Bond enthalpies

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Sarah Rutzick 1L
Posts: 50
Joined: Tue Oct 10, 2017 7:13 am

Bond enthalpies

Postby Sarah Rutzick 1L » Mon Jan 22, 2018 11:07 pm

In lecture on Wednesday of week 2, Dr. Lavelle made the comment that tables of bond enthalpies refer to breaking bonds in gases and therefore we need to add enthalpy of phase change for liquids and solids. I'm confused about what this means/how you would do this, can someone explain or give an example?

Julia Campbell 2F
Posts: 31
Joined: Sat Jul 22, 2017 3:00 am

Re: Bond enthalpies

Postby Julia Campbell 2F » Mon Jan 22, 2018 11:33 pm

So when calculating standard enthalpy using bond enthalpies, the mean bond enthalpies provided will be for those bonds in the gaseous phase. In order to calculate the standard enthalpy of a reaction with something in a liquid or solid form, just use the bond enthalpies as you normally would then after you've added/subtracted the bond enthalpies, add/subtract the enthalpy of vaporization or whatever is given. If a reactant is a liquid for example, add the (delta)H of vaporization for that reactant at the end. If a product is a liquid, subtract the (delta)H of vaporization of that product at the end (because for bond enthalpies, breaking bonds is positive and forming bonds is negative).

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