Unit 4

Question 1

If a chemical reaction causes the temperature of the reaction vessel to decrease, it is a(n) __________ reaction.

a.
endothermic
d.
fast
b.
exothermic
e.
slow
c.
spontaneous

Answer 1

a.

endothermic

Question 2

How much energy is required to convert 2.785 moles of liquid ammonia at 50.00°C to gas at 25.00°C? [Cp(liquid) = 80.80 J/(mol ·°C); Cp(gas) = 35.06J/(mol·°C); Hvap = 23.35 kJ/mol at the normal boiling point of 33.34°C]

a.
74.48 kJ
d.
24.20 kJ
b.
38.11 kJ
e.
69.84 kJ
c.
14.76 kJ

Answer 2

a.

74.48 kJ

Question 3

A 19.5 g piece of titanium [cp = 0.523 J/(g · °C)] is heated and then submerged in 1.50 × 102 grams of water
[cp = 4.18 J/(g · °C)] initially at 22.7°C. The final temperature comes to 23.7°C. What was the initial temperature of the Ti metal?

a.
24.7°C
d.
61.5°C
b.
32.2°C
e.
85.2°C
c.
46.4°C

Answer 3

e.

85.2°C

Question 4

Given the following reactions, what is the overall enthalpy change for the following reaction?
NH3(g) + HCl(g)  NH4Cl(s)

Reaction
H° (kJ)
N2(g) + 3 H2(g)  2 NH3(g)
–92
H2(g) + Cl2(g)  2 HCl(g) 
–185
N2(g) + 4 H2(g) + Cl2(g)  2 NH4Cl(s)
–629

a.
–38 kJ
d.
–445 kJ
b.
–176 kJ
e.
–454 kJ
c.
–352 kJ

Answer 4

b.

–176 kJ

Question 5

The standard enthalpy of combustion of propane gas (C3H8) is 2220 kJ/mol. What is the standard enthalpy of formation of C3H8, given that the standard enthalpies of formation of CO2(g) and H2O() are –394 and 
–286 kJ/mol, respectively?
a.
\+38 kJ/mol
d.
106 kJ/mol
b.
\+60 kJ/mol
e.
466 kJ/mol
c.
–1540 kJ/mol

Answer 5

d.

106 kJ/mol

Question 6

Coal is converted into cleaner, more transportable fuels by burning it with oxygen to produce carbon monoxide. The carbon monoxide then is reacted with hydrogen using a catalyst to produce methane and water. Is the reaction between CO and H2 exothermic or endothermic, and what is the change in enthalpy for it? The enthalpies of formation of the reactants and products are given below.

Molecule
 in kJ/mol
CO
–110
 H2
 0
 CH4  
–75
H2O
–242

a.
exothermic, +317 kJ/mol
d.
endothermic, +427 kJ/mol
b.
endothermic, +207 kJ/mol
e.
exothermic, –207 kJ/mol
c.
exothermic, – 427 kJ/mol

Answer 6

e.

exothermic, –207 kJ/mol

Question 7

Use the bond energies in the table below to estimate the enthalpy change associated with the hydrogenation reaction of acetylene.

Bond 
CC
C—C
CH
HH
Energy (kJ/mol)
839
348
413
436

a.
289 kJ/mol
d.
5363 kJ/mol
b.
289 kJ/mol
e.
5363 kJ/mol
c.
725 kJ/mol

Answer 7

b.

289 kJ/mol

Question 8

Ethanol (CH3CH2OH) has been suggested as an alternative fuel source. Ethanol’s enthalpy of combustion is Hcomb = –1368 kJ/mol. What volume of ethanol (d = 0.789 g/mL) is required to produce the same amount of energy as 1 gallon of gasoline, which can be modeled as isooctane (C8H18, d = 0.703 g/mL) with an energy density of 32.9kJ/mL? (1 gal = 3.785 L)

a.
1.41 gal
d.
0.85 gal
b.
1.00 gal
e.
1.15 gal
c.
0.90 gal

Answer 8

a.

1.41 gal

Question 9

Which of the following processes is NOT spontaneous?

a.
Iron in moist air rusts.
b.
Carbon dioxide and water react to form sugar and oxygen gas.
c.
Liquid water in a freezer turns to ice.
d.
A spark ignites a mixture of propane and air.
e.
Baking soda (sodium hydrogen carbonate) reacts with lemon juice (dilute citric acid).

Answer 9

b.

Carbon dioxide and water react to form sugar and oxygen gas.

Question 10

Determine the value of G° for the reaction 2 NO2(g)  N2O4(g), given

Substance
Hf°(kJ/mol)
S° (J/mol · K)
NO2(g)
33.2
240.0
N2O4(g)
9.2
304.2

a.
–4.8 kJ
d.
–52.3 kJ
b.
\+4.8 kJ
e.
–43.0 kJ
c.
\+52.3 kJ

Answer 10

a.

–4.8 kJ

Question 11

What is the standard entropy change when 10.0 g of methane reacts with 10.0 g of oxygen to form carbon dioxide and liquid water?

Substance
S° (J/mol · K)
CH4(g)
186.2
O2(g)
205.0
H2O()
70.0
CO2(g)
213.6
a.
–121 J/K
d.
–154 J/K
b.
–37.9 J/K
e.
–16.8 J/K
c.
–243 J/K

Answer 11

b.

–37.9 J/K

Question 12

Indicate which one of the following reactions results in a negative Ssys.

a.
H2O(g)  H2O(s)
b.
CaCO3(s)  CaO(s) + CO2(g)
c.
CuSO4 · 5 H2O(s)  CuSO4(s) + 5 H2O(g)
d.
14 O2(g) + 3 NH4NO3(s) + C10H22   3 N2(g) + 17 H2O(g) + 10 CO2(g)
e.
CO2(aq)  CO2(g)

Answer 12

a.

H2O(g)  H2O(s)

Question 13

In a spontaneous process, the entropy of the universe __________
a.
always increases.
b.
always decreases.
c.
does not change.
d.
may decrease if the entropy of the system decreases sufficiently.
e.
may decrease if the entropy of the system increases sufficiently.

Answer 13

a.

always increases.

Question 14

What is the overall standard free energy change for the following two reactions?
A + B  C = G1
C + D  E = G2

a.
G1 + G2
d.
G1G2
b.
G1 – G2
e.
G1/G2
c.
G2 – G1

Answer 14

a.

G1 + G2

Question 15

1. Ammonium nitrate is an ingredient in cold packs used by sports trainers for injured athletes. Calculate the change in temperature when 42 g of ammonium nitrate (NH4NO3, 80.1 g/mol) dissolves in 250 g water. Assume the specific heat of the solution is 4.18 J/(g · °C).
   NH4NO3(s)  NH4+(aq) + NO3–(aq) H = 25.7 kJ/mol

Answer 15

–11°C

Question 16

Estimate the free energy change for the reaction 2 KClO3(s)  3 O2(g) + 2 KCl(s), at 350.0°C from the data given.

Substance
Hf °(kJ/mol)
S° (J/mol · K)
KClO3(s)
391.4
143.1
O2(g)

205.0
KCl(s)
–436.5
82.6

Answer 16

398.0 kJ