CH 223 Practice Exam II

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Multiple Choice

Identify the letter of the choice that best completes the statement or answers the question.

Calculate the standard entropy change for the following reaction,

2 Ag₂O(s) → 4 Ag(s) + O₂(g)

given Sº[Ag₂O] = 121.3 J/K·mol, Sº[Ag(s)] = 42.6 J/K·mol, and Sº[O₂(g)] = 205.1 J/K·mol.

a.	-205.1 J/K
b.	-126.4 J/K
c.	+126.4 J/K
d.	+132.9 J/K
e.	+205.1 J/K

Use the following thermodynamic data

Species	ΔH (kJ/mol)	Sº (J/Kmol)
H₂O₂(l)	-187.78	109.6
H₂O(l)	-285.83	69.91
O₂(g)	0	205.14

to calculate for ΔS

the decomposition of hydrogen peroxide at 25.00 ºC.

2 H₂O₂(l) → 2 H₂O(l) + O₂(g)

a.	-657.94 J/K
b.	-532.35 J/K
c.	+125.75 J/K
d.	+435.85 J/K
e.	+783.75 J/K

If a process is endothermic and spontaneous, which of the following must be true?

a.	ΔG > 0 and ΔH < 0
b.	ΔG < 0 and ΔH < 0
c.	ΔG < 0 and ΔS > 0
d.	ΔH < 0 and ΔS > 0
e.	ΔH > 0 and ΔS < 0

Calculate

for the reaction below at 25.0ºC.

PCl₃(g) + Cl₂(g) → PCl₅(g)

Species	ΔH(kJ/mol)	ΔS(J/K·mol)
PCl₃(g)	-287.0	311.8
Cl₂(g)	0	223.1
PCl₅(g)	-374.9	364.5

a.	-1432.6 kJ
b.	-930.1 kJ
c.	-879.0 kJ
d.	-50.8 kJ
e.	-37.1 kJ

If ΔGº< 0, then

a.	K >1
b.	K = 0
c.	K < 1
d.	K = 1
e.	K < 0

What is the equilibrium constant for formation of carbon dioxide at 25ºC? (R = 8.3145 J/K·mol)

C(s) + O₂(g)

CO₂(g) ΔG

= -3.90 × 10² kJ/mol

a.	5.7 × 10¹
b.	5.4 × 10¹³
c.	2.9 × 10²⁴
d.	4.9 × 10⁴²
e.	2.3 × 10⁶⁸

Calculate ΔGº for the following reaction at 298 K,

N₂O₄(g)

2 NO₂(g)

given K = 0.15. (R = 8.3145 J/K • mol)

a.	+1.15 kJ
b.	+4.70 kJ
c.	+8.13 kJ
d.	+38.1 kJ
e.	+87.0 kJ

Calculate ΔG_fº for CaCO₃ given the following information.

C(s) + O₂(g) → CO₂(g)	ΔGº = -394.4 kJ
CaO(s) + CO₂(g) → CaCO₃(s)	ΔGº = -130.4 kJ
Ca(s) + 1/2 O₂(g) → CaO(s)	ΔGº = -604.0 kJ

a.	-1128.8 kJ
b.	-340.0 kJ
c.	-130.4 kJ
d.	+868.0 kJ
e.	+1128.8 kJ

In the following reaction,

Fe²⁺(aq) + Ag⁺(aq) → Fe³⁺(aq) + Ag(s)

a.	Fe²⁺ is oxidized and Fe³⁺ is reduced.
b.	Fe²⁺ is oxidized and Ag⁺ is reduced.
c.	Ag⁺ is oxidized and Ag(s) is reduced.
d.	Ag⁺ is oxidized and Fe²⁺ is reduced.
e.	Ag⁺ is oxidized and Fe³⁺ is reduced.

10.

The fuel cells used aboard NASA's Space Shuttle are electrochemical cells that generate electricity from which overall chemical reaction?

a.	2 H₂(g) + O₂(g) → 2 H₂O(l)
b.	NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
c.	KOH(aq) + AgNO₃(aq) → AgOH(s) + KNO₃(aq)
d.	CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(g)
e.	HgO(s) + ZnS(s) → HgS(s) + ZnO(s)

11.

Calculate E

for the following electrochemical cell:

Pt | Fe³⁺(aq), Fe²⁺(aq) || Zn²⁺(aq) | Zn

given the following reduction half-reactions.

