Hess’s Law: Two Reactions

Question

Given the standard enthalpy changes for the following two reactions:

(1) 2Ni(s) + O₂(g)2NiO(s)...... ΔH° = -479.4 kJ

(2) 2Zn(s) + O₂(g)2ZnO(s)......ΔH° = -696.6 kJ

what is the standard enthalpy change for the reaction:

(3) NiO(s) + Zn(s)Ni(s) + ZnO(s)......ΔH° = ?

_______ kJ

Answer 1

The second reaction becomes ( after dividing through by 2)

Zn + (1/2) O₂ → Zn O ΔH = -348.3

After reversing the first reaction and dividing through by 2 the first reaction becomes

Ni O → Ni + (1/2) O₂ ΔH = + 239.7

Now add these together ( note the (1/2) O₂ cancels out ) to get

Zn + Ni O → Zn O + Ni ΔH = - 108.6

So the answer is - 108.6 KJ