N2(g) + O2(g)
(3)’ 2NO2(g)
2NO(g) + O2(g)
(4)’ 4CO(g) + 2O2(g)
4CO(g) + 2NO2(g)
4CO2(g)
4CO2(g) + N2(g)
ΔHO = –180.4 kJ
ΔHO = +114 kJ
ΔHO = –1 132 kJ
ΔHOr
ΔHOr = [(–180.4) + (+114) + (–1 132)] kJ
= –1 200 kJ
∴ the standard enthalpy change of the reaction is –1 200 kJ.
42
(2)
(1)
(1)
59 The table below lists the enthalpy changes of four reactions.
–1
Reaction
(1) MgCO3(s) + 2HCl(aq)
(2) Mg(s) + 2HCl(aq)
MgCl2(aq) + CO2(g) + H2O(l)
1
O (g)
2 2
–89.9
–512
MgCl2(aq) + H2(g)
(3) C(graphite) + O2(g)
(4) H2(g) +
ΔH (kJ mol )
–394
CO2(g)
–286
H2O(l)
a) Outline the experimental procedure to determine the enthalpy change of reaction (1) in a school
laboratory.
(3 marks)
Place excess hydrochloric acid in a polystyrene cup. Record the initial temperature of the acid.
(1)
Add a known mass of solid magnesium carbonate to the acid.
(1)
Record the maximum temperature of the reaction mixture.
(1)
Part B
b) Using the given data, construct an enthalpy change cycle and use it to calculate the enthalpy change
of formation of magnesium carbonate.
(5 marks)
ΔHf[MgCO3(s)] refers to the enthalpy change of the following process:
3
Mg(s) + C(graphite) +
O2(g)
MgCO3(s)
2
Mg(s) + C(graphite) +
3
O (g) + 2HCl(aq)
2 2
ΔHf[MgCO3(s)]
ΔH2 + ΔH3 + ΔH4
MgCO3(s) + 2HCl(aq)
ΔH1
MgCl2(aq) + CO2(g) + H2O(l)
(2)
ΔHf[MgCO3(s)] + ΔH1 = ΔH2 + ΔH3 + ΔH4
ΔHf[MgCO3(s)] = ΔH2 + ΔH3 + ΔH4 – ΔH1
(1)
–1
= [(–512) + (–394) + (–286) – (–89.9)] kJ mol
–1
= –1 102 kJ mol
(1)
(1)
–1
∴ the enthalpy change of formation of magnesium carbonate is –1 102 kJ mol .
43
c) Draw an enthalpy level diagram to relate the enthalpy changes depicted in the enthalpy change cycle
in (b).
(2 marks)
Enthalpy (kJ mol–1)
Mg(s) + C(graphite) +
3
O (g) + 2HCl(aq)
2 2
–1
[(–512) + (–394) + (–286)] kJ mol
–1 102 kJ mol–1
MgCO3(s) + 2HCl(aq)
–89.9 kJ mol–1
MgCl2(aq) + CO2(g) + H2O(l)
(1 mark for orre t enthalpy le els; 1 mark for orre t la els)
(2)
Part B
60 Silane (SiH4(g)) is forme in the rea tion of sili on an hy rogen.
The ta le elo lists the stan ar
enthalpy hanges of formation of three ompoun s:
Compound
ΔHOf (kJ mol–1)
H2O(l)
–286
SiO2(s)
–910
SiH4(g)
+34
a) Write a hemi al equation hi h orrespon s to the enthalpy hange of formation of silane.
Si(s) + 2H2(g)
SiH4(g)
(1)
) The om ustion of silane gi es sili on ioxi e an
ater .
i) Write a hemi al equation for the om ustion of silane.
SiH4(g) + 2O2(g)
(1 mark)
SiO2(s) + 2H2O(l)
(1)
ii) Using the a o e ata, al ulate the stan ar enthalpy
assumption ma e in your al ulation.
ΔHO [SiH4(g)] = ΔHOf [SiO2(s)] + 2 x ΔHOf [H2O(l)] – ΔHOf [SiH4(g)]
–1
= [(–910) + 2(–286) – (+34)] kJ mol
–1
= –1 520 kJ mol
Assumption – Hess’s La is follo e .
44
(1 mark)
hange of
om ustion of silane. State ONE
(4 marks)
(1)
(1)
(1)
(1)
61 Hess’s Law can be verified using the reactions shown below.
ΔH1
KOH(s)
KCl(aq)
HCl(aq)
ΔH2
H2O(l)
HCl(aq)
ΔH3
KOH(aq)
a) Complete the following list of required measurements in order to determine ΔH3.
(4 marks)
(1) Volume of KOH(aq)
(1)
(3) Initial temperature of HCl(aq)
(1)
(4) Volume of HCl(aq)
(1)
(5) Final temperature of KCl(aq)
(1)
Part B
(2) Initial temperature of KOH(aq)
b) Hess’s Law can be used to obtain enthalpy changes of reactions that cannot be measured directly.
The table below lists the standard enthalpy changes of formation of three compounds.
Compound
ΔHOf (kJ mol–1)
KClO3(s)
–391
KCl(s)
–437
MgO(s)
–602
Use the above enthalpy changes to construct an enthalpy change cycle and use it to calculate the
standard enthalpy change of the following reaction:
KClO3(s) + 3Mg(s)
KCl(s) + 3MgO(s)
(4 marks)
O
ΔH
KClO3(s) + 3Mg(s)
O
ΔHf [KClO3(s)]
K(s) +
KCl(s) + 3MgO(s)
O
O
ΔHf [KCl(s)] + 3 x ΔHf [MgO(s)]
1
3
Cl (g) +
O (g) + 3Mg(s)
2 2
2 2
(1)
45
ΔHOf [KClO3(s)] + ΔHO = ΔHOf [KCl(s)] + 3 x ΔHOf [MgO(s)]
ΔHO = ΔHOf [KCl(s)] + 3 x ΔHOf [MgO(s)] – ΔHOf [KClO3(s)]
(1)
= [(–437) + 3(–602) – (–391)] kJ
(1)
= –1 850 kJ
(1)
∴ the standard enthalpy change of the reaction is –1 850 kJ.
