[doxasikyrie]

I am having a lot of trouble trying to comprehend this problem. The problem is:

Calculate ΔHrxn for the following reaction:
Ca(s)+CO2(g)-->CaCO3(s)

Use the following reactions and given ΔH's:
Ca(s) + CO2(g) + 1/2O2(g)--> CaCO3(s), ΔH=-812.8 kJ
2Ca(s) + O2(g)--> 2CaO(s), ΔH=-1269.8 kJ

Do I need to find the ΔH of the "Ca(s)+CO2(g)-->CaCO3(s)"? If so how? I have not the slightest idea how to calculate the ΔHrxn. There was another problem on here similar to this one, but it did not make any sense to me at all. Any and all help would be appreciated!

[Borek]

Application of Hess Law.