# Homework Help: Calculate ΔHo for the chemical reaction:

1. Nov 30, 2016

### AMan24

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
2(255) - [(1)(159) + (1)(243)]
= +108KJ

The answer key says its b) -108KJ. Am i doing something wrong or is the book wrong?

Last edited: Nov 30, 2016
2. Dec 1, 2016

### MexChemE

When using bond enthalpies, the way you calculate the heat of reaction is
$$\Delta H° = \sum \Delta H_{\textrm{Broken}} - \sum \Delta H_{\textrm{Formed}}$$
As in, "the sum of enthalpy changes by broken bonds minus the sum of enthalpy changes by formed bonds." So, the correct answer is -108 kJ.

3. Dec 1, 2016

### Staff: Mentor

ΔH° is not the same thing as a bond enthalpy. Quite the opposite - positive bond enthalpy means negative ΔH° (think why).