# Thermochemistry Practice Problems ```Thermochemistry Practice
6.1
1. A system does 3 J of work on the surroundings, and 12 J of work are added to the system.
a. What is the energy change of the system?
b. Of the surroundings?
2. One hundred joules of work are required to compress a gas. At the same time, the gas gives off 23 J of heat
to the surroundings. What is the energy change of the system?
3. A gas expands from 10 L to 20 L against a constant pressure of 5 atm. During this time it absorbs 2 kJ of
heat. Calculate the work done in kJ.
4. A piston expands against 1.00 atm of pressure, from 11.2 L to 29.1 L. This is done without any transfer of
heat.
a. Calculate the change in energy of the system.
b. Calculate the change in energy for the above change if, in addition, the system absorbs 1,037 J of
heat from the surroundings.
5. If the internal energy of a thermodynamic system is decreased by 300 J when 75 J of work is done on the
system, how much heat was transferred, and in which direction, to or from the system?
6. How much work is done by a system where the pressure is kept constant but the volume changes from 20 L
to 0.5 L (1.00 atm)?
6.2
7. A gas is compressed against a constant pressure of 3.4 atm from 27.9 L to 16.3 L. During this process, there
is a heat gain by the system of 122 J. Calculate the change in energy of the system.
8. If 596 J of heat are added to 29.6 g of water at 22.9&deg;C in a coffee cup calorimeter, what will be the final
temperature of the water?
9. A 5.037 g piece of iron heated to 100&deg;C is placed in a coffee cup calorimeter that initially contains 27.3 g of
water at 21.2&deg;C. If the final temperature is 22.7&deg;C, what is the specific heat capacity of the iron (J/g&deg;C)?
10. Calculate the heat necessary to convert 10.0 g of water (just melted) at 0&deg;C to at 20&deg;C, assuming that the
specific heat remains constant at 1 cal/g&deg;C.
11. The specific heat of aluminum is 0.89 J/g&deg;C. How much energy is required to raise the temperature of a
15.0 gram aluminum can 18&deg;C?
12. One liter of an ideal gas at 0&deg;C and 10 atm was allowed to expand to 1.89 L against a constant external
pressure of 1 atm at a constant temperature. The enthalpy change (ΔH) for this process is -901 J. Calculate q,
w, and ΔE.
13. The heat capacity of a bomb calorimet4er was determined by burning 6.79 g of methane (heat of
combustion = -802 kJ/mol) in the bomb. The temperature is changed by 10.8&deg;C.
a. What is the heat capacity of the bomb?
b. A 12.6-g sample of acetylene, C2H2, produced a temperature increase of 16.9&deg;C in the same
calorimeter. What is the heat of combustion of acetylene (kJ/mol)?
14. A sample of C6H5COOH (benzoic acid) weighing 1.221 g was placed in a bomb calorimeter and ignited in a
pure O2 atmosphere. A temperature rise from 25.24&deg;C to 31.67&deg;C was noted. The heat capacity of the
calorimeter was 5.020 kJ/&deg;C, and the combustion products were CO 2 and H2O. Calculate the ΔH in kJ/mol for
the reaction.
15. When 1.50 L of 1.00 M Na2SO4 solution at 30.0&deg;C is added to 1.50 L of 1.00 M Ba(NO3)2 solution at 30.0&deg;C
in a calorimeter, a white solid (BaSO4) forms. The temperature of the mixtures increases to 42.0&deg;C. Assuming
that the specific heat capacity of the solution is 6.37 J/g&deg;C and that the density of the final solution is 2.00
g/mL, calculate the enthalpy change per mole of BaSO4 formed.
6.3
16. Calculate ΔH for
N2(g) +
O2(g)
2NO(g)
2O2(g)
O2(g)
2NO2(g)
2NO2(g)
Given:
a. N2(g)
b. 2NO(g)
+
+
ΔH = 66.4 kJ/mol
ΔH = -114.1 kJ/mol
17. For the reaction:
H2O(l)
H20 (g)
ΔH = +44 kJ
How much heat is evolved when 9.0 grams of water vapor is condensed to liquid water?
