### All MCAT Physical Resources

## Example Questions

### Example Question #41 : Thermochemistry And Energetics

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?

**Possible Answers:**

More information is needed to answer the question.

112kJ

248kJ

–112kJ

**Correct answer:**

–112kJ

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 #1 : Enthalpy

Consider the following processes:

1)

2)

3)

Which for the following expresses for the process ?

**Possible Answers:**

**Correct answer:**

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 .

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