enthalpy change?

Question

Calculate the standard enthalpy of formation of magnesium carbonate, using the following information. the standard enthalpy of combustion of magnesium is - 602 kJ mol -1 and that of carbon is - 394 kJ mol -1. the standard enthalpy change for the decomposition of magnesium carbonate in the reaction:
MgCO3 (s) ----> MgO (s) + CO2 (g)
is 100 kJ mol-1

Answer 1

This problem can again be solved by using the principle of Hess's Law:
http://en.wikipedia.org/wiki/Hess%27s_La...
What is asked for is the standard enthalpy of formation:
Mg + C + 1.5O2 ==> MgCO3
Write out the given conditions:
Mg + 0.5O2 ==> MgO, ∆H = - 602 kJ/mol---(1)
C + O2 ==> CO2, ∆H = - 394 kJ/mol----------(2)
MgCO3 ==> MgO + CO2, ∆H = 100 kJ/mol-(3)
(1)+(2)-(3) and cancel out identical terms from both side:
Mg + C + 1.5O2 ==> MgCO3, ∆H = -1096 kJ/mol.