CHEMISTRY

Chapter 6 Thermochemistry Study Guide

1. A gas absorbs 0.0 J of heat and then performs 30.7 J of work. The change in internal energy of the gas is

A) 61.4 J
B) 30.7 J
C) –61.4 J
D) –30.7 J
E) none of these

ANS: D

2. What is the kinetic energy of a 1.56-kg object moving at 94.0 km/hr?

A) 5.32  102 kJ
B) 6.89  103 kJ
C) 5.32  10–4 kJ
D) 1.06  103 kJ
E) 2.04  101 kJ

ANS: A

3. Which of the following statements correctly describes the signs of q and w for the following exothermic process at P = 1 atm and T = 370 K?

H2O(g)  H2O(l)

A) q and w are negative.
B) q is positive, w is negative.
C) q is negative, w is positive.
D) q and w are both positive.
E) q and w are both zero.

ANS: C

4. Which of the following statements is correct?

A) The internal energy of a system increases when more work is done by the system than heat was flowing into the system.
B) The internal energy of a system decreases when work is done on the system and heat is flowing into the system.
C) The system does work on the surroundings when an ideal gas expands against a constant external pressure.
D) All statements are true.
E) All statements are false.

ANS: C

5. For a particular process q = –17 kJ and w = 21 kJ. Which of the following statements is false?

A) Heat flows from the system to the surroundings.
B) The system does work on the surroundings.
C) E = +4 kJ
D) The process is exothermic.
E) None of the above is false.

ANS: B

6. One mole of an ideal gas is expanded from a volume of 1.00 liter to a volume of 8.93 liters against a constant external pressure of 1.00 atm. How much work (in joules) is performed on the surroundings? Ignore significant figures for this problem. (T = 300 K; 1 L·atm = 101.3 J)

A) 402 J
B) 803 J
C) 2.41  103 J
D) 905 J
E) none of these

ANS: B

7. A fuel-air mixture is placed in a cylinder fitted with a piston. The original volume is 0.310-L. When the mixture is ignited, gases are produced and 935 J of energy is released. To what volume will the gases expand against a constant pressure of 635 mmHg, if all the energy released is converted to work to push the piston?

A) 10.7 L
B) 8.02 L
C) 11.4 L
D) 11.0 L
E) 1.78 L

ANS: C

8. Calculate the work associated with the compression of a gas from 121.0 L to 80.0 L at a constant pressure of 13.1 atm.

A) –537 L atm
B) 537 L atm
C) 3.13 L atm
D) –3.13 L atm
E) 101 L atm

ANS: B

9. A gas absorbs 825 J of heat and then has 841 J of work done upon it. The change in internal energy of the gas is

A) 1666 J
B) 16 J
C) –16 J
D) –1666 J
E) none of these

ANS: A

10. Of energy, work, enthalpy, and heat, how many are state functions?

A) 0
B) 1
C) 2
D) 3
E) 4

ANS: C

11. Which of the following properties is (are) intensive properties?

I. mass

II. temperature

III. volume

IV. concentration

V. energy

A) I, III, and V
B) II only
C) II and IV
D) III and IV
E) I and V

ANS: C

12. For the reaction H2O(l)  H2O(g) at 298 K and 1.0 atm, H is more positive than E by 2.5 kJ/mol. This quantity of energy can be considered to be

A) the heat flow required to maintain a constant temperature
B) the work done in pushing back the atmosphere
C) the difference in the H–O bond energy in H2O(l) compared to H2O(g)
D) the value of H itself
E) none of these

ANS: B

13. Consider the reaction:

C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(l); H = –1.37  103 kJ

Consider the following propositions:

I. The reaction is endothermic

II. The reaction is exothermic.

III. The enthalpy term would be different if the water formed was gaseous.

Which of these propositions is (are) true?

A) I
B) II
C) III
D) I, II
E) II, III

ANS: E

14. How much heat is required to raise the temperature of a 5.75-g sample of iron (specific heat = 0.450 J/g°C) from 25.0°C to 79.8°C?

A) 2.54 J
B) 315 J
C) 700 J
D) 848 J
E) 142 J

ANS: E

15. A 32.5 g piece of aluminum (which has a molar heat capacity of 24.03 J/°C·mol) is heated to 82.4°C and dropped into a calorimeter containing water (specific heat capacity of water is 4.18 J/g°C) initially at 22.3°C. The final temperature of the water is 24.2°C. Ignoring significant figures, calculate the mass of water in the calorimeter.