18. Given
a. 2H2(g)
b. 2H2(g)
c. C(s)
+
+
+
C(s)
O2(g)
O2(g)
CH4(g)
2H20(l)
CO2(g)
ΔH = -74.81 kJ/mol
ΔH = -571.66 kJ/mol
ΔH = -393.52 kJ/mol
Calculate ΔH for
CH4(g)
+
2O2(g)
CO2(g)
+
2H20(l)
19. Given the following thermochemical data, calculate the ΔH&deg; for:
Ca(s)
a.
b.
c.
+
2H2O(l)
Ca(OH)2 (s)
H2(g) +
CaO(s) +
Ca(s) +
&frac12;O2(g)
H20(l)
&frac12;O2(g)
H20 (l)
Ca(OH)2 (s)
CaO(s)
+
H2(g)
ΔH&deg; = ??? kJ
ΔH = -285 kJ
ΔH = -64 kJ
ΔH = -635 kJ
20. Calculate the value for ΔH&deg; for the following reaction
CaCO3(s)
CaO(s)
+
CO2(g)
Use the following data: the heats of formation of calcium carbonate, calcium oxide, and carbon dioxide are
288.6, 151.9, and 94.1 kcal/mol respectively.
6.4
21. Using standard heats of formation (found in the appendix of your book), calculate ΔH for the following
reactions.
a.
2H2O2(l)
2H20(l)
+
O2(g)
ΔHf&deg; for H2O(l) = -187.8 kJ/mol
+
b.
HCl(g)
H (aq)
+
Cl (aq)
c.
2NO2(g)
N2O4(g)
d.
C2H2(g)
+
H2(g)
C2H4(g)
e.
2NaOH(s)
+
CO2(g)
Na2CO3(s)
+
H2O(g)
22. Calculate the standard change in enthalpy for the following thermite reaction by using enthalpies of
formation:
2Al(s) +
Fe2O3(s)
Al2O3(s)
+
2Fe(s)
NOTE: This reaction occurs when a mixture of powdered aluminum and iron (III) oxide are ignited with a
magnesium fuse.
23.
a. The heat released when the HNO3 reacts with NaOH is 56 kJ/mole of water produced. How much
energy is released when 400.0 mL of 0.200 M HNO 3 is mixed with 500.0 mL of 0.150 M NaOH?
b. The enthalpy of neutralization for the reaction of a strong acid with a strong base is -56 kJ/mol of
water produced. How much energy will be released when 200.0 mL of 0.400 M HCl is mixed with
150.0 mL of 0.5 M NaOH? How does this compare with your answer in part a? Why?
Multiple Choice Questions
24. When zinc reacts with hydrochloric acid, hydrogen gas is released. In this system the release of the
hydrogen gas is counteracted by an outside force that results in a smaller volume by the end of the reaction.
The work done by the outside force:
A. Is negative on the system
B. Is positive on the surroundings
C. Is positive on the system
D. Is zero
25. A piano is brought upstairs by two workers. Due to a mistake by one of the workers, the piano rolls down
the stairs and finally comes to rest by the outside door. Which sequence best describes the energy
transformations for the piano from the moment it is being brought upstairs to when it stops by the door?
A.
B.
C.
D.
Potential Energy Kinetic Energy
Ground Energy Potential Energy
Potential Energy Kinetic Energy
Kinetic Energy Potential Energy
Thermal Energy of the Ground and Piano
Thermal Energy Kinetic Energy of the Piano
Potential Energy Thermal Energy of the Piano and the Ground
Kinetic Energy
26. While a piston performs work of 210 L•atm on the surroundings, the cylinder in which it is placed expands
from 10 to 25 L. At the same time, 45 J of heat is transferred from the surroundings to the system. Against
what pressure was the piston working?
A.
14 atm
B.
11 atm
C.
17 atm
D.
254 atm
27. As a system increases in volume, it releases 52.5 J of energy in the form of heat to the surroundings. The
piston is working against a pressure of 10.25 atm. The final volume of the system is 58.0 L. What was the initial
volume of the system if the energy of the system decreased by 102.5 J?
A.
62.9 L
B.
53.1 L
C.
48 L
D.
68 L
28. A 500.0-g sample of an element, at 195&deg;C is dropped into an ice-water mixture. 109.5 g of ice melts and an
ice-water mixture remains. Calculate the specific heat of the element, and determine which element it is.
