[pakiaish]

Ok so here is the question I'm given

Calculate ΔHrxn for
Ca (s) + ½ O2 (g) + CO2 (g) → CaCO3 (s)
Given the following set of reactions:
Ca (s) + ½ O2 (g) → CaO (s) ΔH = -635.1 kJ
CaCO3 (s) → CaO (s) + CO2 (g) ΔH = 178.3 kJ


I understand that in order to find delta H for the reaction it is the Products - Reactants but here I'm given three reactions. I just don't know what to do about the two delta H sums I'm given. If anyone could please please walk me through it, it would be great thanks

[cliverlong]

Google Hess' Law, try a few examples, then tackle the question again.

Clive