Part B
62 An experiment was carried out to determine the enthalpy change of hydration of BaCl2(s), i.e. the enthalpy
change of the following process:
BaCl2(s) + 2H2O(l)
BaCl2•2H2O(s)
0.0300 mole of BaCl2(s) and 0.0300 mole of BaCl2•2H2O(s) were dissolved separately in 50.0 cm3 of water
in a polystyrene cup. The maximum change in temperature of each mixture was determined. The table
below lists the results obtained:
The form of barium chloride used
Maximum change in temperature (°C)
BaCl2(s)
+1.20
BaCl2•2H2O(s)
–3.00
a) Calculate, under the conditions of the experiment, the enthalpy change of solution of each of the
following compounds.
(You may assume that the barium chloride solution formed has a specific heat capacity of 4.18 J g–1 K–1
and a density of 1.00 g cm–3, and that the heat capacity of the polystyrene cup is negligible.)
i) BaCl2(s)
(2 marks)
Heat released when BaCl2(s) dissolved = 50.0 g x 4.18 J g–1 K–1 x 1.20 K
= 251 J
–251 J
Enthalpy change of solution of BaCl2(s) =
0.0300 mol
(1)
–1
= –8 370 J mol
= –8.37 kJ mol–1
46
(1)
ii) BaCl2•2H2O(s)
(2 marks)
Heat taken in when BaCl2•2H2O(s) dissolved = 50.0 g x 4.18 J g–1 K–1 x 3.00 K
= 627 J
+627 J
Enthalpy change of solution of BaCl2•2H2O(s) =
0.0300 mol
(1)
–1
= +20 900 J mol
= +20.9 kJ mol–1
b) From your results in (a), calculate the enthalpy change of hydration of BaCl2(s).
BaCl2(s)
water
BaCl2•2H2O(s)
BaCl2(aq)
water
BaCl2(aq)
(1)
(2 marks)
ΔH1 = –8.37 kJ mol–1
ΔH2 = +20.9 kJ mol–1
Part B
The enthalpy change of the following process is required:
BaCl2(s) + 2H2O(l)
BaCl2•2H2O(s)
ΔH
Looking at the target equation, we need 1 mole of BaCl2(s) as the reactant. So, keep the first equation as it is.
We need 1 mole of BaCl2•2H2O(s) as the product. The second equation has 1 mole of BaCl2•2H2O(s), but it is on the
reactant side. So, reverse the second equation.
ΔH = ΔH1 – ΔH2
= [(–8.37) – (+20.9)] kJ mol–1
–1
= –29.3 kJ mol
(1)
(1)
–1
∴ the enthalpy change of hydration of BaCl2(s) is –29.3 kJ mol .
c) Suggest why the enthalpy change of hydration of BaCl2(s) CANNOT be determined directly.
It is difficult to measure the temperature of a solid.
(1 mark)
(1)
47
63 Solutions of carbonates and hydrogencarbonates react with acids as shown below:
CO32–(aq) + 2H+(aq)
H2O(l) + CO2(g)
Reaction 1
HCO3–(aq) + H+(aq)
H2O(l) + CO2(g)
Reaction 2
a) The standard enthalpy change of Reaction 2 was determined experimentally by mixing known volumes
of 2.00 mol dm–3 NaHCO3(aq) and 2.20 mol dm–3 HCl(aq). The following results were obtained.
Volume of NaHCO3(aq) used = 40.0 cm3
Volume of HCl(aq) used = 40.0 cm3
Change in temperature = –1.0 °C
Use the above data to calculate the standard enthalpy change of Reaction 2.
–3
(Assume that the heat capacity of all solutions = 4.18 J cm
K–1)
(3 marks)
Total volume of solution = (40.0 + 40.0) cm3
Part B
= 80.0 cm3
Amount of heat taken in = 80.0 cm3 x 4.18 J cm–3 K–1 x 1.0 K
= 334 J
(1)
–3
Number of moles of NaHCO3 used = 2.00 mol dm x
40.0
dm3
1 000
= 0.0800 mol
Number of moles of HCl used = 2.20 mol dm–3 x
40.0
dm3
1 000
= 0.0880 mol
+
–
According to the equation of Reaction 2, 1 mole of HCO3 (aq) would react with 1 mole of H (aq).
+
–
i.e. 0.0800 mole of HCO3 (aq) would react with 0.0800 mole of H (aq).
+
–
Hence H (aq) was in excess. HCO3 (aq) was the limiting reactant.
+334 J
Standard enthalpy change of Reaction 2 =
0.0800 mol
(1)
= +4 180 J mol–1
= +4.18 kJ mol–1
48
(1)
b) i) Use the data below to calculate another value for the standard enthalpy change of Reaction 2.
(3 marks)
Species
ΔHOf (kJ mol–1)
H2O(l)
–286
CO2(g)
–394
HCO3–(aq)
–692
H+(aq)
0.0
ΔHOr = ∑ ΔHOf [products] – ∑ ΔHOf [reactants]
+
–
= ΔHOf [H2O(l)] + ΔHOf [CO2(g)] – ΔHOf [HCO3 (aq)] – ΔHOf [H (aq)]
(1)
–1
= [(–286) + (–394) – (–692)] kJ mol
(1)
= +12 kJ mol–1
(1)
Part B
ii) Compare your answers in (a) and (b)(i). Suggest one possible reason for any difference.
(1 mark)
Heat was taken in from the surroundings. Otherwise, the temperature drop could be more.