A) 212 g
B) 5.72 kg
C) 6.42 g
D) 1.68 kg
E) none of these

ANS: A

16. A 45.9 g sample of a metal is heated to 95.2°C and then placed in a calorimeter containing 120.0 g of water (c = 4.18 J/g°C) at 21.6°C. The final temperature of the water is 24.5°C. Which metal was used?

A) Aluminum (c = 0.89 J/g°C)
B) Iron (c = 0.45 J/g°C)
C) Copper (c = 0.20 J/g°C)
D) Lead (c = 0.14 J/g°C)
E) none of these

ANS: B

17. You take 295.5 g of a solid at 30.0°C and let it melt in 425 g of water. The water temperature decreases from 85.1°C to 30.0°C. Calculate the heat of fusion of this solid.

A) 160 J/g
B) 166 J/g
C) 331 J/g
D) 721 J/g
E) cannot solve without the heat capacity of the solid

ANS: C

18. 30.0 mL of pure water at 282 K is mixed with 50.0 mL of pure water at 306 K. What is the final temperature of the mixture?

A) 294 K
B) 297 K
C) 342 K
D) 588 K
E) 24 K

ANS: B

19. Consider the reaction

H2(g) + O2(g)  H2O(l) H° = –286 kJ

Which of the following is true?

A) The reaction is exothermic.
B) The reaction is endothermic.
C) The enthalpy of the products is less than that of the reactants.
D) Heat is absorbed by the system.
E) Both A and C are true.

ANS: E

20. What is the specific heat capacity of a metal if it requires 178.1 J to change the temperature of 15.0 g of the metal from 25.00°C to 32.00°C?

A) 0.590 J/g°C
B) 11.9 J/g°C
C) 25.4 J/g°C
D) 1.70 J/g°C
E) 283 J/g°C

ANS: D

21. A 140.0-g sample of water at 25.0°C is mixed with 111.7 g of a certain metal at 100.0°C. After thermal equilibrium is established, the (final) temperature of the mixture is 29.6°C. What is the specific heat capacity of the metal, assuming it is constant over the temperature range concerned?

A) 0.34 J/g°C
B) 0.68 J/g°C
C) 0.22 J/g°C
D) 2.9 J/g°C
E) none of these

ANS: A

22. If 5.0 kJ of energy is added to a 15.5-g sample of water at 10.°C, the water is

A) boiling
B) completely vaporized
C) frozen solid
D) decomposed
E) still a liquid

ANS: E

23. A chunk of lead at 91.6°C was added to 200.0 g of water at 15.5°C. The specific heat of lead is 0.129 J/g°C, and the specific heat of water is 4.18 J/g°C. When the temperature stabilized, the temperature of the mixture was 17.9°C. Assuming no heat was lost to the surroundings, what was the mass of lead added?

A) 1.57 kg
B) 170 g
C) 204 g
D) 211 g
E) none of these

ANS: D

24. What is the specific heat capacity of silver if it requires 86.3 J to raise the temperature of 15 grams of silver by 25°C?

A) 4.3 J/g°C
B) 0.23 J/g°C
C) 0.14 J/g°C
D) 0.60 J/g°C
E) none of these

ANS: B

25. If a student performs an endothermic reaction in a calorimeter, how does the calculated value of H differ from the actual value if the heat exchanged with the calorimeter is not taken into account?

A) Hcalc would be more negative because the calorimeter always absorbs heat from the reaction.
B) Hcalc would be less negative because the calorimeter would absorb heat from the reaction.
C) Hcalc would be more positive because the reaction absorbs heat from the calorimeter.
D) Hcalc would be less positive because the reaction absorbs heat from the calorimeter.
E) Hcalc would equal the actual value because the calorimeter does not absorb heat.

ANS: D

26. Consider the reaction:

When a 21.1-g sample of ethyl alcohol (molar mass = 46.07 g/mol) is burned, how much energy is released as heat?

A) 0.458 kJ
B) 0.627 kJ
C) 6.27  102 kJ
D) 2.89  104 kJ
E) 2.18 kJ

ANS: C

27. The total volume of hydrogen gas needed to fill the Hindenburg was 2.11  108 L at 1.00 atm and 24.7°C. How much energy was evolved when it burned?

A) 8.64  106 kJ
B) 2.98  1010 kJ
C) 3.02  104 kJ
D) 2.47  109 kJ
E) 4.94  109 kJ

ANS: D

28. What is the enthalpy change when 49.4 mL of 0.430 sulfuric acid reacts with 23.3 mL of 0.309 M potassium hydroxide?

H2SO4(aq) + 2KOH(aq)  K2SO4(aq) + 2H2O(l) H° = –111.6 kJ/mol
A) –0.402 kJ
B) –3.17 kJ
C) –2.37 kJ
D) –0.803 kJ
E) –112 kJ

ANS: A

29. How much heat is liberated at constant pressure when 2.35 g of potassium metal reacts with 5.68 mL of liquid iodine monochloride (d = 3.24 g/mL)?

