Hess's Law states that the enthalpy change for a reaction can be expressed as the sum of enthalpy changes for several individual steps. Therefore, if you're asked to determine the enthalpy change for a reaction under non-standard conditions, and you only have a table for standard conditions, you can construct a more complicated pathway and simply sum the enthalpy changes for the individual steps.
Determine the enthalpy change for cooling 125 g of water from 25 C to -15 C.
This process can be broken down into 3 steps:
1) Cooling liquid water from 25 to 0 C
2) Freezing the liquid to solid ice
3) Cooling the ice from 0 to -15 C
1: The specific heat capacity of liquid water is about 4.18 J/(g-K), so the enthalpy change of the first step is
2: The enthalpy of fusion of water is 333.5 J/g, so the enthalpy change of the second step is
3: The specific heat capacity of ice is about 2.03 J/(g-K), so the enthalpy change of the third step is
The total enthalpy change of the process is the sum of the individual steps: