Practice Quiz

SAT Chemistry

Category - Multiple Choice

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Calculate the bond dissociation energy in the following equation:

CH4(g) + 2O2(g) --> CO2(g) + 2H20 (g)

Given the following:

DHf° C-H = 87 Kcal/mol
DHf° O=O = 118 Kcal/mol
DHf° C=O = 178 Kcal/mol
DHf° O-H = 111 Kcal/mol
  1. +584 KCal
  2. +216 KCal
  3. -584 KCal
  4. -216 KCal
  5. -228 KCal
Explanation
Answer: D, -216 KCal. Solution is as follows:

DHdiss° = [energy of bonds broken] - [energy of bonds formed]

In the equation, you have the following bonds:

4C-H (CH4) + 2 O=O --> O=C=O + 2 H-O-H ***OR***
4C-H + 2O=O --> 2 C=O + 4 O-H

Using the heats of formation provided, the equation becomes

DHdiss° = [(4*87) + (2*118)] - [(2*178) +(4*111)] = -216KCal
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