MCAT Physical : Enthalpy

Study concepts, example questions & explanations for MCAT Physical

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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?

Possible Answers:

More information is needed to answer the question.

112kJ

248kJ

–112kJ

Correct answer:

–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 #1072 : Mcat Physical Sciences

Consider the following processes:

1) 

2) 

3) 

Which for the following expresses  for the process  ?

Possible Answers:

Correct answer:

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 .

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