Download Energetics Past Paper Questions

Chemistry 11 (HL)
Exam Review – Topics 5 and 15
Energetics Past Paper Questions
REMEMBER:
1. No calculators are allowed on Paper 1.
2. You may use an IB Periodic Table on Paper 1.
3. You may find helpful information in the DATA BOOKLET in Paper 2.
PAPER 1 (CORE)
1.
What energy changes occur when chemical bonds are formed and broken?
A.
Energy is absorbed when bonds are formed and when they are broken.
B.
Energy is released when bonds are formed and when they are broken.
C.
Energy is absorbed when bonds are formed and released when they are broken.
D.
Energy is released when bonds are formed and absorbed when they are broken.
2.
The temperature of a 2.0 g sample of aluminium increases from 25°C to 30°C.
–1 –1
How many joules of heat energy were added? (Specific heat of Al = 0.90 J g K )
A.
0.36
B.
2.3
C.
9.0
D.
11
3.
Using the equations below:
C(s) + O2(g) → CO2(g)
Mn(s) + O2(g) → MnO2(s)
∆H = –390 kJ
∆H = –520 kJ
what is ∆H (in kJ) for the following reaction?
MnO2(s) + C(s) → Mn(s) + CO2(g)
A.
910
B.
130
C.
–130
D.
–910
4.
Which combination is correct for a chemical reaction that absorbs heat from the
surroundings?
Type of reaction
ΔH at constant pressure
A.
Exothermic
Positive
B.
Exothermic
Negative
C.
Endothermic
Positive
D.
Endothermic
Negative
5.
According to the enthalpy level diagram below, what is the sign for DH and what term is used
to refer to the reaction?
A.
B.
C.
D.
DH
positive
negative
positive
negative
reaction
endothermic
exothermic
exothermic
endothermic
1
Chemistry 11 (HL)
Exam Review – Topics 5 and 15
6.
Consider the specific heat capacity of the following metals.
–1 –1
Metal
Specific heat capacity / J kg K
Cu
385
Ag
234
Au
130
Pt
134
Which metal will show the greatest temperature increase if 50 J of heat is supplied to a 0.001
kg sample of each metal at the same initial temperature?
A.
Cu
B.
Ag
C.
Au
D.
Pt
7.
Which statement is correct for an endothermic reaction?
A.
Bonds in the products are stronger than the bonds in the reactants.
B.
Bonds in the reactants are stronger than the bonds in the products.
C.
The enthalpy of the products is less than that of the reactants.
D.
The reaction is spontaneous at low temperatures but becomes non-spontaneous at
high temperatures.
8.
For the reaction
9.
Which statement about bond enthalpies is correct?
A.
Bond enthalpies have positive values for strong bonds and negative values for weak
bonds.
B.
Bond enthalpy values are greater for ionic bonds than for covalent bonds.
C.
Bond breaking is endothermic and bond making is exothermic.
D.
The carbon–carbon bond enthalpy values are the same in ethane and ethene.
10.
The following equation shows the formation of magnesium oxide from magnesium metal.
2Mg(s) + O2(g) → 2MgO(s)
∆H˚ = –1204kJ
Which statement is correct for this reaction?
A.
1204 kJ of energy are released for every mol of magnesium reacted.
B.
602 kJ of energy are absorbed for every mol of magnesium oxide formed.
C.
602 kJ of energy are released for every mol of oxygen gas reacted.
D.
1204 kJ of energy are released for every two mol of magnesium oxide formed.
11.
The average bond enthalpy for the C―H bond is 412 kJ mol
enthalpy change closest to this value?
A.
CH4(g) → C(s) + 2H2(g)
B.
CH4(g) → C(g) + 2H2(g)
C.
CH4(g) → C(s) + 4H(g)
D.
CH4(g) → CH3(g) + H(g)
12.
Which statements are correct for an endothermic reaction?
I.
The system absorbs heat.
II.
The enthalpy change is positive.
III.
The bond enthalpy total for the reactants is greater than for the products.
2H2(g) + O2(g) → 2H2O(g)
–1
the bond enthalpies (in kJ mol ) are
H–H
x
O=O
y
O–H
z
–1
Which calculation will give the value, in kJ mol , of ∆H˚ for the reaction?
