Cliffs AP Chemistry, 3rd Edition

Answer: C

Begin this problem by drawing a structural diagram of the reaction.

There are three steps you need to take to do this problem:

Step 1:Decide which bonds need to be broken on the reactant side of the reaction. Add up all
the bond energies for the bonds that are broken. Call this subtotal ∆H 1. Assign ∆H 1 a positive
value because energy is required when bonds are broken. In the example given, a C==O and
aHH bond need to be broken. This becomes xkcal/mole + 104 kcal/mole, or ∆H 1 = 104 +
xkcal/mole.

Step 2:Decide which bonds need to be formed on the product side of the reaction. Add up all
of the bond energies that are formed. Call this subtotal ∆H 2. Assign ∆H 2 a negative value be-
cause energy is released when bonds are formed. In the example given, a CH, a CO, and a
OH bond need to be formed. This becomes 99 kcal/mole + 84 kcal/mole + 111 kcal/mole, or
294 kcal/mole. Remember to assign a negative sign, which makes ∆H 2 = −294 kcal/mole.

Step 3:Apply Hess’s law: ∆H°= ∆H 1 + ∆H 2. You know that ∆H°is −17 kcal/mole, so Hess’s
law becomes

−17 kcal/mole = 104 kcal/mole + xkcal/mole −294 kcal/mole

x= 173 kcal/mole

which represents the bond energy of the C==O bond.

4. Given the following heats of formation:

Substance ∆H°f

acetic acid −120 kcal/mole

carbon dioxide −95 kcal/mole

water −60 kcal/mole

Find ∆H°of combustion for acetic acid (CH 3 COOH).

A. −430 kcal/mole

B. −190 kcal/mole

C. −45 kcal/mole

D. 45 kcal/mole

E. 190 kcal/mole

HCCOHH

H

H H

HCC O H

H H

H H

+

Thermochemistry