(1)
c) i) The standard enthalpy change of Reaction 1 is –2.3 kJ mol–1. Calculate the ΔHOf of CO32–(aq).
(3 marks)
ΔHOr = ∑ ΔHOf [products] – ∑ ΔHOf [reactants]
+
2–
ΔHOr = ΔHOf [H2O(l)] + ΔHOf [CO2(g)] – ΔHOf [CO3 (aq)] – 2 x ΔHOf [H (aq)]
–1
–2.3 kJ mol
(1)
= [(–286) + (–394)] kJ mol–1 – ΔHOf [CO32–(aq)]
–1
2–
ΔHOf [CO3 (aq)] = [(–286) + (–394) – (–2.3)] kJ mol
(1)
= –678 kJ mol–1
(1)
ii) Use your answer to (c)(i) to calculate the standard enthalpy change of the following reaction:
2HCO3–(aq)
CO32–(aq) + H2O(l) + CO2(g)
(3 marks)
ΔHOr = ∑ ΔHOf [products] – ∑ ΔHOf [reactants]
2–
–
= ΔHOf [CO3 (aq)] + ΔHOf [H2O(l)] + ΔHOf [CO2(g)] – 2 x ΔHOf [HCO3 (aq)]
(1)
= [(–678) + (–286) + (–394) – 2(–692)] kJ
(1)
= +26 kJ
(1)
49
64 Zinc reacts with copper(II) sulphate solution in an exothermic reaction.
Zn(s) + CuSO4(aq)
ZnSO4(aq) + Cu(s)
In an experiment to find the enthalpy change for the reaction, a student weighed out 3.17 g of zinc
3
–3
powder. The student then placed 50.0 cm of 1.00 mol dm copper(II) sulphate solution into a polystyrene
cup. The temperature of the solution in the cup was measured every minute for 10.0 minutes, with the
zinc being added at 3.5 minutes. The solution was continuously stirred during the experiment.
The temperature readings obtained are shown in the table below.
Time (min)
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Temperature (°C)
20.0
20.0
20.0
20.0
56.5
58.0
59.0
57.0
55.5
53.0
52.0
a) i) Plot a graph of temperature (°C) against time (min).
(1 mark)
Part B
5FNQFSBUVSFž$
5Gž$
å5ž$
5Jž$
5JNFNJO
(1 mark for correctly plotted points)
(1)
ii) Find, from the graph, the maximum temperature rise of the mixture. (You should show your
working on the graph.)
(2 marks)
Working on the graph to show temperatures at 3.5 minutes.
(1)
Calculated temperature change must be in the range 43.5 °C – 44.5 °C.
(1)
iii) Suggest ONE reason why a series of temperature readings was taken rather than just the initial and
final temperatures.
(1 mark)
To take into account the heat loss to the surroundings.
50
(1)
b) i) Explain why a polystyrene cup, rather than a glass beaker, was used in the experiment.
(1 mark)
Low heat capacity / good insulator of heat / low mass / absorbs less heat
ii) Explain why the solution was continuously stirred in the experiment.
(1)
(1 mark)
To ensure uniform temperature.
(1)
iii) Suggest a piece of apparatus suitable for measuring the 50.0 cm3 of copper(II) sulphate solution in
the experiment.
(1 mark)
Burette / pipette / volumetric flask
(1)
c) Show, by calculations, which of the two reactants is in excess.
(2 marks)
(Relative atomic mass: Zn = 65.4)
Number of moles of Zn =
3.17 g
–1
65.4 g mol
Part B
= 0.0485 mol
(0.5)
Number of moles of CuSO4 = 1.00 mol dm–3 x
50.0
dm3
1 000
= 0.0500 mol
(0.5)
According to the equation, 1 mole of Zn reacts with 1 mole of CuSO4. During the reaction, 0.0485 mole of Zn reacts
with 0.0485 mol of CuSO4.
∴ CuSO4 is in excess.
(1)
d) Assuming that the specific heat capacity and the density of the mixture are 4.18 J g –1 K –1 and
1.00 g cm–3 respectively, calculate the enthalpy change of this reaction, in kJ mol–1.
(2 marks)
Amount of heat released during the reaction = 50.0 g x 4.18 J g–1 K–1 x 43.8 K
= 9 150 J
= 9.15 kJ
(1)
–9.15 kJ
Enthalpy change of reaction =
0.0485 mol–1
= –189 kJ mol–1
(1)
–1
∴ the enthalpy change of the reaction is –189 kJ mol .
51
e) Suggest TWO improvements to the procedure that may give more accurate results.
(2 marks)
Any two of the following:
•
Put a lid on the polystyrene cup.
(1)
•
Put the polystyrene cup in another polystyrene cup / in a beaker.
(1)
•
Determine the heat capacities of the polystyrene cup and the thermometer. Take these values into account in the
calculations.
(1)
65 Cans of ‘self-heating’ coffee were not available until recently. Inside the can, in separate compartments,
are calcium oxide and water. When a button is pressed, these react together to give enough heat to warm
up the coffee.
a) The reaction between calcium oxide and excess water forms calcium hydroxide solution.
Write a balanced chemical equation for the reaction. Include state symbols.
Part B
CaO(s) + H2O(l)
Ca(OH)2(aq)
(1 mark)
(1)
b) A group of students determined the enthalpy change of this reaction by placing a known mass of
calcium oxide into 250.0 cm3 of water in an insulated flask and measuring the temperature rise.
The group of students recorded the measurements shown in the table below.
Mass of calcium oxide used
7.40 g
250.0 cm3
Volume of water used
Initial temperature of water
20.0 °C
Final temperature of water
57.5 °C
Calculate the amount of heat transferred to the water by the reaction of 1.00 mole of CaO(s).