A.
2x + y –2z
B.
4z – 2x – y
C.
2x + y – 4z
D.
2z –2x – y
A.
I and II only
B.
I and III only
C.
–1
. Which process has an
II and III only
D.
I, II and III
2
Chemistry 11 (HL)
Exam Review – Topics 5 and 15
13.
For which of the following is the sign of the enthalpy change different from the other three?
A.
CaCO3(s) → CaO(s) + CO2(g)
+
–
B.
Na(g) → Na (g) + e
C.
CO2(s) → CO2(g)
D.
2Cl(g) → Cl2(g)
14.
Separate solutions of HCl(aq) and H2SO4(aq) of the same concentration and same volume
were completely neutralized by NaOH(aq). X kJ and Y kJ of heat were evolved respectively.
Which statement is correct?
A.
X=Y
B.
Y = 2X
C.
X = 2Y
D.
Y = 3X
15.
How much energy, in joules, is required to increase the temperature of 2.0 g of aluminium
–1 –1
from 25 to 30°C? (Specific heat of Al = 0.90 J g K ).
A.
0.36
B.
4.5
C.
9.0
D.
54
16.
When the solids Ba(OH)2 and NH4SCN are mixed, a solution is produced and the
temperature drops.
Ba(OH)2(s) + 2NH4SCN(s) → Ba(SCN)2(aq) + 2NH3(g) + 2H2O(l)
Which statement about the energetics of this reaction is correct?
A.
The reaction is endothermic and DH is negative.
B.
The reaction is endothermic and DH is positive.
C.
The reaction is exothermic and DH is negative.
D.
The reaction is exothermic and DH is positive.
17.
Which statements about exothermic reactions are correct?
I.
They have negative DH values.
II.
The products have a lower enthalpy than the reactants.
III.
The products are more energetically stable than the reactants.
A.
18.
B.
I and III only
C.
II and III only
D.
I, II and III
A sample of a metal is heated. Which of the following are needed to calculate the heat
absorbed by the sample?
I.
The mass of the sample
II.
The density of the sample
III.
The specific heat capacity of the sample
A.
19.
I and II only
I and II only
B.
I and III only
C.
II and III only
The average bond enthalpies for O—O and O==O are 146 and 496 kJ mol
What is the enthalpy change, in kJ, for the reaction below?
H—O—O—H(g) → H—O—H(g) + ½O==O(g)
A.
– 102
B.
+ 102
C.
+ 350
D.
–1
D.
I, II and III
respectively.
+ 394
3
Chemistry 11 (HL)
Exam Review – Topics 5 and 15
PAPER 2 (CORE)
20.
Two reactions occurring in the manufacture of sulfuric acid are shown below:
reaction I
S(s) +O2(g) → SO2(g)
∆H˚ = –297 kJ
reaction II
(i)
(ii)
(iii)
SO2(g) +
O2(g) → SO3(g)
∆H˚ = –92 kJ
State the name of the term ∆H˚. State, with a reason, whether reaction I would be
accompanied by a decrease or increase in temperature. (3)
At room temperature sulfur trioxide, SO3, is a solid. Deduce, with a reason, whether
the ∆H˚ value would be more negative or less negative if SO3(s) instead of SO3(g)
were formed in reaction II. (2)
Deduce the ∆H˚ value of this reaction:
S(s) +
O2(g) → SO3(g)
(1)
(Total 6 marks)
21.
In aqueous solution, potassium hydroxide and hydrochloric acid react as follows.
KOH(aq) + HCl(aq) → KCl(aq)+ H2O(l)
The data below is from an experiment to determine the enthalpy change of this reaction.
3
–3
50.0 cm of a 0.500 mol dm solution of KOH was mixed rapidly in a glass beaker with
3
–3
50.0 cm of a 0.500 mol dm solution of HCl.
Initial temperature of each solution = 19.6°C
Final temperature of the mixture = 23.1°C
(a)
(b)
(c)
(d)
(e)
22.
State, with a reason, whether the reaction is exothermic or endothermic. (1)
Explain why the solutions were mixed rapidly. (1)
–1
Calculate the enthalpy change of this reaction in kJ mol . Assume that the specific
heat capacity of the solution is the same as that of water. (4)
Identify the major source of error in the experimental procedure described above.
