Question

Given the standard reaction enthalpies for the following two reactions: (1) 2Pb(s) + O2(g) = 2PbO(s)...... ΔrH° = -434.6 kJ/mol

(2) 2Zn(s) + O2(g) = 2ZnO(s)........ΔrH° = -696.6 kJ/mol

calculate the standard enthalpy change for the reaction: (3) PbO(s) + Zn(s) =Pb(s) + ZnO(s)......ΔrH° = ?

Answer 1

Lets number the reaction as 1, 2, 3 from top to bottom

required reaction should be written in terms of other reaction

This is Hess Law

required reaction can be written as:

reaction 3 = -0.5 * (reaction 1) +0.5 * (reaction 2)

So, deltaHo rxn for required reaction will be:

deltaHo rxn = -0.5 * deltaHo rxn(reaction 1) +0.5 * deltaHo rxn(reaction 2)

= -0.5 * (-434.6) +0.5 * (-696.6)

= -131.0 KJ

Answer: -131.0 KJ