# Part III: Thermodynamics and Electrochemistry

Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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Part III: Thermodynamics and Electrochemistry

1. Calculate the amount of heat released in the complete combustion of 8.17 grams of Al to form Al
2
O
3

(s) at 25
o
C

and 1 atm pressure. The standard heat of formation for Al
2
O
3

is 1646 kJ/mol.

4 Al(
s) + 3O
2
(g)
--
> 2Al
2
O
3
(s)

a) 254 kJ

b) 203 kJ

c) 127 kJ

d) 249 kJ

e) 101 kJ

2. Calculate the

H
o

at 25
o
C for the reaction below.

2ZnS (s) + 3O
2
(g)
--
> 2ZnO(s) + 2SO
2
(g)

H
o
f
(kJ/mol)

-
205.6

-
348.3

-
296.8

a)
-
257.1 kJ

b)
-
879.0 kJ

c) +257.1 kJ

d)
-
582.2 kJ

e) +879.0

3. Estimate the temperature above wh
ich the reaction below is nonspontaneous. First calculate

S
o
.

Pb(s) + 2HCl(g)
--
> PbCl
2
(s) + H
2
S(g)

H
o
f
(kJ/mol)

-
100.4

-
92.31

-
359.4

-
20.6

G
o
f
(kJ/mol)

-
98.7

-
95.30

-
314.1

-
33.6

4. For w
hich set of values of

H and

S will a reaction be spontaneous (product
-
favored) at all temperatures?

a)

H = +10 kJ,

S =
-
5 J/K

b)

H =
-
10 kJ,

S =
-
5 J/K

c)

H =
-
10 kJ,

S = +5 J/K

d)

H = +10 kJ,

S = +5 J/K

e) no such values exist

5. For the

reaction given below,

H
o

=
-
1516 kJ at 25
o
C and

S
o

=
-
432.8 J/K at 25
o
C. This reaction is spontaneous
_____________.

SiH
4

(g) + 2O
2

(g)
--
> SiO
2

(s) + 2H
2
O(l)

a) only below a certain temperature

b) only above a certain temperature

c) at
all temperatures

d) at no temperatures

e) cannot tell from the information available

6. Which response contains all the processes below for which

S is
positive

and none for w
hich

S is negative?

I. CCl
4

(l)
--
> CCl
4

(g)

II. N
2

(g) + 3H
2

(g)
--
> 2NH
3

(g)

III. 2N
2
O(g)
--
> 2N
2

(g) + O
2

(g)

IV. I
2

(g)
--
> I
2

(s)

a) I and II

b) III and IV

c) II and IV

d) I and III

e) I, II, and IV

7. The heat of vaporization

of methanol, CH
3
OH, is 35.20 kJ/mol. Its boiling point is 64.6
o
C. What is the change in

entropy for the vaporization of methanol?

a)
-
17.0 J/mol K

b) 3.25 J/mol K

c) 17.0 J/mol K

d) 104 J/mol K

e) 543 J/mol K

8. Which one of the followin
g statements is
not

correct?

a) When

G for a reaction is negative, the reaction is spontaneous.

b) When

G for a reaction is positive, the reaction is nonspontaneous.

c) When

G for a reaction is zero, the system is at equilibrium.

d) When

H for a reaction is negative, the reaction is never

spontaneous.

e) When

H for a reaction is very positive, the reaction is not expected to be spontaneous

9. For which of the following substances is

G
o
f
zero?

a) H
2
O (l)

b) Br
2

(s)

c) O (g)

d) N
2

(g)

10. The half
-
reaction that occurs at th
e cathode during the electrolysis of molten sodium bromide is
________________________.

a) 2Br
-

--
> Br
2
+ 2e
-

b) Br
2

+ 2e
-

--
> 2Br
-

c) Na
+

+ e
-

--
> Na

d) Na
--
> Na
+

+ e
-

e) 2H
2
O + 2e
-

--
> 2OH
-

+ H
2

11. Balance the following redox reacti
on:

Cr
2
O
7
2
-

+ S
2
O
3
2
-

--
> Cr
3+

+ S
4
O
6
2
-

12. Given the standard electrode potentials below, calculate K
c

at 25
o
C for the following reaction.

(F = 96,500 coul/Far and R = 8.314 J/mol K) 2Fe
2+
(aq)

+ I
2

(s)
--
> 2F
e
3+
(aq)

+ 2I
-

(aq)

E
o

Fe
3+
(aq) + e
-

--
>

Fe
2+
(aq)

+0.771 V

I
2

(s)

+ 2e
-

--
>

2I
-

(aq)

+0.535 V

a) 1.6 x 10
12

b) 1.1 x 10
-
8

c) 1.0 x 10
-
4

d) 9.6 x 10
17

e) 9.7 x 10
3

13. Calculate E
cell

for the following voltaic cell.

Ag | Ag
+
(1.0 x 10
-
5

M) || Au
3+

(1.0 x 10
-
1

M) | Au

a) +0.78 V

b) +0.46 V

c) +0.88 V

d) +0.98 V

e) +2.58 V

14. Calculate the Gibbs free energy change for the reaction below when initial concentrations of Cr
3+

and Cu
2+
are 1.00
M
.

(F

= 96,500 coul/Far)

2 Cr
3+

(aq) + 3Cu(s)
--
> 2Cr(s) + 3 Cu
2+

a)
-
232 kJ

b) 623 kJ

c) 313 kJ

d) 232 kJ

e)
-
523 kJ

15. Calculate the [H
+
] for the hydrogen half
-
cell of the following voltaic cell if the observed cell voltage is 2.02 V.

Mg | Mg
+2

(1.00 M) || H
+

(?
M
), H
2
(1.00 atm) | Pt

a) 8.0 x 10
5

M

b) 1.1 x 10
-
3

M

c) 9.8 x 10
-
1

M

d) 8.0 x 10
-
3

M

e) 1.2 x 10
-
6

M

16. In the table of standard reduction potentials which substance is the best oxidizing agent. ____________

17
. How many minutes would a 5.00 ampere current have to be applied to plate out 8.00 grams of copper metal from

aqueous copper(II) sulfate solution?

a) 14.6 minutes

b) 33.3 minutes

c) 81.0 minutes

d) 124 minutes

e) 188 minutes

18. Oxidati
on occurs at the _________ in a voltaic cell and oxidation occurs at the _________ in an electrolytic cell.

a) anode, anode

b) cathode, cathode

c) anode, cathode

d) cathode, anode

e) anode, salt bridge