Download Unit 5 Student Packet

AP Chemistry Work and Energy Calculations
(Show ALL work!)
1. *Calculate the change in energy for a system undergoing an endothermic process in which 15.6kJ
of heat flows and where 1.4 kJ of work is done on the system.
2. Calculate the E for each of the following cases:
a. q= + 51kJ, w= -15kJ
b. q=+100kJ, w= -65kJ
c. q=-65kJ, w=-20kJ
d. In which of these cases does the system do work on the surroundings?
3. * Calculate the E for each of the following cases:
a. q= - 47kJ, w= +88kJ
b. q=+82kJ, w= + 47kJ
c. q= + 47kJ, w=0 kJ
d. In which of these cases do the surroundings do work on the system?
4. A gas absorbs 45kJ of heat and does –29kJ of work. Calculate the change in internal energy.
5. A system releases 125 kJ of heat and does 29 kJ of work. Calculate the change in internal
energy.
6. *Calculate the work associated with the expansion of a gas from 46L to 64L at a constant
external pressure of 15 atm.
7. The volume of an ideal gas is decreased from 5.0L to 5.0mL at a constant pressure of 2.00 atm.
Calculate the work associated with this process.
8. *Consider a mixture of air and gasoline vapor in a cylinder with a piston. The original volume is
40.0 cm3. If the combustion of this mixture released 950.0J of energy, to what volume will the
gases expand against a constant pressure of 650.0 torr if all of the energy of combustion is
converted into work to push back the piston?
9. A balloon is being inflated to its full extent by heating the air inside it. In the final stages of
this process, the volume of the balloon changes from 4.00 x 106L to 4.50 x 106L by the addition of
1.3 x 108J of energy as heat. Assuming that the balloon expands against constant pressure of 1.0
atm, calculate the E for the process (to convert between L·atm and J, use 1 L·atm =101.325 J)
10. *A balloon filled with 39.1 mol helium has a volume of 876 L at 0.0 ͦC and 1.00 atm pressure. The
temperature of the balloon is increased to 38.0 ͦC as it expands to a volume of 998 L, the
pressure remaining constant. Calculate q,w and E for the helium in the balloon. (the molar heat
capacity for helium gas is 20.8 J/ ͦC mol)
11. *One mole of H2O (g) at 1.00 atm and 100.0 ͦC occupies a volume of 30.6 L. When one mole of H2O
(g) is condensed to one mole of H2O (l) at 1.00 atm and 100. ͦC, 40.66kJ of heat is released. If
the density of liquid water at this temperature and pressure is 0.996 g/cm3. Calculate the E for
the condensation of one mole of water at 1.00 atm and 100. ͦC.
Answers:
1. 17.0 kJ 2. 36 kJ, 35 kJ, -85 kJ, all
3. 41 kJ, 129 kJ, 47 kJ, a and b 4. 16 kJ
kJ 6. -27000
7. 1010 J
8. 11.00 L
9. 7.93 x 104 kJ
10. 30900J, -12400J, 18500J
37.56 kJ
5. -154
11. -
AP Chemistry: Heat Transfer Problem Set /Lecture Notes
Ex: What is the final temperature when 35 g of water at 22C is added to 64 g of water at 88C?
Ex: The heat of fusion of water is 40.1kJ/mol. How much heat will be transferred when 50.10g of
water melts? What is the sign for “q” in this process?
1.
What is the specific heat of 50.00g of a metal that raises 100.0mL of water in a calorimeter from
34.0ºC to 36.0ºC. The initial temp of the metal was 95ºC. The final temp of the water and the metal
is 36.0ºC.
2. A block of rhenium metal (c= 0.0329 cal/g C) is heated to 88.2 ºC and then dropped into 100.0g water
initially at 26.4ºC. The final temp of the water and the metal is 32.4ºC. What is the mass of the
block of rhenium?
3. 28.0g of a substance with a molecular weight of 52.0 g/mol is placed in a bomb calorimeter with a
heat capacity of 2.60 J/°C and combusted. What is the molar heat of combustion for the substance
if the temperature of the calorimeter went from 25.0°C to 142.0°C? Convert to J and Cal.
4. The temperature of a 95.4 g piece of copper increases from 25.0ºC to 48.0ºC when the metal
absorbs 849 J of heat. What is the specific heat of copper?
5. How much heat is required to change 52.9gof ice at -15.0ºC into steam at 230.0ºC?
6. How much heat is required to convert 55.0g of liquid water at 22.0ºC to steam at 105ºC?
AP CHEMISTRY: HESS’S LAW
Guided Practice: “Enthalpy is a state function.”
As a reaction proceeds from A to B, the overall enthalpy change is the same whether it occurs in one
step or a series of steps.
1. Given:
OR:
N2 (g)
+
2 O2 (g) 
N2 (g) + 2 O2 (g)
2 NO (g) + O2 (g) 
N2 (g) + 2 O2 (g) 
2 NO2 (g)

