Use Hess’s Law to calculate the heat of reaction for the following reaction:

A + 3B → 3C + 2D

A + 2B → C + D H= -20 kJ

B → 2C + D H= -45 kJ

Question 10 options:

+25 kJ

-65 kJ

-25 kJ

+65 kJ

-65kJ

Explanation:

It is possible to calculate the heat of a reaction by the sum of half-reactions, thus:

(a) A + 2B → C + D   ΔH = -20 kJ

(b) B → 2C + D    ΔH = -45 kJ

The sum of (a) + (b) gives:

(a) + (b) A + 3B → 3C + 2D ΔH = -20kJ - 45kJ = -65kJ

Thus, heat of reaction is -65kJ