(2 marks)
(Relative atomic masses: O = 16.0, Ca = 40.1; specific heat capacity of water = 4.18 J g–1 K–1; density
of water = 1.00 g cm–3)
Amount of heat released by 7.40 g of CaO(s) = 250.0 g x 4.18 J g–1 K–1 x 37.5 K
= 39 200 J
= 39.2 kJ
7.40 g
Number of moles of CaO(s) used =
56.1 g mol–1
(1)
= 0.132 mol
Amount of heat transferred to water by the reaction of 1.00 mole of CaO(s) =
39.2 kJ
0.132 mol
= 297 kJ mol–1
∴ 297 kJ of heat are transferred to the water by the reaction of 1.00 mole of CaO(s).
52
(1)
c) The reaction will produce solid calcium hydroxide if the exact molar ratio of water to calcium oxide is
used.
i) Suggest ONE reason why it is very difficult to measure this enthalpy change directly.
(1 mark)
It is hard to prevent the calcium hydroxide formed from dissolving.
(1)
ii) This enthalpy change (ΔH) can be measured indirectly by using common apparatus, solid calcium
oxide, solid calcium hydroxide and dilute hydrochloric acid.
Outline the data you need to obtain and the data treatment involved.
(5 marks)
Allow a known mass of solid calcium oxide to react with excess hydrochloric acid. Record the maximum temperature
rise.
(1)
Calculate the enthalpy change for the reaction between solid calcium oxide and hydrochloric acid, in kJ mol–1.
(1)
CaCl2(aq) + H2O(l)
ΔH1
Part B
(1) CaO(s) + 2HCl(aq)
Allow a known mass of solid calcium hydroxide to react with excess hydrochloric acid. Record the maximum
temperature rise.
(1)
Calculate the enthalpy change for the reaction between solid calcium hydroxide and hydrochloric acid,
–1
in kJ mol .
(2) Ca(OH)2(s) + 2HCl(aq)
(1)
CaCl2(aq) + 2H2O(l)
ΔH2
Combine equation (1) and the reverse form of equation (2) (i.e. equation (2)’).
(1) CaO(s) + 2HCl(aq)
(2)’ CaCl2(aq) + 2H2O(l)
CaO(s) + H2O(l)
CaCl2(aq) + H2O(l)
Ca(OH)2(s) + 2HCl(aq)
Ca(OH)2(s)
ΔH1
–ΔH2
ΔH = ΔH1 – ΔH2
(1)
53
66 a) The table below lists the enthalpy changes of formation of three compounds under standard conditions.
Compound
ΔHOf (kJ mol–1)
H2O(l)
–286
CO2(g)
–394
C4H10(g)
–125
i) State the meaning of the term ‘standard conditions’.
(3 marks)
•
At a temperature of 25 °C (298 K).
(1)
•
At a pressure of 1 atmosphere (atm).
(1)
•
All the substances involved are in their standard states, i.e. each in its most stable physical state at 25 °C and
1 atm.
(1)
ii) Write a chemical equation for the complete combustion of butane (C4H10(g)).
Part B
13
O2(g)
C4H10(g) +
2
4CO2(g) + 5H2O(l)
(1 mark)
(1)
iii) Calculate the standard enthalpy change of combustion of C4H10(g).
(3 marks)
ΔHOc [C4H10(g)] = ∑ ΔHOf [products] – ∑ ΔHOf [reactants]
= 4 x ΔHOf [CO2(g)] + 5 x ΔHOf [H2O(l)] – ΔHOf [C4H10(g)]
= [4(–394) + 5(–286) – (–125)] kJ mol
–1
–1
(1)
(1)
(1)
= –2 880 kJ mol
iv) Draw an enthalpy level diagram for the standard enthalpy change of combustion of butane. Label
your diagram fully.
(1 mark)
Enthalpy (kJ mol–1)
C4H10(g) +
13
O (g)
2 2
–1
ΔHc [C4H10(g)] = –2 880 kJ mol
O
4CO2(g) + 5H2O(l)
(1)
54
b) The enthalpy changes of combustion of some compounds in kJ g–1 and kJ cm–3 are given below:
Compound
ΔHc (kJ g–1)
ΔHc (kJ cm–3)
Butane, C4H10(g)
?
–0.12
Ethanol, C2H5OH(l)
–30
–21
2,2,4-trimethylpentane, C8H18(l)
–48
–33
–1
i) Use your answer to (a)(iii) to calculate the enthalpy change of combustion of butane in kJ g .
(Relative atomic masses: H = 1.0, C = 12.0)
Enthalpy change of combustion of butane =
(1 mark)
–2 880 kJ mol–1
58.0 g mol–1
= –49.7 kJ g–1
(1)
Part B
ii) Use the information in the table to compare the advantages and disadvantages of using these three
compounds as fuels for a motor vehicle.
(3 marks)
•
A comparison of any two or three fuels by ΔHc in kJ g–1
(1)
e.g. butane gives out the greatest amount of heat per g.
•
–3
A comparison of any two or three fuels by ΔHc in kJ cm
(1)
3
e.g. C8H18 gives out the greatest amount of heat per cm .
•
A comparison of states, and consequence of state on use as fuel in motor vehicles
(1)
e.g. C4H10 is a gas; it needs a big fuel tank to be stored at high pressure.
or C2H5OH / C8H18 is a liquid; it needs a smaller fuel tank.
55
67 Some students were measuring the enthalpy changes for the neutralization of 1 mole of different acids
with different alkalis.
a) In the first experiment, a student investigated the enthalpy change of neutralization of hydrochloric acid
with potassium hydroxide solution in the following way:
• 50.0 cm3 of 2.00 mol dm–3 hydrochloric acid were placed in a polystyrene cup and 50.0 cm3 of 2.00
mol dm–3 potassium hydroxide solution were transferred to a beaker. Both solutions were allowed to
stand for five minutes before their temperatures were noted.