Fe³⁺(aq) + e^- → Fe²⁺(aq)	Eº = +0.771 V
Zn²⁺(aq) + 2 e^- → Zn(s)	Eº = -0.763 V

a.	-1.534 V
b.	-0.008 V
c.	+0.008 V
d.	+1.802 V
e.	+2.305 V

12.

Calculate E for the following electrochemical cell:

Pt | H₂(g) | H⁺(aq) || Pb²⁺(aq) | PbSO₄(s) | Pb

given the following standard reduction potentials.

2 H⁺(aq) + 2 e^- → H₂(g)	Eº = 0.000 V
PbSO₄(s) + 2 e^- → Pb(s) + SO₄^2-(aq)	Eº = -0.356 V

a.	-0.712 V
b.	-0.356 V
c.	-0.178 V
d.	+0.356 V
e.	+0.712 V

13.

Calculate E for the following electrochemical cell at 25ºC

Ag | Ag⁺(aq, 0.150 M) || Sn²⁺(aq, 0.500 M), Sn⁴⁺(aq, 0.500 M) | Pt

given the following standard reduction potentials.

Ag⁺(aq) + e^- → Ag(s)	Eº = +0.80 V
Sn⁴⁺(aq) + 2 e^- → Sn²⁺(aq)	Eº = +0.14 V

a.	-0.915 V
b.	-0.61 V
c.	+0.89 V
d.	+0.915 V
e.	+0.99 V

14.

for the following reaction is -1.73 × 10³ kJ, calculate

Cr₂O₇^2-(aq) + 2 Al(s) + 14 H⁺(aq) → 2 Cr³⁺(aq) + 2 Al³⁺(aq) + 7 H₂O(l)

a.	+1.49 V
b.	+2.18 V
c.	+2.99 V
d.	+4.48 V
e.	+5.98 V

15.

Which of the following equations is the solubility product for Ca(IO₃)₂?

a.	K_sp = [Ca²⁺][I^-]²[O^2-]⁶
b.	K_sp = [Ca²⁺][I^-]²[3O^2-]²
c.	K_sp = [Ca²⁺][IO]
d.	K_sp = [Ca²⁺][IO]²
e.	K_sp = [Ca²⁺]²[IO]

16.

The solubility of BaSO₄ is 1.05 × 10^-5 mol/L. What is the value of K_sp for BaSO₄?

a.	3.24 × 10^-3
b.	6.48 × 10^-3
c.	2.10 × 10^-5
d.	1.10 × 10^-10
e.	2.20 × 10^-10

17.

What is the molar solubility of Ag₂CrO₄ in 0.20 M K₂CrO₄? The value of K_sp for Ag₂CrO₄ is 9.0 × 10^-12.

a.	4.5 × 10^-11 mol/L
b.	1.3 × 10^-10 mol/L
c.	3.6 × 10^-8 mol/L
d.	3.4 × 10^-6 mol/L
e.	1.3 × 10^-4 mol/L

18.

At what pH will a solution 0.150 M Cu²⁺ begin to precipitate as Cu(OH)₂? The K_sp for Cu(OH)₂ is 1.6 × 10^-19.

a.	1.80
b.	5.01
c.	7.23
d.	8.99
e.	13.18

19.

A solution contains 0.10 M Cl^- and 0.10 M Br^-. If Ag⁺ is added until AgCl just begins to precipitate, what are the concentrations of Ag⁺ and Br^-? (K_sp for AgCl = 1.8 × 10^-10, K_sp for AgBr = 3.3 × 10^-13)

a.	[Ag⁺] = 1.8 × 10^-9 M, [Br^-] = 3.3 × 10^-12 M
b.	[Ag⁺] = 1.8 × 10^-9 M, [Br^-] = 1.8 × 10^-4 M
c.	[Ag⁺] = 3.3 × 10^-12 M, [Br^-] = 1.0 × 10^-1 M
d.	[Ag⁺] = 3.3 × 10^-12 M, [Br^-] = 1.8 × 10^-4 M
e.	[Ag⁺] = 1.8. × 10^-9 M, [Br^-] = 1.8 × 10^-3 M

20.

Consider the reaction

Ni(OH)₂(s) + 4 CN^-(aq)

Ni(CN)₄^2-(aq) + 2 OH^-(aq) K = 2.8 × 10¹⁴

If K_sp for Ni(OH)₂ is 2.8 × 10^-16, what is the value of the formation constant, K_form, for the reaction below?