2 NO (g)
2 NO2 (g)
2 NO2 (g)
H = 68 kJ
+
O2 (g) H = 180 kJ
H = -112 kJ
H = 68 kJ
Student Practice: For any reaction that can be written as a series of reactions, the standard heat of
reaction, Hrxn, is the sum of the standard heats of reaction for each step.
H rxn =  (H 1, 2, 3 …)
2. Given:
N2O4 (g)

2 NO2 (g)
H = + 57.93 kJ
2 NO (g) + O2 (g)

2 NO2 (g)
H = - 113.14 kJ
?Calculate H for:
2 NO (g) + O2 (g)

N2O4 (g)
3. Given:
2 CO (g) + O2 (g)

2 CO2 (g)
H = - 566.1 kJ
2 Cu (s) + O 2(g)

2 CuO (s)
H = - 310.5 kJ
?Calculate the value of H for:
CuO (s) + CO (g)

Cu (s) + CO2 (g)
4. Barium oxide can be used to neutralize pure sulfuric acid.
BaO (s) + H2SO4 (l) 
BaSO4 (s) + H2O (l)
What is the standard enthalpy change of this reaction? Use thermochemical equations below.
SO3 (g) + H2O (l)

H2SO4 (l)
H = - 18.7 kcal
BaO (s) + SO3 (g)

BaSO4 (s)
H = - 51.0 kcal
5. Given:
C2H4 (g) + 3 O2 (g)  2 CO2 (g) + 2 H2O (l)
H = - 845.2 kJ
2 H2 (g) + O2 (g)

2 H2O (l)
H = - 573.2 kJ
2 C2H6 (l) + 7 O2 (g) 
4 CO2 (g) + 6 H2O (l)
H = -1987.4 kJ
?Calculate H for:
C2H4 (g) + H2 (g)  C2H6 (l)
6. What is the value of H for the neutralization of lithium hydroxide by hydrochloric acid by the
following equation?
LiOH (aq) + HCl (aq)

LiCl (aq) + H2O (l)
Given:
Li (s) + ½ O2 (g) + ½ H2 (g)
2 Li (s) + Cl2 (g)

LiOH (s)

HCl (g)

LiCl (s)

2 H2O (l)

H2 (g) + Cl2 (g)


