Hess’s Law: One Reaction

Question

The standard enthalpy change for the following reaction is 832 kJ at 298 K.

1. 2 Na₂O(s)  4 Na(s) + O₂(g)    ΔH° = 832 kJ

What is the standard enthalpy change for this reaction at 298 K?

2 Na(s) + 1/2 O2(g)  Na2O(s)

 _______kJ

Answer 1

Note that the second equation is the reverse of the first with the coefficients halved.

If heat is gained in one direction, then it must be lost in the opposite direction. Thus for the second equation

ΔH° = -832kJ/2 = -416kJ