Explain how it could be minimized. (2)
–3
The experiment was repeated but with an HCl concentration of 0.510 mol dm
–3
instead of 0.500 mol dm . State and explain what the temperature change would be.
(2)
(Total 10 marks)
(a)
(b)
Define the term average bond enthalpy. (2)
Use the information from Table 10 in the Data Booklet to calculate the enthalpy change
for the complete combustion of but-1-ene according to the following equation
C4H8(g) → 4CO2(g) + 4H2O(g)
(c)
Predict, giving a reason, how the enthalpy change for the complete combustion of but2-ene would compare with that of but-1-ene based on average bond enthalpies. (1)
The enthalpy level diagram for a certain reaction is shown below.
(3)
(d)
State and explain the relative stabilities of the reactants and products.
(2)
(Total 8 marks)
4
Chemistry 11 (HL)
23.
Exam Review – Topics 5 and 15
Calculate the enthalpy change, ∆H4 for the reaction
C + 2H2 +
O2 → CH3OH
using Hess’s Law, and the following information.
CH3OH +
O2 → CO2 + 2H2O
∆H4
∆H1 = -676 kJ mol
C + O2 → CO2
∆H2 = -394 kJ mol
H2 +
∆H3 = -242 kJ mol
O 2 → H 2O
-1
-1
-1
(Total 4 marks)
24.
The reaction between ethene and hydrogen gas is exothermic.
(i)
Write an equation for this reaction. (1)
(ii)
Deduce the relative stabilities and energies of the reactants and products. (2)
(iii)
Explain, by referring to the bonds in the molecules, why the reaction is exothermic. (2)
(Total 5 marks)
25.
The data below is from an experiment used to measure the enthalpy change for the
combustion of 1 mole of sucrose (common table sugar), C12H22O11(s). The timetemperature data was taken from a data-logging software programme.
Mass of sample of sucrose, m = 0.4385 g
Heat capacity of the system, Csystem = 10.114 kJ K
–1
(a)
Calculate ΔT, for the water, surrounding the chamber in the calorimeter. (1)
(b)
Determine the amount, in moles, of sucrose. (1)
(c)
(i)
Calculate the enthalpy change for the combustion of 1 mole of sucrose. (1)
(ii)
Using Table 12 of the Data Booklet, calculate the percentage experimental error
based on the data used in this experiment. (1)
5
Chemistry 11 (HL)
(d)
Exam Review – Topics 5 and 15
A hypothesis is suggested that TNT, 2-methyl-1,3,5-trinitrobenzene, is a powerful
explosive because it has:
• a large enthalpy of combustion
• a high reaction rate
• a large volume of gas generated upon combustion
Use your answer in part (c)(i) and the following data to evaluate this hypothesis:
Equation for combustion
Enthalpy of
Relative
combustion
rate of
–1
combustion
/ kJ mol
C12H22O11(s) + 12O2(g) ® 12CO2(g) + 11H2O(g)
Sucrose
Low
2C7H5N3O6(s) ® 7CO(g) + 7C(s) + 5H2O(g) + 3N2(g)
TNT
High
3406
(3)
(Total 7 marks)
26.
27.
But–1–ene gas, burns in oxygen to produce carbon dioxide and water vapour according to
the following equation.
C4H8 + 6O2 → 4CO2 + 4H2O
Ө
(a)
Use the data below to calculate the value of ∆H for the combustion of but-1-ene. (3)
Bond
C-C
C=C
C-H
O=O
C=O
O–H
Average bond
348
612
412
496
743
463
–1
enthalpy / kJ mol
(b)
State and explain whether the reaction above is endothermic or exothermic. (1)
(Total 4 marks)
(a)
Define the term average bond enthalpy, illustrating your answer with an equation for
methane, CH4. (3)
(b)
The equation for the reaction between methane and chlorine is
CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g)
Use the values from Table 10 of the Data Booklet to calculate the enthalpy change for
this reaction. (3)
(c)
Explain why no reaction takes place between methane and chlorine at room
temperature unless the reactants are sparked, exposed to UV light or heated. (2)
(d)
Draw an enthalpy level diagram for this reaction. (2)
(Total 10 marks)
28.
