BitchAss, One of the key ideas of looking at state functions like the enthalpy is that the changes are the same regardless of the process taken from the initial to the final state.

This allows us to take advantage of existing data to calculate the enthalpy changes for reactions we may not have been able to perform or study. This is because even if we cannot directly measure the change in the enthalpy, we can imagine a new process which involves a number of steps that can (or have) been measured. The enthalpy change for the overall process is the sum of the enthalpy change of the steps in the process. This is known as Hess's Law and is given in the following equation.

ΔHrxn=ΔH1+ΔH2+ΔH3+⋯

CH4(g)+2O2(g)→CO2(g)+2H2O(l)
CH4(g)+2O2(g)→CO2(g)+2H2O(l)ΔHrxn=−890kJmol−1
CO2(g)+2H2O(l)→CH4(g)+2O2(g)ΔHrxn=+890kJmol−1
2CH4(g)+4O2(g)→2CO2(g)+4H2O(l)ΔHrxn=2(−890)=−1780kJmol−1