Calculate ∆H for the reaction (j)

Question

Calculate ∆H for the reaction

Ca(s) + ½ O₂(g) + CO₂(g) → CaCO₃(s)

given the following data:

Equation∆H
Ca(s) + ½ O2(g) → CaO(s)	-635 kJ
CaCO3(s) → CaO(s) + CO2(g)	178 kJ

∆H =  kJ

Answer 1

Target equation is Ca + 1/2 O₂ + CO₂ ===> CaCO₃

Use Hess' Law to find ∆H of the reaction:

Ca + 1/2 O₂ ==> CaO ∆H = -635 kJ (equation #1)

CaO + CO₂ ==> CaCO₃ ∆H = -178 kJ (reversed equation 2)

Add the two equations and cancel like items on opposite sides to get:

Ca + 1/2 O₂ + CaO + CO₂ ==> CaO + CaCO₃ ∆H = -635 kJ + -178 kJ = -813 kJ

Ca + 1/2 O₂ + CO₂ ==> CaCO3 ∆H = -813 kJ