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Postby donnanguyen1d » Sat Jan 20, 2018 9:05 am

What is the difference between bond enthalpies and enthalpies of formation? When do we use each?

Luke Bricca 1H
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Re: Enthalpy

Postby Luke Bricca 1H » Sat Jan 20, 2018 9:34 am

Bond enthalpies are the specific quantities of energy needed to break or form a bond between two atoms (ex: C--C or C--H). Bond enthalpies can be summed (since enthalpy is a state property) to find the total enthalpy of a reaction.

Enthalpies of formation are the standard reaction enthalpies required to form 1 mol of a product. This is essentially the energy required to form 1 mol of a product in any chemical reaction.

Lily Sperling 1E
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Re: Enthalpy

Postby Lily Sperling 1E » Sat Jan 20, 2018 9:38 am

Formation enthalpy refers to the formation of a compound while bond enthalpy is used when forming a molecule from isolated atoms. I believe the difference is just the creation of a compound vs a molecule.

Justin Folk 3I
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Re: Enthalpy

Postby Justin Folk 3I » Sat Jan 20, 2018 10:52 am

Bond enthalpy is equal to enthalpy of formation in a diatomic molecule; for the others it is the sum of the bond enthalpies i believe. Correct me if wrong

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Re: Enthalpy

Postby 905022356 » Sat Jan 20, 2018 1:03 pm

Bond enthalpies refer to the energy required to break a specific bond. The values given for diatomic molecules are absolute values, while the values given for any other bond enthalpy are averages.

The enthalpy of formation of a given substance represents the heat released or absorbed when one mol of a substance is formed from its constituent elements in their standard states.

To find ΔH of a reaction, you can use either the enthalpies of formation of the reactants and the products, or the bond enthalpies of all of the bonds in the reactants and the product. It just depends on what the question is asking you to use, or what information you've been given.

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