# Question #557f1

##### 1 Answer

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.

Example:

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: