# Calculate ΔHo for the chemical reaction:

## 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?

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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.

AMan24
Borek
Mentor
ΔH° is not the same thing as a bond enthalpy. Quite the opposite - positive bond enthalpy means negative ΔH° (think why).

AMan24