Cliffs AP Chemistry, 3rd Edition

Answer: B

Begin this problem by realizing that ∆H°of combustion for acetic acid represents the amount
of heat released when acetic acid burns in oxygen gas. The products are carbon dioxide and
water. According to the following balanced equation,

CH COOH 3222 ^,,h++22 2O __ggii" CO H O^h

Because ∆H°= ΣH°f products−ΣH°f reactants, you can substitute at this point to give

∆H°= 2(−95 kcal/mole) + 2(−60 kcal/mole) −(−120 kcal/mole)

∆H°= −190 kcal/mole

5. For H C CH22 2 33=+___ggiiiH "H C CH-g, predict the enthalpy given the following
   bond dissociation energies:

HC, 413 kJ/mole HH, 436 kJ/mole

C==C, 614 kJ/mole CC, 348 kJ/mole

A. −656 kJ/mole

B. −343 kJ/mole

C. −289 kJ/mole

D. −124 kJ/mole

E. −102 kJ/mole

Answer: D

Begin this problem by drawing a structural diagram:

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 , and assign it a positive
value because when energy is required, bonds are broken. In the example given, a C==C and a
HH bond need to be broken. This becomes 614 kJ/mole + 436 kJ/mole = ∆H 1 = 1050 kJ/mole.

HCCHHH

H H

HCC H

H H

H H

+

Part II: Specific Topics