LiOH (s)
2 LiCl (s)
LiOH (aq)
HCl (aq)
LiCl (aq)
2 H2 (g) + O2 (g)
2 HCl (g)
H
H
H
H
=
=
=
=
H = - 487.0 kJ
- 815.0 kJ
- 19.2 kJ
- 77.0 kJ
- 36.0 kJ
H = + 571.8 kJ
H = - 184.6 kJ
AP CHEMISTRY: ENTHAPLY PROBLEM SET (SHOW ALL WORK!)
1. When 2.80 g of calcium chloride dissolves in 100.0 g of water, the temperature of the water rises
from 20.5 C to 25.4 C. Assume that the solution absorbs all of the heat and that the specific heat
of the solution is the same as that of water.
a) Write a balanced equation for the solution process.
b) What is q for the process described above?
c) Is the solution process exothermic or endothermic?
d) How much heat would be absorbed by the water if one mole of calcium chloride were dissolved?
2. When 1.34 g of KBr dissolves in 74.0 g of water in a coffee cup calorimeter, the temperature drops
from 18.000 C to 17.279 C. Assume that all of the heat absorbed in the dissolving process comes
from the water and that the specific heat of the solution is the same as that of water.
a) Write a balanced equation for the dissolving process.
b) Is this process exothermic or endothermic?
c) Calculate q for this process.
d) Determine H for dissolving one mole of KBr.
3. A sample of sucrose, C12H22O11, with a mass of 4.50 g is burned in a bomb calorimeter with a heat
capacity of 24.11 kJ/C. The temperature rises from 22.15 C to 25.22 C. Calculate q for the
calorimeter and Hcomb for the sucrose.
4. When 5.00 mL of ethanol (d= 0.789 g/mL) is burned in a bomb calorimeter, the temperature changes
from 19.75 C to 28.67 C. The calorimeter has a heat capacity of 13.24 kJ/C. Determine Hcomb
for one mole of ethanol and write a thermochemical equation for the combustion of ethanol.
5. The combustion of one mole of benzene (liquid), in oxygen liberates 3268 kJ of heat.
a) Write the thermochemical equation for the combustion of benzene.
b) Is the reaction endothermic or exothermic?
c) How much heat will be released when 10.0 g of benzene is burned?
d) What mass of benzene must be burned to produce 1.00 kJ of heat?
6. Consider the dissolving of silver bromide:
AgBr (s)  Ag + (aq) + Br – (aq)
H = 84.4 kJ
a) Calculate H when one mole of silver bromide precipitates.
b) Calculate the heat transferred when 1.00 g of silver bromide dissolves.
7. Consider the decomposition of ammonium dichromate:
(NH4)2Cr2O7 (s) 
N2 (g) + 4 H2O (l) + Cr2O3 (s)
H = 300. kJ
Determine the amount of heat released/absorbed for the decomposition of 14.6 g of ammonium
dichromate.
8. When magnesium and carbon dioxide react to form carbon and magnesium oxide, 16.7 kJ of heat is
evolved per gram of magnesium reacted.
a) Write the thermodynamic equation for the reaction.
b) How much heat is produced of 1.00 g of MgO is formed?
9. Complete combustion of 8.62 g of benzoic acid, C6H5COOH (s), at constant pressure released 227.8
kJ of thermal energy. Write a thermochemical equation for the combustion of benzoic acid.
Answers:
1. a) 2.0 kJ c) -79.2 kJ
2. c) 223 J d) 19.8 kJ/mol
3. q = 74.0 kJ, Hcomb = -5630 kJ/mol
4. –1377 kJ
5.
6.
7.
8.
c) 419 kJ, d) 0.0239 g
a) –84.4 kJ , b) 449 J
17.4kJ
b) 10.1 kJ
AP CHEMISTRY:
1. Given:
2 ZnO (s)
+
More Enthalpy Problems
2 S (s)