2K(s) + ICl(l)  KCl(s) + KI(s) H° = –740.71 kJ/mol
A) 2.22  103 kJ
B) 8.40  101 kJ
C) 1.28  102 kJ
D) 2.23  101 kJ
E) 7.41  102 kJ

ANS: D

30. Which of the following statements is/are true?

I. q (heat) is a state function because is a state function and q = H.
II. When 50.0 g of aluminum at 20.0°C is placed in 50.0 mL of water at 30.0°C, the H2O will undergo a smaller temperature change than the aluminum. (The density of H2O = 1.0 g/mL, specific heat capacity of H2O = 4.18 J/g°C, specific heat capacity of aluminum = 0.89 J/g°C)
III. When a gas is compressed, the work is negative since the surroundings are doing work on the system and energy flows out of the system.
IV. For the reaction (at constant pressure) 2N2(g) + 5O2(g)  2N2O5(g), the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.
A) I, II, IV
B) II, III
C) II, III, IV
D) II, IV
E) All of the above statements are true.

ANS: D

31. Consider the following processes:

2A  (1/2)B + C H1 = 5 kJ/mol
(3/2)B + 4C  2A + C + 3D H2 = –15 kJ/mol
E + 4A  C H3 = 10 kJ/mol

Calculate H for: C  E + 3D

A) 0 kJ/mol
B) 10 kJ/mol
C) –10 kJ/mol
D) –20 kJ/mol
E) 20 kJ/mol

ANS: C

32. At 25°C, the following heats of reaction are known:

H (kJ/mol)
2ClF + O2  Cl2O + F2O 167.4
2ClF3 + 2O2  Cl2O + 3F2O 341.4
2F2 + O2  2F2O –43.4

At the same temperature, calculate H for the reaction: ClF + F2  ClF3

A) –217.5 kJ/mol
B) –130.2 kJ/mol
C) +217.5 kJ/mol
D) –108.7 kJ/mol
E) none of these

ANS: D

33. Given the heats of the following reactions:

H°(kJ)
I. P4(s) + 6Cl2(g)  4PCl3(g) –1225.6
II. P4(s) + 5O2(g)  P4O10(s) –2967.3
III. PCl3(g) + Cl2(g)  PCl5(g) –84.2
IV. PCl3(g) + O2(g)  Cl3PO(g) –285.7

Calculate the value of H° for the reaction below:

P4O10(s) + 6PCl5(g)  10Cl3PO(g)

A) –110.5 kJ
B) –610.1 kJ
C) –2682.2 kJ
D) –7555.0 kJ
E) None of these is within 5% of the correct answer.

ANS: B

34. One of the main advantages of hydrogen as a fuel is that:

A) The only product of hydrogen combustion is water.
B) It exists as a free gas.
C) It can be economically supplied by the world’s oceans.
D) Plants can economically produce the hydrogen needed.
E) It contains a large amount of energy per unit volume of hydrogen gas.

ANS: A

35. Consider the following standard heats of formation:

P4O10(s) = –3110 kJ/mol

H2O(l) = –286 kJ/mol

H3PO4(s) = –1279 kJ/mol

Calculate the change in enthalpy for the following process:

P4O10(s) + 6H2O(l)  4H3PO4(s)

ANS:

–290 kJ

36. For the complete combustion of 1.000 mole of ethane gas at 298 K and 1 atm pressure, H° = -1560 kJ/mol. What will be the heat released when 4.42 g of ethane is combusted under these conditions?

A) –230 kJ
B) 230 kJ
C) 10588 kJ
D) –10588 kJ
E) none of these

ANS: B

37. For the complete combustion of 1.000 mole of propane gas at 298 K and 1 atm pressure, H° = -2220 kJ/mol. What will be the heat released when 4.13 g of propane is combusted under these conditions?

A) –208 kJ
B) 208 kJ
C) 23651 kJ
D) –23651 kJ
E) none of these

ANS: B

38. A 36.2 g piece of metal is heated to 81°C and dropped into a calorimeter containing 50.0 g of water (specific heat capacity of water is 4.18 J/g°C) initially at 21.7°C. The empty calorimeter has a heat capacity of 125 J/K. The final temperature of the water is 29.7°C. Ignoring significant figures, calculate the specific heat of the metal..

A) 1.439 J/gK
B) 0.900 J/gK
C) 0.360 J/gK
D) 0.968 J/gK
E) none of these

ANS: A

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