# 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 .