2 ZnS (s)
+
O2 (g) H = + 154.4 kJ
Compute the heat required to react 1.00 g of zinc oxide with excess sulfur.
2. A 0.150 g sample of octane (liquid) was burned in a bomb calorimeter causing the temperature to
change from 25.246C to 26.386C. If the heat capacity of the calorimeter was 7.15 kJ/C,
calculate H comb for octane.
3. Calcium oxide (lime) reacts with water to give calcium hydroxide. A 5.40 g sample of calcium oxide
was added to 500. mL of water in a calorimeter of heat capacity 350. J/C. The observed
temperature increase was 2.60C. What is the standard enthalpy change for the reaction of one
mole of calcium oxide? (Surroundings = calorimeter & solution) Assume that the resulting solution
has the same specific heat as water.
4. A 50.0 mL sample of 0.20 M HCl was mixed with 50.0 mL of 0.20 M NaOH in a coffee cup
calorimeter. Use the equation below to predict the temperature change that should be observed.
H3O + + OH -  2 H2O (l)
H = - 54 kJ
5. Propane gas is often used as a home fuel in areas where natural gas is not available. When 3.00 g of
propane is burned in a calorimeter whose heat capacity is 32.7 kJ/C, the temperature increases by
4.25C. Write the thermochemical equation for the combustion of octane.
6. When 1.00 g of oxalic acid, H2C2O4, is burned in a calorimeter, the temperature increases 0.312C. If
the H comb of oxalic acid is - 246 kJ/mol, determine the heat capacity of the calorimeter.
7. Fructose, C6H12O6, a sugar found in fruits, is a source of energy for the body. When 5.00 g of
fructose is burned, 78.3 kJ of heat are released. Write the thermochemical equation for the
combustion of fructose.
8. A sample of potassium chloride is dissolved in water in a coffee cup calorimeter. Use the data shown
below to determine the enthalpy change for the dissolving process.
mass of KCl
2.80 g
volume of water
100.0 mL
initial temperature of water
22.6C
final temperature of solution
19.9C
_______________________________________________________________________
Answers:
1) 948 J
2) -6200 kJ
3) -66.4 kJ/mol
4) 1.29C
5)
6)
7)
8)
C3H8 (g) + 5 O2 (g)  3 CO2 (g) + 4 H2O (l) H = 2040 kJ
8.75 kJ/C
C6H12O6 (s) + 6 O2 (g)  6 CO2 (g) + 6 H2O (l) H
= - 2830 kJ
Hsoln = +30.9 kJ/mol
**Entropy Problem Set from Zumd Textbook – CH 16
Heat of Neutralization Worksheet
A calorimetry investigation is to be performed to determine the heat of neutralization when
hydrochloric acid and sodium hydroxide solutions are mixed.
1. Write the complete chemical equation for the reaction between hydrochloric acid and sodium
hydroxide.
2. Write the net ionic equation for the reaction between hydrochloric acid and sodium hydroxide.
3. The procedure then calls for measuring 50.00 mL of each 0.10 molar solution. Why is it important to
measure the volumes as accurately as possible?
4. What is the most accurate measuring instrument in your lab for measuring liquid volumes? Explain.
5. The measured volumes of each solution are put into separate containers and allowed to set for several
minutes. The initial temperature of each solution is recorded. The solutions are mixed in the coffee cup
calorimeter and the highest temperature is recorded. Assume that the acid and base solutions have the
same density and specific heat as water. Explain how the measurements are used to calculate the heat
produced during the procedure. Use the equation q = mc∆T
6. Use the net ionic equation from #2 to calculate the number of moles of product formed during the
procedure.
7. Use your answers to #5 and #6 to calculate the value of the heat of reaction, ∆H, for the procedure
in kilojoules per mole.