A.
Zn
B.
Ba
C.
Pb
D.
Ag
29. What is the final temperature, in &deg;C, when 20.0 g of water at 80&deg;C is mixed with 20.0 g of water at 25&deg;C?
A.
12&deg;C
B.
7.0&deg;C
C.
8.8&deg;C
D.
52.5&deg;C
30. Benzoic acid, C7H6O2, is a standard used in determining the heat capacity of a calorimeter. ΔH&deg; of
combustion of benzoic acid is 3.22 x 10 3 kJ/mol. 0.5 g of benzoic acid was burned in a calorimeter containing
1000.0 g of water. The change in temperature of the calorimeter was 3&deg;C. Calculate the heat capacity of the
calorimeter in J/K?
A.
450 J/K
B.
210 J/K
C.
4025 J/K
D.
2307 J/K
2H2O(g)
ΔH&deg; = ??? kJ
31. Silane, SiH4, is highly combustible and creates a fire hazard.
SiH4(g)
Si(s) +
Si(s) +
H2(g) +
A.
+
2H2(g)
O2(g)
&frac12; O2(g)
2O2(g)
SiO2(s) +
SiH4(g)
SiO2(s)
H2O (g)
-1429 kJ
B.
ΔH&deg; = 34 kJ/mol
ΔH&deg; = -911 kJ/mol
ΔH&deg; = -242 kJ/mol
-733 kJ
C.
733 kJ
D.
-1143 kJ
32. Ethane, C2H6, may be produced by using the following method
C(s)
+
CH4(g)
+
H2(g)
C2H6
ΔH&deg; = -10 kJ/mol
Calculate the ΔH&deg; for the following reaction given the information below:
C(s)
+
2H2(g)
CH4(g)
2CH4(g)
+
2CH4(g)
2C2H6(g) + 2H2(g)
ΔH&deg; = ??? kJ
ΔH&deg; = 130 kJ
A.
D.
-150 kJ
B.
-75 kJ
C.
70 kJ
10 kJ
33. How much heat will be evolved if 56.08 g of calcium oxide reacts with sulfuric acid according to the
following reaction?
CaO(s) +
Ca(s)
Ca(s)
H2(g)
H2(g)
+
+
+
+
&frac12; O2(g)
S(s)
&frac12; O2(g)
S(s)
+
A.
-2960 kJ
H2SO4(g)
+
CaSO4(s)
2O2(g)
2O2(g)
B.
-754 kJ
+
H2O(l)
ΔH&deg; = ??? kJ
CaO(s)
CaSO4(s)
H2O (l)
H2SO4(g)
ΔH&deg; = -152 kJ/mol
ΔH&deg; = -1434 kJ/mol
ΔH&deg; = -286 kJ/mol
ΔH&deg; = -814 kJ/mol
C.
D.
-10744 kJ
754 kJ
34. Calculate the heat of formation of carbon monoxide based on the reaction below:
2CO(g)
+
C(s)
C3O2(g)
ΔH&deg; = 127.3 kJ
-110.5 kJ/mol
D.
The heat of formation for carbon suboxide is -93.7 kJ/mol.
A.
116.8 kJ/mol
B.
-348.3 kJ/mol
C.
93.7 kJ/mol
35. Calculate the heat of reaction for the following reaction given the information below:
2KIO3(s)
+
12HCl(g)
2ICl(l)
+
2KCl(s)
+ 6H20 + 4Cl2(g)
ΔH&deg; = ??? kJ
The heats of formation for the reactants and products, respectively, are:
-501.0, -92.0, -24.0, -435.0, and -286 kJ/mol.
A.
-2634 kJ
B.
-528 kJ
C.
-3227 kJ
D.
-152 kJ
36. How much ozone is used if the following reaction releases 568 kJ of energy in the form of heat?
Pb(s) +
A.
None
C(s)
+
O3(g)
PbCO3(s)
B.
192 g
C.
ΔH&deg; = ???
4g
D.
48 g
37. The carbon dioxide and water in the atmosphere have all of the following effects except:
A.
B.
Increasing the temperature of the earth
Allowing visible light to escape the earth
C.
D.