Cliffs AP Chemistry, 3rd Edition

Samples: Multiple-Choice Questions

1. Given the following information:

Reaction (1): HOHO 22 __ggii+^12 " 2 ^,h ∆H°= −286 kJ

Reaction (2): CO 22 _ggi"C s^h+O _ i ∆H°= 394 kJ

Reaction (3): 2 CO 22 _gggi++H O^,h"C H 22 __ii 25 O 2 ∆H°= 1300 kJ

Find ∆H°for the reaction C 2 H 2 (g) →2 C(s) + H 2 (g).

A. −226 kJ

B. −113 kJ

C. 113 kJ

D. 226 kJ

E. 452 kJ

Answer: A

Recognize that this is a Hess’s law problem, which basically requires that you rearrange the
three reactions listed if necessary (remembering to reverse the sign of ∆H°if you reverse the
reaction) and then add up the reactions and the ∆H°’s for the answer.

Looking at the first reaction, you notice that H 2 (g) is on the wrong side. This requires that you
reverse the reaction, thus changing the sign of ∆H°. In the second reaction, C is on the correct
side of the equation, so the second equation should be left as is. In the third reaction, C 2 H 2 (g) is
on the wrong side of the equation, so the reaction and the sign of ∆H°must be reversed. At this
point the set-up should look like

HO 22 ^,h"H__ggii+ 12 / O 2 ∆H°= +286 kJ

CO 22 _gs gi"C^h+O _ i ∆H°= +394 kJ

CH 22 ___gggiii++ 52 / O 22 " 2 CO HO 2 ^,h ∆H°= −1300 kJ

Before you add up the three reactions, note that there is no CO 2 (g) in the final reaction. The 2
CO 2 (g) in the third reaction must be able to cancel with two CO 2 (g)’s in the second reaction. In
order to get 2 CO 2 (g) in the second reaction, multiply everything in the second equation by 2,
which yields

/

/

()

gg

sg

ggg

ggs

g

12

222

52 2

2

HO H O

CO C O

CH O CO HO

CH H C

22

22

22

2

2

22 2

22

"

"

"

"

,

,

+

+

++

+

^ __

^ _

___^

__^

h ii

h i

iiih

iih

H

H

H

H

kJ

kJ

kJ

∆

∆

∆

∆

286

788

1300

=+

=+

=-

=- 226 kJ

%

%

%

%

Thermochemistry