Given the following data:
–1
C(s) + F2(g) → CF4(g); ∆H1 = –680 kJ mol
–1
F2(g) → 2F(g); ∆H2 = +158 kJ mol
–1
C(s) → C(g); ∆H3 = +715 kJ mol
–1
calculate the average bond enthalpy (in kJ mol ) for the C––F bond.
(Total 4 marks)
6
Chemistry 11 (HL)
Exam Review – Topics 5 and 15
ADDITIONAL HIGHER LEVEL MATERIAL – Paper 1 and Paper 2
29.
Which combination of ionic charge and ionic radius give the largest lattice enthalpy for an
ionic compound?
Ionic charge
Ionic radius
A.
high
large
B.
high
small
C.
low
small
D.
low
large
30.
What is the energy change (in kJ) when the temperature of 20 g of water increases by 10°C?
A.
20×10×4.18
B.
20×283×4.18
C.
D.
31.
Which type of reaction is referred to in the definition of standard enthalpy change of
formation?
A.
the formation of a compound from its elements
B.
the formation of a crystal from its ions
C.
the formation of a molecule from its atoms
D.
the formation of a compound from other compounds
32.
Which is a correct equation to represent the lattice enthalpy of magnesium sulfide?
A.
MgS(s) → Mg(s) + S(s)
B.
MgS(s) → Mg(g) + S(g)
+
–
C.
MgS(s) → Mg (g) + S (g)
2+
2–
D.
MgS(s) → Mg (g) + S (g)
33.
For the reaction
2H2(g) + O2(g) → 2H2O(g)
the bond enthalpies (in kJ mol
–1
) are
H–H
x
O=O
y
O–H
z
–1
Ө
Which calculation will give the value, in kJ mol , of ∆H for the reaction?
A.
2x + y –2z
B.
4z – 2x – y
C.
2x + y – 4z
D.
2z –2x – y
34.
The lattice enthalpy values for lithium fluoride and calcium fluoride are shown below.
ο
–1
LiF(s)
∆H = +1022 kJ mol
ο
–1
CaF2(s) ∆H = +2602 kJ mol
Which of the following statements help(s) to explain why the value for lithium fluoride is less
than that for calcium fluoride?
I.
The ionic radius of lithium is less than that of calcium.
II.
The ionic charge of lithium is less than that of calcium.
A.
I only
B.
II only
C.
I and II
D.
Neither I nor II
7
Chemistry 11 (HL)
35.
36.
ο
Which reaction has the most negative ∆H value?
+
–
A.
LiF(s) → Li (g) + F (g)
+
–
B.
Li (g) + F (g) → LiF(s)
+
–
C.
NaCl(s) → Na (g) + Cl (g)
+
–
D.
Na (g) + Cl (g) → NaCl(s)
(a)
(b)
37.
38.
39.
Exam Review – Topics 5 and 15
The lattice enthalpy of an ionic compound can be calculated using a Born-Haber cycle.
Using lithium fluoride as the example, construct a Born-Haber cycle, labelling the cycle
with the formulas and state symbols of the species present at each stage. (6)
Two values of the lattice enthalpies for each of the silver halides are quoted in the Data
Booklet. Discuss the bonding in silver fluoride and in silver iodide, with reference to
these values (2)
(i)
Define the terms lattice enthalpy and electron affinity. (2)
(ii)
Use the data in the following table and from the data booklet to construct the BornHaber cycle for sodium chloride, NaCl, and determine the lattice enthalpy of NaCl(s).
(4)
ο
–1
Na(s) + ½ Cl2(g) → NaCl(g)
∆H = –411 kJ mol
ο
–1
Na(s) → Na(g)
∆H = +108 kJ mol
(iii)
Describe the structure of sodium chloride. (2)
(i)
Define the term average bond enthalpy. (2)
(ii)
(iii)
The equation for the reaction of ethyne and hydrogen is:
C2H2(g) + 2H2(g) → C2H6(g)
Use information from Table 10 of the Data Booklet to calculate the change in enthalpy
for the reaction. (2)
State and explain the trend in the bond enthalpies of the C–Cl, C–Br and C–I bonds.(2)
(i)
Ө
Define the term standard enthalpy change of formation, ∆Hf .
