what is the standard enthalpy change for the reaction

Question

Given the standard enthalpy changes for the following two reactions:

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

(2) Ni(s) + Cl2(g)NiCl2(s)......ΔH° = -305.3 kJ

what is the standard enthalpy change for the reaction:

(3) 2NiCl2(s) + O2(g)2NiO(s) + 2Cl2(g)......ΔH° = ?

Answer 1

This is an example of Hess' Law

The target equation is  2NiCl₂(s) + O₂(g) ==> 2NiO(s) + 2Cl₂(g)

Eq (1) has 2NiO on the right as does the target, so simply copy it down...

2Ni(s) + O2(g) ==> 2NiO(s) ... ∆H = -479.4 kJ

Eq (2) Has NiCl2 on the right, but target has it on the left and w of them, so reverse it and x2 to get ...

2NiCl2(s) ==> 2Ni(s) + 2Cl2(g) ... ∆H = 2x+305.3 kJ (note change in sign when you reverse the rx) = +610.6 kJ

Add them together to get....

2Ni(s) + O2(g) + 2NiCl2(s) ===> 2NiO(s) + 2Ni(s) +2Cl2(g).  Cancel appropriate species on each side to get...

2NiCl2(s) + O2 ===> 2NiO(s) + 2Cl2(g) This is the target equation.

Now, add the ∆H values :  -479.4 kJ + 610.6 kJ = 131.2 kJ