Question

Question #557f1

Answer 1

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
`DeltaH_1=(125g)(4.18J/(g-K))(0-25)=-13062J`

2: The enthalpy of fusion of water is 333.5 J/g, so the enthalpy change of the second step is
`DeltaH_2=(125 g)(-333.5)=-41687 J`

3: The specific heat capacity of ice is about 2.03 J/(g-K), so the enthalpy change of the third step is
`DeltaH_3=(125g)(2.03J/(g-K))(-15-0)=-3806J`

The total enthalpy change of the process is the sum of the individual steps:
`DeltaH_(Total)=-13,062J-41,687J-3,806J=-58,555J`