MCAT Physical : Enthalpy
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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?
More information is needed to answer the question.
–112kJ
248kJ
112kJ
–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 #2 : Enthalpy
Consider the following processes:
1)
2)
3)
Which for the following expresses 
1)
2)
3)
We need to add the given processes in such a way that leaves 
Since there are no 
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 
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 
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