(2)
(ii)
Ө
Construct a simple enthalpy cycle and calculate the value of ∆Hf (C2H5OH(l)) given
the following data.
Ө
–1
Ө
–1
Compound
DHf / kJ mol
ΔH comb/ kJ mol
–286
H2O(l)
–394
CO2(g)
–1371
C2H5OH(l)
(5)
40.
Define the term standard enthalpy of formation, and write the equation for the standard
enthalpy of formation of ethanol. (5)
8
Chemistry 11 (HL)
Exam Review – Topics 5 and 15
ANSWERS:
PAPER 1 (CORE)
1.
6.
12.
17.
D
C
A
D
2.
7.
13.
18.
C
B
D
B
3.
8.
14.
19.
B
C
B
A
4.
10.
15.
C
D
C
5.
11.
16.
B
D
B
PAPER 2 (CORE)
20.
(a)
(i)
(ii)
(iii)
21.
(a)
(b)
(c)
(d)
(e)
22.
(a)
(b)
(c)
(d)
standard enthalpy (change) of reaction; (temperature) increase;
reaction is exothermic/sign of ∆H° is negative;
more (negative);
heat given out when gas changes to solid/solid has less enthalpy than gas
–389 kJ;
exothermic because temperature rises/heat is released;
to make any heat loss as small as possible
q = mc∆T
=100×4.18×3.5;
= 1463 J/1.463 kJ;
amount of KOH/HCl used = 0.500×0.050 = 0.025 mol;
–1
∆H = (1.463÷0.025) = –58.5 (kJ mol
heat loss (to the surroundings);
insulate the reaction vessel/use a lid/draw a temperature versus time graph;
3.5°C/temperature change would be the same;
amount of base reacted would be the same/excess acid would not react/
KOH is the limiting reagent
amount of energy needed to break one mole of (covalent) bonds in the gaseous state;
average calculated from a range of compounds;
-1
bonds broken: 161 + 2×348 + 8×412 + 6×496/7580 kJ mol ;
-1
bonds made: 8×743 + 8×463/9648 kJ mol ;
-1
(bonds broken - bonds made =) ∆H = -2068(kJ mol );
same/equal, because the same bonds are being broken and formed;
products more stable than reactants;
bonds are stronger in products than reactants
23.
-1
∆H4 = -202 (kJ mol );
24.
(a)
(i)
(ii)
(iii)
25.
(a)
ΔT = 23.70 – 23.03 = 0.67 (°C/K);
(b)
C2H4(g) + H2(g) → C2H6(g);
products more stable than reactants; products lower in energy
(overall) bonds in reactants weaker/(overall) bonds in product stronger;
less energy needed to break weaker bonds/more energy produced
to make stronger bonds (thus reaction is exothermic
= 1.281×10
–3
mol
9
Chemistry 11 (HL)
(c)
26.
Exam Review – Topics 5 and 15
3
–1
= –5.3×10 kJ mol ;
(i)
ΔHc = (C ΔT)/n =
(ii)
Percentage experimental error =
= 5.4%
(d)
enthalpy change of combustion of sucrose > TNT, and therefore not important;
rate of reaction for TNT is greater than that of sucrose, so this is valid;
amount of gas generated (in mol) for sucrose > than that of TNT
(according to the given equation), so this is not important;
(a)
(Amount of energy required to break bonds of reactants)
-1
= 8×412 + 2×348 + 612 + 6×496/7580 (kJ mol );
(Amount of energy released during bond formation)
-1
= 4×2×743 + 4×2×463/9648 (kJ mol );
-1
∆H = -2068 (kJ or kJ mol );
27.
(b)
exothermic and ∆H˚ is negative/energy is released;
(a)
energy for the conversion of a gaseous molecule into (gaseous) atoms;
(average values) obtained from a number of similar bonds/compounds
CH4(g) → C(g) + 4H(g); (State symbols needed)
(bond breaking) = 1890/654;
(bond formation) = 2005/769;
–1
enthalpy = –115(kJ mol )
(b)
(c)
molecules have insufficient energy to react (at room temperature)/
wrong collision geometry/unsuccessful collisions;
extra energy needed to overcome the activation energy/Ea for the reaction;
(d)
exothermic shown;
28.
