## practice midterm question

505166714
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### practice midterm question

Can somebody explain how to calculate reaction enthalpy using enthalpy of combustion? Why is the enthalpy of the reaction negative?
Attachments Erin Kim 2G
Posts: 75
Joined: Fri Sep 28, 2018 12:26 am

### Re: practice midterm question

You know the general equation for combustion is "molecule" + O2= CO2 + H20. So you would write out the combustion equations for all three of the molecules for which the heat of combustion is given. Then you would rearrange those equations to have the correct molecules cancel out to form the given equation. Then you would use Hess's Law and the resulting H values added all together to get the final H. You are able to use Hess Law because enthalpy is a state function.

David Effio 1H
Posts: 38
Joined: Wed Feb 14, 2018 3:01 am

### Re: practice midterm question

Enthalpy change of combustion and enthalpy change of the reaction are not the same thing. To solve the change in enthalpy of the reaction, you have to use the enthalpy change of combustion.

In this case this would be:

DeltaH rxn = DeltaH of C2H6 - (DeltaH of C2H2 + 2 * DeltaH of H2)
= -1560 - (-1300 + 2(-286))
=-1560 + 1872
= 312 kj

This is the change in enthalpy of combustion, which is always positive as the reaction is exothermic.
The enthalpy change of the reaction will, therefore, be negative because the process is exothermic, hence the -312 kJ/mol.

I hope that helps!

Irene Zhou 1E
Posts: 73
Joined: Fri Sep 28, 2018 12:18 am

### Re: practice midterm question

This question is a version of Hess' Law except that the question does not explicitly state each equation. Thus, you need to write out each of the equations and subsequently manipulate them and their respective enthalpy to calculate the overall enthalpy.

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

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