# MCAT Physical : Enthalpy

## Example Questions

### Example Question #1 : Enthalpy

The formation of nitrous oxide is a 2-step process.   The overall enthalpy of the reaction is +68kJ.

If the enthalpy change of step 1 is 180kJ, what is the enthalpy change of step 2?

248kJ

112kJ

–112kJ

–112kJ

Explanation:

Hess's law states that the enthalpies of the steps in an overall reaction can be added in order to find the overall enthalpy. Since the overall enthalpy of the reaction is +68kJ, we can use the following equation to find step 2's enthalpy.   ### Example Question #2 : Enthalpy

Consider the following processes:

1) 2) 3) Which for the following expresses for the process ?      Explanation:

1) 2) 3)  We need to add the given processes in such a way that leaves on the left side, and on the right.

Since there are no in the final equation, begin by combining processes 1 and 2 to eliminate.

First, reverse process 2 and multiply by two:

2a) 2a) 2a) Add this result to process 1.

1) 2a) 1+2a) Now we need to combine with process 3 to eliminate the . Start by multiplying process 3 by two.

3a) 3a) Add this to the combined result from processes 1 and 2.

1+2a) 3a) 1+2a+3a) This gives our final combination. The elemental reaction is , and the enthalpy of the process is . 