Chemistry: heat of formation?

Question

Please help with this chemistry question,

The heat of reaction when 1 mole of calcium carbonate (CaCO3) is decompossed into lime and carbon dioxide is +176.9 kj/mol. Given that the heat of formation of CaCO3 and CO2 (in their standard states are -1206 and -393.5 kj/mol, calculate the heat of formation of calcium oxide.

thanks

Answer 1

First, let's write the reaction:

CaCO3 -> CO2 + CaO

We know the heat of the overall reaction, as well as the heats of formation of two of the three components. Thus, we can calculate the heat of formation of the third.

(Sum of Heats of Formation of Products) - (Sum of Heats of Formation of Reactants) = Heat of Reaction

Substituting in what we know:

(-393.5 kJ/mol + X) - (-1206 kJ/mol) = 176.9 kJ/mol

Solving for X:

X = 176.9 - 1206 + 393.5 = -635.6 kJ/mol

That would be the heat of formation of the calcium oxide, or lime.