• The potassium hydroxide solution was then added to the acid, the solution mixture was stirred
thoroughly and the highest temperature was noted.
• A temperature change of +13.5 °C was noted.
i) Why was it necessary to allow the two solutions to stand before mixing?
To obtain steady temperatures.
(1 mark)
(1)
Part B
ii) Calculate the enthalpy change of neutralization, in kJ mol–1.
(3 marks)
(Density of solution mixture = 1.00 g cm–3; specific heat capacity of solution mixture = 4.18 J g–1 K–1)
Total volume of the solution mixture = 50.0 cm3 + 50.0 cm3
= 100.0 cm3
Mass of the solution mixture = 100.0 g
Amount of heat released during neutralization = 100.0 g x 4.18 J g–1 K–1 x 13.5 K
= 5 640 J
= 5.64 kJ
(1)
Number of moles of HCl reacted = number of moles of KOH reacted
50.0
–3
dm3
= 2.00 mol dm x
1 000
= 0.100 mol
(1)
0.100 mole of HCl reacted with 0.100 mole of KOH to produce 0.100 mole of H2O.
–5.64 kJ
Enthalpy change of neutralization =
0.100 mol
= –56.4 kJ mol–1
–1
∴ the enthalpy change of neutralization for the reaction is –56.4 kJ mol .
56
(1)
iii) State ONE assumption made when calculating this enthalpy change, other than those stated in (a)
(ii).
(1 mark)
Any one of the following:
•
No heat was lost to the surroundings.
(1)
•
The polystyrene cup and thermometer had negligible heat capacity.
(1)
•
All the alkali was transferred from the beaker to the polystyrene cup.
(1)
b) The enthalpy changes of neutralization of three acids are given below.
• Reaction 1
HNO3(aq) with NaOH(aq)
ΔH = –57.1 kJ mol–1
• Reaction 2
HCl(aq) with NaOH(aq)
ΔH = –57.1 kJ mol–1
• Reaction 3
CH3COOH(aq) with NaOH(aq)
ΔH = –55.2 kJ mol–1
Part B
Explain why the enthalpy change is the same for Reactions 1 and 2 but different for Reaction 3.
(3 marks)
Both Reactions 1 and 2 involve the neutralization between a strong monobasic acid and sodium hydroxide solution.
The same chemical change occurs in both cases.
+
–
H (aq) + OH (aq)
(1)
H2O(l)
Hence Reactions 1 and 2 have the same enthalpy change of neutralization.
The enthalpy change for Reaction 3 is less because ethanoic acid is a weak acid.
(1)
Some energy is consumed when the acid dissociates to give hydrogen ions before neutralization.
(1)
68 Lactic acid has the structure shown below.
H
H
H
O
C
C
C
H
OH
a) Name the functional groups in lactic acid.
OH
(2 marks)
Hydroxyl group
(1)
Carboxyl group
(1)
57
b) In an experiment, the concentration of a solution of lactic acid was determined by measuring the
temperature change in its reaction with sodium hydrogencarbonate.
25.0 cm3 of the lactic acid solution were placed in a polystyrene cup with negligible heat capacity,
and the temperature of the solution was recorded every half minute for 2.5 minutes. At precisely 3.0
minutes, excess sodium hydrogencarbonate was added to the cup. The mixture was stirred and its
temperature was recorded for an additional 6 minutes. The graph below shows the plot of temperature
against time.
5Gž$
Part B
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i) Find, from the graph, the maximum temperature rise of the mixture. (You should show your
working on the graph.)
(2 marks)
Working on the graph to show temperatures at 3.0 minutes.
(1)
Maximum temperature rise of the mixture = 13.0 °C
(1)
ii) Assuming that the specific heat capacity and the density of the solution are 4.18 J g–1 K–1 and
1.00 g cm–3 respectively, calculate the enthalpy change occurring during this experiment.
(1 mark)
Enthalpy change = +25.0 g x 4.18 J g–1 K–1 x 13.0 K
= +1.36 kJ
58
(1)
iii) A data book gives the enthalpy change of this reaction as +45.2 kJ mol–1. Use this information and
your answer to (b) to calculate the concentration of the lactic acid solution.
(2 marks)
Number of moles of lactic acid in the cup =
+1.36 kJ
–1
+45.2 kJ mol
= 0.0301 mol
0.0301 mol
Concentration of the lactic acid solution =
25.0
3
dm
1 000
= 1.20 mol dm–3
(
(1)
)
(1)
c) As a check on the results from the experiment involving the measurement of temperature change,
the concentration of the lactic acid solution was also determined by titration with a standard sodium
hydroxide solution.
The equation for this reaction is
Part B
CH3CHOHCOOH(aq) + NaOH(aq)
CH3CHOHCOONa(aq) + H2O(l)
25.0 cm3 of the lactic acid solution required 20.0 cm3 of 1.60 mol dm–3 sodium hydroxide solution for
neutralization.
Calculate the concentration of the lactic acid solution.
Number of moles of NaOH = 1.60 mol dm–3 x
(2 marks)
20.0
dm3
1 000
= 0.0320 mol
(1)
According to the above equation, 1 mole of CH3CHOHCOOH requires 1 mole of NaOH for neutralization.
i.e. number of moles of CH3CHOHCOOH = 0.0320 mol
0.0320 mol
Concentration of the lactic acid solution =
25.0
3
dm
1 000
= 1.28 mol dm–3
(
)
(1)
d) Suggest TWO reasons why the value for the concentration of a lactic acid solution obtained by
measuring the temperature change is usually lower than that determined by titration.
(2 marks)
Any two of the following:
–1
K–1.
(1)
•
The specific heat capacity of the solution is greater than 4.18 J g
•
–3
The density of the solution is greater than 1.00 g cm .
