Chemistry I-Honors
Thermodynamics Practice Test
Solution Set
1.
2.
3.
C
A
D
Which of the following is not directly measurable?
A) heat of formation; B) heat of combustion; C) heat content;
In an endothermic reaction, H is:
A) positive; B) negative; C) equal to zero;
D) undefined;
D) change of heat content.
E) determined by entropy.
Compounds that have a highly negative heat of formation:
A) do not exist; B) are highly unstable; C) are moderately stable; D) are highly stable;
E) only exist in the gaseous state.
In calculations involving the Hf, elements in their most stable form at standard conditions have a
Hf: A) of exactly one; B) of zero; C) equal to their group number; D) that is undefined.
.......................................................................................................................................
o
Consider the decomposition of calcium carbonate (chalk) into carbon dioxide and calcium oxide at 25 C
and 1.0 atm. when answering the next set of questions. (Use the thermodynamic tables provided.)
CaCO3(solid)
CaO(solid) + CO2(gas)
4.
B
5.
+177.9 kJ
What is the molar heat of decomposition for calcium carbonate at standard
conditions?
Hdecomp = [(-635.5 kJ) + (-393.5 kJ)] - (-1206.9 kJ) = +177.9 kJ
6
6.22 kJ released
If only 3.50 grams of calcium carbonate is decomposed at these conditions, how much
heat is either absorbed or released?
3.50 g (1 mol / 100.1 g cpd)(177.9 kJ / 1 mol cpd) = 6.22 kJ
7.
+160.7 J
What is the molar entropy for the decomposition for calcium carbonate at standard
conditions?
Sdecomp = [(40. joules) + (213.6 joules)] - (92.9 joules) = +160.7 J
8.
+130.0 kJ
What is the molar free energy for the decomposition for calcium carbonate at
standard conditions?
G = H - TS
= +177.9 kJ - (298 K)(0.1607 kJ) = +130.0 kJ
If the temperature of this reaction were increased to 500oC, would this reaction become more
spontaneous or less spontaneous? Justify your answer.
G = H - TS
+177.9 kJ - (773 K)(0.2568 kJ)
= -20.6 kJ
Reaction becomes spontaneous at the higher temperature.
9.
-2-
10. A
In the water gas formation reaction: C(g) + H2O(g)  CO(g) + H2(g)
the energies involved in bond formation and bond breaking are such that, overall:
A) 131.3 kJ is absorbed by the system;
B) 131.3 kJ is released by the system;
C) 393.5 kJ is released by the system;
D) the system neither releases nor absorbs energy.
11. D
Reactions that occur spontaneously in nature are:
A) always exothermic;
B) usually endothermic; C) always endothermic;
D) usually exothermic;
E) always at very high temperatures.
12. F
If a reaction proceeds spontaneously, the only thing that is definitely known is that:
A) H is positive; B) H is negative;
C) S is positive; D) S is negative;
E) G is positive; F) G is negative.
13. B
With which factors will a nonspontaneous reaction become spontaneous as the reaction
temperature is increased?
A) H is positive and S is negative;
B) both H and S are positive;
C) S is positive and H is negative;
D) both H and S are negative.
14.
+ 16.0 kcal
If a gaseous system absorbs 56.0 kcal of heat and does 40.0 kcal of work by expanding,
what is the change in energy for this system?
E = q - w = +56.0 kcal - 40.0 kcal = + 16.0 kcal
Consider the combustion of liquid ethanol (C2H5OH) at standard conditions for the next set of questions:
For ethanol (ethyl alcohol), Hf = -277.7 kJ/mol; So = +160.7 joules; Gf = -274.76 kJ/mol;
15. Write the balanced equation for this reaction.
C2H5OH(liq) +
16.
3 O2(g)
2 CO2(g) + 3 H2O(liq)
-1366.8 kJ What is the heat of combustion for this reaction is exactly one mole of ethanol is reacted?
Hcomp = [2(-393.5 kJ) + 3(-285.83 kJ)] - (-277.7 kJ) = -1366.8 kJ
17.
-1
18. - 1364.3 kJ
What is the n for this reaction?
What is the E for this reaction?
ngas = 2 - 3 = -1
E = H = nRT
E = -1366.8 kJ - (-1)(8.314 x 10-3 kJ/mol-K)(298 K) = - 1364.3 kJ
-3-
19.
2.96 x 103 K
What is the boiling point (in Kelvin) of a substance if its heat of vaporization is known to
be 225.6 kJ and its entropy for the gaseous state is 76.2 joules/mole-K?
G = H - TS
@ equilibrium, G = 0, so H = TS
T = H / S = 225.6 kJ / 0.0762 kJ = 2960.6 K
20.
C
Which of the following has a positive value for its change of entropy?
A) water in the liquid state freezing;
B) sodium acetate crystallizing out of a supersaturated
solution; C) mixing of equal moles of carbon dioxide and argon gas; D) condensation of steam.
21. If a system is at equilibrium at standard conditions, what is definitely known about H, S, and/or G?
Only thing definitely known is that G = 0.
OMITTHIS PROBLEM -THERE ARE TYPOS!!!
22. Given:
2 NH3(g) + 3 N2O(g)  4 N2(g) + 3 H2O(liq)
2 NH3(g) + 3 O2(g) 
2 N2(g) + 6 H2O(liq)
H = -1010 kJ
H = -1531 kJ
calculate the enthalpy change (H) for the following reaction:
2 N2(g) +
O2(g) 
2 N2O(g)
H = ______________