8. A follow-up activity uses nitric acid and potassium hydroxide solutions as the reactants. The same
amounts and the same concentrations of the reactants are used and the same measurements are made.
Write the complete equation for this reaction.
9. Write the net ionic equation for this reactants given in #8.
10. Predict how the calculated value for the heat of reaction for nitric acid and potassium
hydroxide would compare to the calculated value calculated in #7.
Sample AP Problem: 2002 AP Examination Free-Response
Question 5
2.
3.
4.
5.
6.
Thermochemistry Unit Review
1. Determine the internal energy change for a system if a gas expands slowly while absorbing
350. J of heat while doing 200. J of work.
Define “state function”.
Consider: N2O5 (g) + H2O (l)  2 HNO3 (l)
H = - 76.6 kJ
a) Is this reaction exothermic or endothermic?
b) How much heat is released or absorbed when 84.0 g of HNO3 (l) is decomposed?
Write the thermochemical equation for the combustion of propane gas.
Write the thermochemical equation for the formation (Hf) of solid sodium nitrate.
Given:
Cu2O (s) + ½ O2 (g)  2 CuO (s)
H = - 144 kJ
CuO (s)
+ Cu (s)
 Cu2O (s)
H = - 11 kJ
Calculate Hf for CuO (s).
7. Calculate the molar heat of vaporization (liquid to vapor) of water.
8. The specific heat of liquid ammonia is 4.38 J/g C. Determine the amount of heat used to raise the
temperature of 1.50 g of NH3 (l) from – 60. C to – 55 C.
9. When 0.80 g of sulfur is burned to form only sulfur dioxide gas. All of the heat released is used to
raise the temperature of 100. g of water by 17.8 C. Determine the value of H for the equation: S
 SO2 (g)
(s) + O2 (g)
10. Calculate H for the reaction: MgO (s) + H2O (l)  Mg(OH)2 (s)
11. Determine the value of Hf for calcium carbide given that:
CaC2 (s) + 2 H2O (l)  Ca(OH)2 (s) + C2H2 (g)
H = - 127.2 kJ
12. When a 4.82 g sample of LiBr (s) is dissolved in 100.00 g of water in a coffee cup calorimeter, the
temperature changes from 19.8 C to 26.0 C. Determine the value of H soln for lithium bromide in
kJ/mol.
13. When 1.00 g of acetylene, C2H2 (g), burns in a bomb calorimeter, the temperature rises 10.94 C. In a
separate experiment, it is found that when 8.16 kJ of heat is added to the calorimeter, the
temperature increases by 1.79 C.
a) Determine the heat capacity of the calorimeter.
b) Calculate H comb in kJ/mol for acetylene.
14. H comb for sucrose is – 5644 kJ/mol. Determine the value of Hf for sucrose, C12H22O11 (s)
15. The combustion of one mole of benzene in oxygen liberates 3268 J of heat.
a. Write a thermochemical equation for the combustion of benzene.
b. Is the reaction endothermic or exothermic?
c. How much heat is absorbed/released when 10.00 g of benzene are burned?
d. How many grams of benzene must burn to produce one kilojoule of heat?
16. When ignited, ammonium dichromate decomposes in a fiery display. This is the reaction for the
“laboratory volcanoes” often demonstrated in science fair projects. The decomposition of 1.00 g of
ammonium dichromate produces 1.19 KJ of energy, nitrogen gas, steam and chromium(III) oxide.
a. Write a balanced thermochemical equation for the reaction.
b. How much heat is evolved for the formation of 11.2L of nitrogen at 0°C and 1.00 atm?
17. To produce silicon used for semiconductors from sand (SiO2), a reaction is used which can be broken
down into three steps:
SiO2 (s) + 2 C (s)
→ Si (s) + 2 CO (g)
∆H = 689.9KJ
Si(s) + 2Cl2(g)
→ SiCl4 (g)
∆H= -657.0KJ
SiCl4 (g) + 2 Mg (s)
→ 2 MgCl2 (s) + Si (s)
∆H= -652.6 KJ
a. Write the the thermochemical equation for the overall reaction for the formation of Si from
SiO2.
b. What is ∆H for the formation of one mole of silicon? Is the reaction endothermic or
exothermic?
18. Calculate the ∆Hf for dinitrogen pentoxide from the equations shown below.
2H2(g) +
O2(g)
→ 2H2O(l)
∆H = -51.6 kJ
N2O5 (g) +
H2O (l)
→ 2HN03(l)
∆H = -73.7 kJ
½ N2(g) + 3/2 O2 (g) + ½ H2(g)
→ HNO3(l)
∆H = -174.1 kJ
19. The space shuttle orbiter utilized the oxidation of methyl hydrazine by dinitrogen tetroxide for
propulsion:
5N2O4 (l) + 4 N2H3CH3(l) → 12 H2O (g) + 9 N2(g) + 4CO2(g)
Calculate ∆Hrxn
Thermodynamics June 2012 Free Response Questions
#1.
Substance
________________
Standard Heat of
Formation, ∆Hf˚,
in kJ mol–1
Absolute
Entropy, S˚,
in J mol–1 K–1
_____________________________
_______________________
C(s)
0.00
CO2(g)
–393.5
H2(g)
0.00
H2O(l)
–285.85
O2(g)
0.00
C3H7COOH(l)
?
5.69
213.6
130.6
69.91
205.0
226.3
The enthalpy change for the combustion of butyric acid at 25˚C, ∆H˚comb, is –2,183.5 kilojoules per mole. The
combustion reaction is
C3H7COOH(l) + 5 O2(g) √ 4 CO2(g) + 4 H2O(l)
(a) From the above data, calculate the standard heat of formation, ∆Hf˚, for butyric acid.
(b) Write a correctly balanced equation for the formation of butyric acid from its elements.
(c) Calculate the standard entropy change, ∆Sf˚, for the formation of butyric acid at 25˚C. The entropy change,
∆S˚, for the combustion reaction above is –117.1 J K–1 at 25˚C.
(d) Calculate the standard free energy of formation, ∆G˚f, for butyric acid at 25˚C.
#2.
Standard Free Energies of
Formation at 298 K
Substance
C2H4Cl2(g)
C2H5Cl(g)
HCl(g)
Cl2(g)
∆G˚f 298 K, kJ mol–1
–80.3
–60.5
–95.3
0
Average Bond Dissociation
Energies at 298 K
Bond
Energy, kJ mol–1
C–H
414
C–C
347
C–Cl
377
Cl–Cl
243
H–Cl
431
The tables above contain information for determining thermodynamic properties of the reaction below.
C2H5Cl(g) + Cl2(g) <=> C2H4Cl2(g) + HCl(g)
(a) Calculate the ∆H˚ for the reaction above, using the table of average bond dissociation energies.
(b) Calculate the ∆S˚ for the reaction at 298 K, using data from either table as needed.
(c) Calculate the value of Keq for the reaction at 298 K.
(d) What is the effect of an increase in temperature on the value of the equilibrium constant? Explain your
answer.
#3. Propane, C3H8, is a hydrocarbon that is commonly used as fuel for cooking.
(a) Write a balanced equation for the complete combustion of propane gas, which yields CO2(g) and H2O(l).
(b) Calculate the volume of air at 30˚C and 1.00 atmosphere that is needed to burn completely 10.0 grams of
propane. Assume that air is 21.0 percent O2 by volume.
o
(c) The heat of combustion of propane is –2,220.1 kJ/mol. Calculate the heat of formation, ∆H f , of propane
o
o
given that ∆H f of H2O(l) = –285.3 kJ/mol and ∆H f of CO2(g) = –393.5 kJ/mol.
(d) Assuming that all of the heat evolved in burning 30.0 grams of propane is transferred to 8.00 kilograms of
water (specific heat = 4.18 J/g.K), calculate the increase in temperature of water.
#4.
CO(g) + 2H2(g) ↔ CH3OH
Substance
CO
CH3OH
ΔHf (kJ/mol)
-110.5
-238.6
ΔH = -128.1 kJ
ΔGf (kJ/mol)
-137.3
-166.2
S (J/mol K)
+197.9
+126.8
a. Calculate ΔG for the reaction above at 25C.
b. Calculate ΔS for the reaction above at 25C.
c. In the table above, there is no data for H2(g). What are the values of ΔHf, ΔGf and S for H2 at 25C?
#5.
C2H2(g) + 2H2(g)  C2H6(g)
Substance
C2H2(g)
H2(g)
C2H6
S (J/mol K)
200.9
130.7
Bond
C–C
C=C
C–H
H–H
ΔHf (kJ/mol)
226.7
0
-84.7
Bond Energy
(kJ/mol)
347
611
414
436
a. If ΔSrxn = -232.7 J/K, calculate S for C2H6(g).
b. Calculate ΔG for the reaction. What does the sign of ΔG indicate about the reaction?
c. Calculate the bond energy of the carbon-carbon triple bond in C2H2 in kJ/mol.