Bond Enthalpies vs. Mean Bond Enthalpy

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Ian Hurst 3L
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Bond Enthalpies vs. Mean Bond Enthalpy

Postby Ian Hurst 3L » Sun Jan 17, 2016 5:06 pm

Can someone explain to me why bonds have an enthalpy and a mean bond enthalpy? (What is the mean bond enthalpy an average of?) Also, which one do we use when solving problems?

Brenda Melano 3H
Posts: 27
Joined: Fri Sep 25, 2015 3:00 am

Re: Bond Enthalpies vs. Mean Bond Enthalpy

Postby Brenda Melano 3H » Sun Jan 17, 2016 5:15 pm

I think mean bond enthalpy is the average enthalpy needed to break a certain type of bond (say a C-H bond) for all molecules with that (C-H) bond. Bond enthalpy or bond dissociation enthalpy is the energy needed to break a C-H bond in one particular molecule. I would assume that we use the average bond dissociation energy for problems because it would be too much work and data to keep in a textbook to use specific dissociation energies for one bond for all the molecules.

904564128
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

Re: Bond Enthalpies vs. Mean Bond Enthalpy

Postby 904564128 » Sun Jan 17, 2016 5:16 pm

Bond dissociation enthalpy is the energy needed to break one mole of the bond to give separated atoms - everything being in the gas state.
Now consider methane, CH4. It contains four identical C-H bonds, and it seems reasonable that they should all have the same bond enthalpy. However, if you took methane to pieces one hydrogen at a time, it needs a different amount of energy to break each of the four C-H bonds. Every time you break a hydrogen off the carbon, the environment of those left behind changes. And the strength of a bond is affected by what else is around it. In cases like this, the bond enthalpy quoted is an average value and is thus called a mean bond enthalpy.
You don't need to worry about which to choose, just use the values that you are given.


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