(1)
•
The heat capacity of the apparatus is not taken into account.
(1)
59
69 A student conducts the following experiment to determine the enthalpy change of solution of ammonium
nitrate.
• Add 50.0 cm3 of distilled water to a polystyrene cup.
• Measure the steady temperature of the water.
• Add a sample of ammonium nitrate to the polystyrene cup, with stirring.
• Measure the final temperature of the water.
Data:
Heat capacity of polystyrene cup and water
Initial temperature of water
Final temperature of water
Mass of ammonium nitrate
228 J K–1
28.6 °C
25.3 °C
2.55 g
a) Define the term ‘standard enthalpy change of solution’.
(2 marks)
Part B
The standard enthalpy change of solution of a substance is the enthalpy change when one mole of the substance
dissolves
(1)
in an infinite volume of solvent (or enough solvent so that further dilution has no additional effect) under standard
conditions.
(1)
b) Calculate the enthalpy change of solution of ammonium nitrate.
(2 marks)
(Relative atomic masses: H = 1.0, N = 14.0, O = 16.0)
Amount of heat taken in = 228 J K–1 x (28.6 – 25.3) K
= 752 J
Number of moles of NH4NO3 =
(1)
2.55 g
80.0 g mol–1
= 0.0319 mol
Enthalpy change of solution of NH4NO3 =
+752 J
0.0319 mol
= +23 600 J mol–1
= +23.6 kJ mol–1
–1
∴ the enthalpy change of solution of ammonium nitrate is +23.6 kJ mol .
60
(1)
c) The dissolving process of this type may be slow and heat may be lost while the process is still going
on. This can make it difficult to get an accurate measurement of the temperature change in the
method above.
Suggest how, using a clock, the method can be modified to find the maximum temperature change
more accurately.
(4 marks)
Record the temperature of the water at intervals; then add the solid and stir; continue recording the temperature.
(1)
Plot the temperature of the water against time.
(1)
Join the points before solid addition using a straight line and extrapolate to the time at which the solid is added.
(1)
Join the points after solid addition using a straight line and extrapolate back to the time at which the solid is added. (1)
The separation of the lines at the time of solid addition corresponds to the maximum temperature change for the
process.
Part B
70 The heat capacity of a calorimeter and its contents can be determined by performing in the calorimeter a
reaction which produces a known amount of heat, and measuring the rise in temperature.
In one experiment, 50.0 cm3 of 0.600 mol dm–3 lead(II) nitrate solution were added to 50.0 cm3 of 1.14
mol dm–3 potassium iodide solution in a calorimeter encased in an insulating jacket. The mixture was
stirred continuously and the maximum temperature rise was 3.10 °C as the following reaction occurred.
Pb(NO3)2(aq) + 2KI(aq)
PbI2(s) + 2KNO3(aq)
–1
ΔH = –49.0 kJ mol
a) Define the term ‘heat capacity’.
(1 mark)
The heat capacity of a substance is the amount of heat required to raise the temperature of the substance by 1 K
(or 1 °C)
(1)
b) Calculate the heat capacity of the calorimeter and its contents.
Number of moles of Pb(NO3)2 = 0.600 mol dm
–3
(4 marks)
50.0
dm3
x
1 000
= 0.0300 mol
Number of moles of KI = 1.14 mol dm–3 x
(0.5)
50.0
dm3
1 000
= 0.0570 mol
(0.5)
According to the equation, 1 mole of Pb(NO3)2 reacts with 2 moles of KI. During the reaction, 0.0285 mole of
Pb(NO3)2 reacted with 0.0570 mole of KI.
61
∴ Pb(NO3)2 was in excess.
(1)
–1
Amount of heat released by the reaction = 49 000 J mol
0.0570
x
mol
2
= 1 400 J
1 400 J
Heat capacity of calorimeter and its contents =
3.10 K
(1)
= 452 J K–1
(1)
–1
∴ the heat capacity of the calorimeter and its contents is 452 J K .
c) Two errors that might have occurred during this experiment are listed below.
Part B
i) For each error, indicate the likely effect on the calculated heat capacity by placing a tick in the
appropriate box.
(2 marks)
Error
Calculated heat
capacity too low
(1) The concentration of the lead(II)
nitrate solution had been
incorrectly recorded and was
–3
actually 0.410 mol dm .
No effect on
calculated heat
capacity
Calculated heat
capacity too high
✔
(2) The calorimeter was not placed
in its insulating jacket.
✔
ii) Give an explanation for your answer in each case.
(2 marks)
Error (1)
Pb(NO3)2 was already in excess.
(1)
Error (2)
A lower temperature change leads to a higher heat capacity.
(1)
71 Volumes of nickel(II) chloride solution and 1.00 mol dm–3 sodium carbonate solution were mixed, with the
total volume always being 100.0 cm3. A precipitate formed in each case and the change in temperature
was determined. The results are shown below.
62
Volume of nickel(II) chloride solution (cm3)
10.0
30.0
50.0
70.0
80.0
90.0
Change in temperature (°C)
–0.7
–2.0
–3.4
–4.2
–2.8
–1.4
a) Plot a graph of change in temperature (°C) against volume of nickel(II) chloride solution (cm3). (1 mark)
7PMVNFPGOJDLFM**
DIMPSJEFTPMVUJPODN $IBOHFJOUFNQFSBUVSFž$
m
m
m
m
m
Part B
(1 mark for correctly plotted points)
(1)
b) Find, from the graph, the volumes of the two solutions that would produce the maximum temperature
fall when mixed. (You should show your working on the graph.)
(2 marks)
Mark two straight lines on the graph, one through the first three points and (0,0) while the other through the last three
points and (100.0,0).
(1)
3
3
3
The two straight lines intersect at 67.0 cm , i.e. 67.0 cm of nickel(II) chloride solution and 33.0 cm of sodium
carbonate solution would produce the maximum temperature fall when mixed.
