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° = ?
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
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