Name
1.) Which of the following reactions is not thermodynamically favored at low
temperatures but becomes favored as the temperature increases?
Reaction
ΔHo (KJ/mol)
ΔSo(J/mol*K)
(A) 2 CO(g) + O2(g) --> 2 CO2(g)
-566
-173
(B) 2 H2O(g) --> 2 H2(g) + O2(g)
484
90.0
(C) 2 N2O(g) --> 2 N2(g) + O2(g)
-164
149
(D) PbCl2(s) --> Pb2+(aq) + Cl-(aq)
23.4
-12.5
Questions 2-6 refer to the following
K(s) + ½ Cl2(g) --> KCl(s)
ΔHo = -437 kJ/mol
The elements K and Cl react directly to form the compound KCl according to the
equation above. Refer to the information above and the table below to answer the
questions that follow.
Process
K(s) --> K(g)
K(g) --> K+(g) + eCl2(g) --> 2 Cl(g)
Cl(g) + e- --> Cl-(g)
+
K (g) + Cl-(g) --> KCl(s)
ΔHo (kJ/mol)
V
W
X
Y
Z
2.) How much heat is released or absorbed when 0.050 mol of Cl2(g) is formed from
KCl?
(A) 87.4 kJ is released
(B) 43.7 kJ is released
(C) 43.7 is absorbed
(D) 87.4 kJ is absorbed
3.) What remains in the reaction vessel after equal masses of K(s) and Cl2(g) have
reacted until either one or both of the reactants have been completely consumed?
(A) KCl only
(B) KCl and K only
(C) KCl and Cl2 only
(D) KCl, K, and Cl2
4.) Which of the values of ΔHo for a process in the table is (are) less than zero (i.e.
indicates an exothermic process)?
(A) z only
(B) y and z only
(C) x, y, and z only
(D) w, x, y, and z
5.) It is observed that the reaction producing KCl form its elements goes essentially
to completion. Which of the following is a true statement about the thermodynamic
favorability of the reaction?
(A) The reaction is favorable and driven by an enthalpy change only.
(B) The reaction is unfavorable and driven by an entropy change only.
(C) The reaction is favorable and driven by both enthalpy and entropy changes.
(D) The reaction is unfavorable due to both enthalpy and entropy changes.
Cl2(g) + 2 e- --> 2 Cl-(g)
6.) Which of the following expressions is equivalent to ΔHo for the reaction
represented above?
(A) x + y
(B) x - y
(C) x + 2y
(D) x/2 – y
7.) Which of the following equations represents a reaction for which the standard
entropy change is positive (ΔSo > 0)?
(A) 3 O2(g) --> 2 O3(g)
(B) 2 H2(g) + O2(g) --> 2 H2O(l)
(C) CaCO3(s) --> CaO(s) + CO2(g)
(D) I2(g) + 2 K(s) --> 2 KI(s)
N2(g) + 3 H2(g) <----> 2 NH3(g)
ΔHo298 = -92 kJ/mol
ΔGo298 = -33 kJ/mol
8.) Consider the reaction represented above at 298 K. When equal volumes of N2(g)
and H2(g), each at 1 atm, are mixed in a closed container at 298 K, no formation of
NH3(g) is observed. Which of the following best explains the observation?
(A) The N2(g) and the H2(g) must be mixed in a 1:3 ratio for a reaction to occur.
(B) A high activation energy makes the forward reaction extremely slow at 298 K.
(C) The reaction has an extremely small equilibrium constant, thus almost no
product will form.
(D) The reverse reaction has a lower activation energy than the forward reaction, so
the forward reaction does not occur.
Questions 9-11 refer to the following information
XY2 --> X + Y2
The equation above represents the decomposition of a compound XY2. The diagram
below shows two reaction profiles (path one and path two) for the decomposition of
XY2.
9.) Which of the following most likely accounts for the difference between reaction
path one and reaction path two?
(A) A higher temperature in path one
(B) A higher temperature in path two
(C) The presence of a catalyst in path one
(D) The presence of a catalyst in path two
10.) Which of the following best describes the flow of heat when 1.0 mol of XY2
decomposes.
(A) 50 kJ of heat is transferred to the surroundings.
(B) 50 kJ of heat is transferred from the surroundings.
(C) 100 kJ of heat is transferred to the surroundings.
(D) 100 kJ of heat is transferred from the surroundings.
11.) The reaction is thermodynamically favorable under standard conditions at
298K. Therefore, the value of ΔSo for the reaction must be
(A) equal to zero
(B) equal to ΔHo/298 K
(C) greater than ΔHo/298 K
(D) less than ΔHo/298 K
12.) Which of the following processes involves the greatest increase in entropy?
(A) SO3(g) + H2(g) --> SO2(G) + H2O(g)
(B) N2(g) + 3 H2(g) --> 2 NH3(g)
(C) Ag+(aq) + Cl-(aq) --> AgCl(s)
(D) MgSO3(s) --> MgO(s) + SO2(g)
2 NH3(g) --> 3 H2(g) + N2(g)
ΔHo298 = 92 kJ/mol
13.) According to the information above, what is the standard enthalpy of formation,
ΔHof, for NH3(g) at 298 K?
(A) -92 kJ/mol
(B) -46 kJ/mol
(C) 46 kJ/mol
(D) 92 kJ/mol
(E) 184 kJ/mol
2 H2(g) + O2(g) --> 2 H2O(g)
14.) For the reaction represented above at 25oC, what are the signs of ΔHo, ΔS0, and
ΔGo?
(A)
ΔHo
+
ΔSo
+
ΔGo
+
(B)
+
+
-
(C)
+
-
-
(D)
-
-
-
(E)
-
-
+
Free Response
XClO3(s) --> XCl(s) + 3/2 O2(g)
The equation above represents the decomposition of a compound containing an
unknown element, X. A 1.39 g sample of XClO3(s) was completely decomposed by
heating. The gas produced by the reaction was captured over water in a gascollection tube at 24.0oC. The total volume of gas in the tube was 506 mL, and the
total pressure inside the tube was determined to be 739.5 torr. The vapor pressure
of water is 22.4 torr at 24.0 oC.
(a) calculate the partial pressure, in torr, of the O2(g) that was collected at 24.0 oC.
(b) Calculated the total number of moles of O2(g) collected at 24.0 oC.
(c) Determine the moles of XClO3(s) that decomposed.
(d) Determine the molar mass of the compound.
(e) Determine the identity of element X
In a different experiment, 0.470 mol of XClO3(s) decomposes at 1.0 atm in the
presence of a catalyst as a total of 21.1 kJ of heat is released. The value of ΔGo298 for
the reaction is -121.5 kJ/mol.
(f) Calculate the value of ΔHo298 for the decomposition reaction.
(g) Which is larger: the sum of the bond energies of the products or the sum of the
bond energies of the reactants? Justify your answer.
(h) How does the presence of a catalyst affect the value of ΔGo298 for the reaction?
Justify your answer.