Question

Using the following equation
Ca(s) + 2C(s) --> CaC2(s)
^ H = -62.8 kJ

CO2(g) --> C(s) + O2(g)
^ H = 393.5 kJ

CaCO3(s) + CO2 (g) --> CaC2(s) + 2 1/2O2(g)
^ H = 1538 kJ

Determine the heat of the reaction (in kJ) for: Ca(s) + C(s) + 3/2O2(g) --> CaCO3(s)

Answer 1

Consider these three reactions happening together:
CaC2(s)+ 2 1/2O2(g)-> CaCO3(s)+ CO2 (g)
CO2 (g) -> C(s) + O2(g)
Ca(s) +2C(s) -> CaC2(s)

The net reaction is
Ca(s) + C(s) + (3/2)O2 (g) -> CaCO3 (s)

Add or subtract the three heats of formation, ^H, as appropriate. Some are the reverse of the reactions you wrote. Reverse the sign of ^H in those cases.

You should end up the the heat of reaction for the final "net" reaction

I am surprised to see a question like this in ninth grade. I was taught it in graduate school!