Heat of Combustion vs. Heat of Formation

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Mayvera Doaran 2H
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Joined: Fri Sep 25, 2015 3:00 am

Heat of Combustion vs. Heat of Formation

Postby Mayvera Doaran 2H » Sat Jan 09, 2016 12:47 pm

What is the difference between the heat of combustion and the heat of formation? How do they relate to each other?

Shawn Barman 3C
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Joined: Fri Sep 25, 2015 3:00 am

Re: Heat of Combustion vs. Heat of Formation

Postby Shawn Barman 3C » Sat Jan 09, 2016 2:19 pm

The heat of combustion will always be negative since the reaction will be giving off heat as it breaks the chemical bonds. The heat of formation should always be positive since it will be taking in heat in order to produce the chemical bonds. They can be related to one another by using the sum of the heat of formation for the products and the sum of the heat of formation for the reactants and taking the difference (Hess's law) will give you the heat of combustion.

Mayvera Doaran 2H
Posts: 23
Joined: Fri Sep 25, 2015 3:00 am

Re: Heat of Combustion vs. Heat of Formation

Postby Mayvera Doaran 2H » Mon Jan 11, 2016 7:03 pm

So to clarify, the heat of combustion is the opposite of heat of formation? So if you know the heat of combustion, you know the heat of formation?

Chem_Mod
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Re: Heat of Combustion vs. Heat of Formation

Postby Chem_Mod » Mon Jan 11, 2016 9:36 pm

They are not opposites.

The heat of combustion is just the enthalpy of reaction for any combustion reaction.

To find the enthalpy of any reaction, you look up enthalpies of formation and do products minus reactants.


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