Calculate ΔHrxn CaO(s)+CO2(g)→CaCO3(s) given ΔH values: Ca(s)+CO2(g)+12O2(g)→CaCO3(s), ΔH= -812.8 kJ 2Ca(s)+O2(g)→2CaO(s), ΔH= -1269.8 kJ?

Refer to the given ΔH values. The following thermochemical equations are written.
CaO(s) → Ca(s) + (1/2)O₂(g) .... ΔH = -(1/2)(-1269.8 kJ) = (+634.9 kJ)
Ca(s) + CO₂(g) + (1/2)O₂(g) → CaCO₃(s) …. ΔH= -812.8 kJ

Add the above two thermochemical equations, and cancel Ca(s) and (1/2)O₂(g) on the both sides.
CaO(s) + CO₂(g) → CaO(s) …. ΔH = (+634.9 kJ) + (-812.8 kJ) = -177.9 kJ

CaO(S) → Ca(s) + ½O2(g), ΔH= + 634. 9kj.mole-1 (i)

Ca(s) + CO2(g) + ½O2(g) → CaCO3(s), ΔH= - 812.8 kj.mole-1 (ii)

Adding both the equations,

CaO(s) + CO2(g) → CaCO3(s), ΔH= - 177.9 kj.mole-1