(1)
c) i) Write an ionic equation for the reaction between nickel(II) chloride solution and sodium carbonate
solution.
(1 mark)
Ni2+(aq) + CO32–(aq)
NiCO3(s)
(1)
ii) Calculate the concentration of the nickel(II) chloride solution.
–3
Number of moles of Na2CO3 = 1.00 mol dm
(1 mark)
33.0
3
x
dm
1 000
= 0.0330 mol
According to the equation, 1 mole of Na2CO3 reacts with 1 mole of NiCl2.
i.e. number of moles of NiCl2 = 0.0330 mol
0.0330 mol
Concentration of NiCl2(aq) =
67.0
3
dm
1 000
= 0.493 mol dm–3
(
)
(1)
63
∴ the concentration of the nickel(II) chloride solution is 0.493 mol dm–3.
d) Calculate the enthalpy change of the reaction between nickel(II) chloride solution and sodium carbonate
solution.
(2 marks)
(Density of solution mixtures = 1.00 g cm–3, specific heat capacity of solution mixtures = 4.18 J g–1 K–1)
Amount of heat taken in = 100.0 g x 4.18 J g–1 K–1 x 4.50 K
= 1 880 J
= 1.88 kJ
+1.88 kJ
Enthalpy change of reaction =
0.0330 mol
Part B
= +57.0 kJ mol–1
–1
∴ the enthalpy change of the reaction is +57.0 kJ mol .
72 Read the following passage and answer the questions that follow.
Propellants for space shuttles
NASA and commercial launch space shuttles use four types of propellants:
•
petroleum;
•
cryogenic propellant;
•
hypergolic propellant; and
•
solid propellant.
The petroleum used as a rocket fuel is a type of kerosene similar to the sort burnt in heaters and lamps.
However, the rocket petroleum is highly refined. It is burnt with liquid oxygen (the oxidizer) to provide
thrust.
A cryogenic propellant is a mixture of liquid hydrogen (the fuel) and liquid oxygen (the oxidizer).
For a hypergolic propellant, the fuel is CH3NHNH2(l) and the oxidizer is N2O4(l). The fuel and oxidizer
react on mixing and need no ignition source. CO2(g), H2O(g) and N2(g) are produced.
A solid propellant is the oldest and simplest, dating back to the ancient Chinese. It is simply a casing filled
with a mixture of Al(s) and NH4ClO4(s) which burns at a high rate. Al2O3(s), AlCl3(s), NO(g) and H2O(g)
are produced.
64
(1)
(1)
a) The following equation represents the combustion reaction of a cryogenic propellant:
1
H2(g) +
O2(g)
H2O(g)
2
Calculate the enthalpy change at 298 K per gram of the fuel-oxygen mixture in the mole ratio as
indicated in the equation.
(Molar mass of water = 18.0 g mol–1; ΔHOf [H2O(g)] = –242 kJ mol–1)
(1 mark)
–1
Enthalpy change =
–242 kJ mol
18.0 g mol–1
= –13.4 kJ g–1
(1)
b) i) Write a chemical equation for the reaction of CH3NHNH2(l) with N2O4(l).
4CH3NHNH2(l) + 5N2O4(l)
4CO2(g) + 12H2O(g) + 9N2(g)
(1 mark)
(1)
Compound
Molar mass (g mol–1)
ΔHOf (kJ mol–1)
CH3NHNH2(l)
46.0
+53
N2O4(l)
92.0
–20
CO2(g)
44.0
–394
H2O(g)
18.0
–242
ΔH = 4 x ΔHOf [CO2(g)] + 12 x ΔHOf [H2O(g)] – 4 x ΔHOf [CH3NHNH2(l)] – 5 x ΔHOf [N2O4(l)]
Part B
ii) Given the following standard enthalpy changes of formation, calculate the enthalpy change of the
above reaction.
(3 marks)
(1)
= [4(–394) + 12(–242) – 4(+53) – 5(–20)] kJ
(1)
= –4 590 kJ
(1)
iii) Calculate the enthalpy change at 298 K per gram of CH3NHNH2(l) / N2O4(l) fuel mixture in the mole
ratio indicated in the equation written in (b)(i).
(2 marks)
Enthalpy change per gram of fuel mixture =
–4 590 kJ
[4(46.0) + 5(92.0)] g
= –7.13 kJ g–1
c) Suggest why a cryogenic propellant is considered to be environmentally friendly.
The propellant produces H2O(g) only.
(1)
(1)
(1 mark)
(1)
d) Suggest an advantage of using a hypergolic propellant in manoeuvring a shuttle.
(1 mark)
CH3NHNH2(l) reacts with N2O4(l) on mixing. The propulsion can easily be started and restarted.
(1)
65
73 Read the following passage and answer the questions that follow.
Self-heating meals
Self-heating meals come with their own self-contained heat source. Each meal contains a package of food,
a tray that holds a porous heater pad containing a supercorroding alloy made of magnesium and iron mixed
with sodium chloride, and a pouch filled with water. A string is attached to the water pouch, which when
pulled, releases water onto the heater pad. The following reaction occurs:
Mg(s) + 2H2O(l)
Mg(OH)2(s) + H2(g)
This reaction does not normally occur according to the activity series. However, in the presence of iron,
magnesium will react with water, generating heat. The heat released heats the meal in 10 to 15 minutes.
Part B
a) A self-heating meal can heat 300.0 g of water from 4.0 °C to 76.0 °C in 12 minutes. Determine the
amount of heat released by the reaction between magnesium and water.
(Specific heat capacity of water = 4.18 J g–1 K–1)
(1 mark)
Amount of heat released = 300.0 g x 4.18 J g–1 K–1 x (76.0 – 4.0) K
= 90 300 J
= 90.3 kJ
(1)
b) Given the following data, calculate the enthalpy change of the reaction of a heater pad.