Ni²⁺(aq) + 4 CN^-(aq)

Ni(CN)₄^2-(aq)

a.	1.0 × 10^-30
b.	7.8 × 10^-2
c.	1.3 × 10¹
d.	2.8 × 10²
e.	1.0 × 10³⁰

21.

Only 1.06 g of Ca(NO₃)₂ will dissolve per liter of a solution that is buffered to a pH of 13.00. What is the value of K_sp for Ca(OH)₂? The molar mass of Ca(NO₃)₂ is 164.1 g/mol.

a.	1.8 × 10^-10 M
b.	4.2 × 10^-7 M
c.	1.1 × 10^-6 M
d.	4.2 × 10^-5 M
e.	6.5 × 10^-5 M

22.

Write a balanced half-reaction for the reduction of NO₃^-(aq) to NO(g) in an acidic solution.

a.	NO₃^-(aq) + H⁺(aq) + e^- → NO(g) + HO₂(aq)
b.	NO₃^-(aq) + 2 H⁺(aq) + e^- → NO(g) + 2 OH^-(aq)
c.	NO₃^-(aq) + 3 e^- → NO(g) + 2 O₂(g)
d.	NO₃^-(aq) + 4 H⁺(aq) + 3 e^- → NO(g) + 2 H₂O(l)
e.	2 HNO₃(aq) + 6 e^- → NO(g) + H₂(g) + 3 O₂(g)

23.

Write a balanced chemical equation for the following reaction in a basic solution.

H₂O₂(aq) + Cr(OH)₃(s) → H₂O(l) + CrO₄^2-(aq)

a.	2 H₂O₂(aq) + 3 Cr(OH)₃(s) → H₂O(l) + 3 CrO₄^2-(aq) + ¹¹/₂ H⁺(aq)
b.	2 H₂O₂(aq) + Cr(OH)₃(s) → H₂O(l) + CrO₄^2-(aq) + 2 OH^-(aq)
c.	H₂O₂(aq) + 2 Cr(OH)₃(s) → 2 CrO₄^2-(aq) + 5 H₂O(l)
d.	3 H₂O₂(aq) + 2 Cr(OH)₃(s) + 4 OH^-(aq) → 2 CrO₄^2-(aq) + 8 H₂O(l)
e.	4 H₂O₂(aq) + 2 Cr(OH)₃(s) → 2 H₂O(l) + 2 CrO₄^2-(aq) + 4 OH^-(aq)

24.

Use the standard reduction potentials below to determine which compound or ion is the best reducing agent?

Hg²⁺(aq) + 2 e^- → Hg(l)	Eº = +0.855 V
Cu²⁺(aq) + 2 e^- → Cu(s)	Eº = +0.337 V
Cd²⁺(aq) + 2 e^- → Cd(s)	Eº = -0.40 V

a.	Hg²⁺
b.	Hg(l)
c.	Cu²⁺
d.	Cd²⁺
e.	Cd

25.

At what temperature would you expect a reaction to become spontaneous if ΔH = +67.0 kJ and ΔS = -131 J/K?

a.	T < -511 K
b.	T > 238 K
c.	T > 511 K
d.	The reaction will be spontaneous at any temperature.
e.	The reaction will NOT be spontaneous at any temperature.

Completion

Complete each sentence or statement.

The total energy of the universe is constant. This is a statement of the ________ law of thermodynamics.

A chemical reaction with an equilibrium constant greater than one is said to be ________-favored.

For any process, the change in entropy of the universe equals the sum of the entropy changes to the system and the ________.

When a secondary battery provides electrical energy, it is acting as a voltaic cell. When the battery is recharging, it is operating as a(n) ________ cell.

Essay

Does the formation of complex molecules such as proteins and nucleic acids from more simple molecules break the second law of thermodynamics?

At the boiling point, liquid and gas phases exist at equilibrium. In addition, for a system at equilibrium ΔGº = 0. Calculate the enthalpy of vaporization of water at its normal boiling point if ΔSº [H₂O(l)] = 69.9 J/K • mol and ΔSº [H₂O(g)] = 188.8 J/K • mol.

Explain the function of a salt bridge in a voltaic cell.

Why do salts containing basic anions have a greater solubility in water than predicted from calculations using K_sp values? Examples of such salts are CaCO₃, PbF₂, Ca₃(PO₄)₂.