GENERAL CHEMISTRY I
CHEM 1311.001
EXAM 4
Thursday July 3, 1997


1. Endothermic reactions cause their immediate surroundings to
a) become coolerb) become warmerc) neither
2. Exothermic reactions cause their immediately surroundings to
a) become coolerb) become warmerc) neither
3. The value of q, when defined with respect to an endothermic reaction is
a) a positive numberb) a negative numberc) zero
4. The value of q, when defined with respect to an exothermic reaction is
a) a positive numberb) a negative numberc) zero
5. How much heat is required to raise the temperaure of a 9.179 g sample of silver metal from 22.4 oC to 88.1 oC? Note that the specific heat of silver is 0.235 J / (g oC).
a) 48.9 Jb) 83.6 Jc) 102.4 Jd) 141.7 J
e) 173.5 J
6. A 533.18 g sample of lead at 12.6 oC absorbs 2.176 kJ of heat. What is the temperature of the lead after absorbing this heat? Note that the specific heat of lead is 0.128 J / (g oC).
a) 15.2 oCb) 31.9 oCc) 44.5 oCd) 60.0 oC
e) 72.3 oC
7. A 95.50 g sample of aluminum at 98.7 oC loses 5.132 kJ of heat. What is the temperature of the aluminum after losing this heat? Note that the specific heat of aluminum is 0.901 J / g oC.
a) 21.4 oCb) 39.1 oCc) 59.6 oCd) 74.2 oC
e) 88.2 oC
8. A 5.193 g sample of copper at 100.0 oC is dropped into 24.016 g of water at 24.3 oC. What will the final temperature be after the system has reached equilibrium? Assume that all of the heat lost by the copper is gained by the water. Note that the specific heat of copper is 0.387 J / (g oC) and the specific heat of water is 4.1796 J / (g oC). Tip to avoid errors: Remember that the specific heats of copper and water are different, so you can't ignore them. Use the "full formula" for calculating the final temperature.
a) 25.8 oCb) 26.5 oCc) 27.0 oCd) 28.3 oC
e) 29.4 oC
9. A 48.66 g sample of water at 85.2 oC was mixed with a 21.15 g sample of water at 21.1 oC. What was the final temperature of the combined masses of water? Assume that all of the heat lost by the hot water is gained by the cold water. Note that the specific heat of water (whether hot or cold) is 4.1796 J / (g oC). Hint: Since the specific heats are the same, you can ignore them by using the "condensed fromula" if you like.
a) 24.9 oCb) 36.7 oCc) 50.0 oCd) 65.8 oC
e) 74.4 oC
10. Given the thermochemical equation

2H2(g)   +   O2(g)   ----->   2H2O(l)      DH = -571.8 kJ

what is DH for the following reaction?

H2O(l)----->   H2(g)   +   (1/2)O2(g)      DH = ?
a) -1146.3 kJb) -571.8 kJc) -285.9 kJ
d) 285.9 kJe) 571.8 kJ
11. Given the thermochemical equation

4Fe(s)   +   3O2(g)   ----->      2Fe2O3(s)      DH = -1644.4 kJ

what is DH for the following reaction?

8Fe(s)   +   6O2(g)   ----->   4Fe2O3(s)      DH = ?
a) -3288.8 kJb) -411.1 kJc) 822.2 kJ
d) 1644.4 kJe) 3288.8 kJ
12. Given the thermochemical equation

N2(g)   +   3H2(g)   ----->   2NH3(g)      DH = -92.38 kJ

what is DH for the following equation

2NH3(g)   ----->   N2(g)   +   3H2(g)      DH = ?
a) -184.76 kJb) -92.38 kJc) -46.19 kJ
d) 46.19 kJe) 92.38 kJ
13. The balanced equation (in lowest terms) for the combustion of methane is

CH4(g)   +   2O2(g)   ----->   CO2(g)   +   2H2O(g)      DH = -802.3 kJ

What is DH for the combustion of 8.195 g of CH4?
a) -621.0 kJb) -409.8 kJc) -312 kJ
d) 312 kJe) 409 kJ
14. When 18.75 g NaOH was dissolved in water in a calorimeter, the temperature rose from 24.89 oC to 29.55 oC. What is DH for the reaction

NaOH(s)   ----->   Na+(aq)   +   OH-(aq)

The heat capacity for the calorimeter and its contents was 4330 J / oC. Hint: Use q = C . Dt rather than q = s . m . Dt for this problem. Remember that what you are calculating initially is qcalorimeter but what you want is qreaction. Note that qreaction = -qcalorimeter.
a) +88.2 kJb) -88.2 kJc) +43.0 kJ
d) -43.0 kJe) +11.1 kJ
15. The following are the thermochemical equations for the formation of lead(II) sulfate from lead, lead(IV) oxide and sulfuric acid, and the formation of sulfuric acid from sulfur trioxide and water:

Pb(s)   +   PbO2(s)   +   2H2SO4(l)   ----->   2PbSO4(s)   +   2H2O(l)      DH = -509.2 kJ

SO3(g)   +   H2O(l)   ----->   H2SO4(l)      DH = -130 kJ

Using the above thermochemical equations, and applying Hess's Law, calculate the DH for the following reaction:

Pb(s)   +   PbO2(s)   +   2SO3(g)   ----->   2PbSO4(s)      DH = ?
a) -3.77 x 103 kJb) 3.77 x 103 kJc) -639 kJ
d) -521 kJe) -769 kJ
Note: The target equation (the one you are to calculate DH for) has been modified to include a coefficeint of 2 in front of SO3. This coefficient was missing in the original version. The correction was made Wednesday June 9, 1999. Your printed version of this exam is outdated if it does not include this correction notice.

16. Use standard enthalpies of formation listed in Table 5.2 to calculate DH for the following reaction:

Fe2O3(s)   +   3CO(g)   ----->   2Fe(s)   +   3CO2(g)      DH = ?
a) 26.8 kJb) -26.8 kJc) 145.7 kJ
d) -145.7 kJe) 218.3 kJ
Note: Table 5.2 refers to the table on page 209 of Chemistry the Study of Matter and its Changes by Brady and Holum (John Wiley, 1996), the textbook in use at the time this test was written. If you use a different text to solve this problem, you may get a slightly different answer, due to minor differences in various published sources of thermochemical information.

ANSWERS:

1 a      2 b      3 a      4 b      5 d     

6 c      7 b      8 a      9 d      10 d     

11 a      12 e      13 b      14 d      15 e     

16 b

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