C(s) + 2F2(g) → CF4(g)
4F(g) → 2F2(g)
C(g) → C(s)
∆H1 = –680 kJ;
∆H2 = 2(–158) kJ;
∆H3 = –715 kJ;
C(g) + 4F(g) → CF4(g)
∆H = –1711 kJ,
so average bond enthalpy =
= –428 kJ mol
–1
10
Chemistry 11 (HL)
Exam Review – Topics 5 and 15
ADDITIONAL HIGHER LEVEL MATERIAL – PAPER 1 and PAPER 2
29.
34.
B
B
36.
(a)
30. C
35. B
31.
A
32.
D
33.
C
Award [6] for completely correct cycle, with endothermic
processes in any order.
Deduct [1] for each line in which species symbol and/or state
symbol is incorrect or missing.
Penalize missing electrons once only.
(b)
bonding in AgF more ionic than in AgI/bonding in AgI more covalent than in AgF;
values closer/in better agreement in AgF/big(ger) difference in values for AgI
(i)
lattice enthalpy for a particular ionic compound is defined as ΔH for the
+
–
process, MX(s) → M (g) + X (g);
[8]
37.
electron affinity is the energy change that occurs when an electron is added to a
gaseous atom or ion;
(ii)
lattice enthalpy = –[(–411) – (+108) – (+494) – (+121) – (–364)]
–1
= 770 (kJ mol )
Award [2] for all correct formulas in correct positions on cycle
diagram.
1 incorrect or missing label award [1].
11
Chemistry 11 (HL)
(iii)
Exam Review – Topics 5 and 15
Award [1] for all correct values in correct positions on cycle
diagram.
–1
calculation of lattice enthalpy of NaCl(s) = 770 (kJ mol );
4
lattice/network/regular structure;
each chloride ion is surrounded by six sodium ions and each sodium ion is
surrounded by six chloride ions/6:6 coordination;
2
[8]
38.
(i)
(ii)
(iii)
energy required to break (a mole of) bonds in the gaseous state
/energy given out when (a mole of) bonds are made in the
gaseous state;
average value from a number of similar compounds;
Ө
(∆H reaction = (∑BEbreak - BEmake))
= [(837) + 2(436)] - [(348 + 4(412)];
-1
= - 287kJ (or kJ mol );
(BE): C-Cl > C-Br > C-I/C-X bond becomes weaker;
halogen size/radius increases/bonding electrons further away from the
nucleus/bonds become longer;
2
2
2
[6]
39.
(i)
change in energy for the formation of (1 mol) of a substance from its
elements; under standard conditions/1 atm pressure or 101 kPa and
298 K/25°C;
2
(ii)
States not required.
Correct cycle showing:
Ө
DHcomb
Ө
DHf (C2H5OH(l));
Ө
Ө
2DHf (CO2(g)) and 3DHf (H2O(l));
Ө
Ө
Ө
Ө
(DHf (C2H5OH(l)) = (2DHf (CO2(g)) + 3DHf (H2O(l)) - DHcomb
= 2(-394) + 3(-286) + 1371;
-1
= -275 kJ mol ;
If values are substituted for symbols in the enthalpy cycle
diagram to give correct answer, award last [2] marks.
If no enthalpy cycle drawn but equation written and Hess’s
5
12
Chemistry 11 (HL)
Exam Review – Topics 5 and 15
Law applied or calculated as follows, then [3 max]
(DHr = ∑DHf (products) - ∑DHf (reactants))
-1371 = (-394×2) + (-286×3) -DHf (ethanol);
DHf (ethanol) = -788 - 858 + 1371;
-1
= - 275(kJ mol );
Award [2] for correct answer without enthalpy cycle and
without working and [1] for 275 or + 275.
[7]
40.
enthalpy change associated with the formation of one mole of a
compound/substance; from its elements;
in their standard states/under standard conditions;
2C(s) + 3H2(g) + O2(g) → C2H5OH(l);
5
Award [1] for formulas and coefficients, [1] for state symbols.
[5]
13