Substance
ΔHOf (kJ mol–1)
H2O(l)
–286
Mg(OH)2(s)
–925
ΔHOr = ΔHOf [Mg(OH)2(s)] – 2 x ΔHOf [H2O(l)]
(3 marks)
(1)
= [(–925) – 2(–286)] kJ
(1)
= –353 kJ
(1)
c) The other substance in the heater pad is sodium chloride. Suggest why it can speed up the production
of heat.
(1 mark)
Increase the rate of redox reaction. / Facilitate electron transfer.
66
(1)
d) The following table lists the temperature changes of a self-heating meal stored just below 21 °C upon
the release of water onto the heater pad.
Time elapsed
(min)
1
2
3
4
5
6
7
8
9
10
Food
temperature (°C)
21.1
23.9
37.8
54.4
62.8
64.4
65.6
65.6
65.6
64.4
Time elapsed
(min)
11
12
13
14
15
16
17
18
19
20
Food
temperature (°C)
62.8
62.2
61.7
61.1
60.0
60.0
60.0
58.9
57.2
56.7
i) Plot a graph of temperature of the self-heating meal against time.
(2 marks)
Part B
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(1 mark for correctly plotted points; 1 mark for connecting the points smoothly)
(2)
ii) Food between 57 °C and 63 °C is suitable for eating. According to your graph, how soon would
this meal be ready to eat?
(1 mark)
Within about 4.5 minutes after the release of water
iii) Once it reaches eating temperature, how long will the meal stay within that range?
About 15 minutes
(1)
(1 mark)
(1)
67
74 The enthalpy change of formation of solid magnesium hydroxide can be estimated in a school laboratory
by using common apparatus, magnesium metal, solid magnesium hydroxide and dilute hydrochloric acid.
Outline the experimental procedure and data treatment involved.
(You may assume that the enthalpy change of formation of water is known.)
(For this question, you are required to give answers in paragraph form.)
(9 marks)
•
Place excess dilute hydrochloric acid in a polystyrene cup with a lid.
•
Measure the steady temperature of the acid.
(0.5)
•
Add solid magnesium hydroxide of mass M1 to the polystyrene cup, with stirring.
(0.5)
•
Measure the highest temperature reached.
(0.5)
•
Repeat the above steps using magnesium of mass M2 instead of magnesium hydroxide.
(0.5)
Let
C1 be the heat capacity of the Mg(OH)2 / HCl system;
Part B
C2 be the heat capacity of the Mg / HCl system;
ΔT1 be the temperature rise of the Mg(OH)2 / HCl system; and
ΔT2 be the temperature rise of the Mg / HCl system.
M1
Number of moles of Mg(OH)2(s) used =
molar mass of Mg(OH)2
= n1
(0.5)
Enthalpy change of reaction between Mg(OH)2(s) and HCl(aq)
C1 x ΔT1
ΔH1 =
n1
M2
Number of moles of Mg(s) used =
molar mass of Mg
(0.5)
= n2
(0.5)
Enthalpy change of reaction between Mg(s) and HCl(aq)
C2 x ΔT2
ΔH2 =
n2
(0.5)
O
Mg(s) + 2HCl(aq) + H2(g) + O2(g)
ΔHf [Mg(OH)2(s)]
O
ΔH2 + 2 x ΔHf [H2O(l)]
Mg(OH)2(s) + 2HCl(aq)
ΔH1
MgCl2(aq) + 2H2O(l)
By Hess’s Law, ΔHOf [Mg(OH)2(s)] = ΔH2 + 2 x ΔHOf [H2O(l)] – ΔH1
(1)
(1)
(3 marks for organization and presentation)
68
75 The enthalpy change of the following decomposition cannot be measured directly.
2NaHCO3(s)
Na2CO3(s) + H2O(l) + CO2(g)
Given solid sodium hydrogencarbonate, solid sodium carbonate and dilute hydrochloric acid, outline the
procedure for finding the enthalpy change of the decomposition. Briefly describe the data treatment
involved.
(You may assume that the enthalpy changes of formation of water and carbon dioxide are known.)
(For this question, you are required to give answers in paragraph form.)
(9 marks)
•
Place excess dilute hydrochloric acid in a polystyrene cup with a lid.
•
Measure the steady temperature of the acid.
(0.5)
•
Add solid sodium hydrogencarbonate of mass M1 to the polystyrene cup, with stirring.
(0.5)
•
Measure the highest temperature reached.
(0.5)
•
Repeat the above steps using solid sodium carbonate of mass M2 instead of sodium hydrogencarbonate.
(0.5)
Part B
Let
C1 be the heat capacity of the NaHCO3 / HCl system;
C2 be the heat capacity of the Na2CO3 / HCl system;
ΔT1 be the temperature rise of the NaHCO3 / HCl system; and
ΔT2 be the temperature rise of the Na2CO3 / HCl system.
M1
Number of moles of NaHCO3(s) used =
molar mass of NaHCO3
= n1
(0.5)
Enthalpy change of reaction between NaHCO3(s) and HCl(aq)
C1 x ΔT1
ΔH1 =
n1
M2
Number of moles of Na2CO3(s) used =
molar mass of Na2CO3
(0.5)
= n2
(0.5)
Enthalpy change of reaction between Na2CO3(s) and HCl(aq)
C2 x ΔT2
ΔH2 =
n2
2NaHCO3(s) + 2HCl(aq)
2 x ΔH1
ΔH
(0.5)
Na2CO3(s) + H2O(l) + CO2(g) + 2HCl(aq)
ΔH2
2NaCl(aq) + 2H2O(l) + 2CO2(g)
By Hess’s Law, ΔH = 2 x ΔH1 – ΔH2
(1)
(1)
(3 marks for organization and presentation)
69