Enthalpy changes

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Matt Sanruk 2H
Posts: 131
Joined: Wed Sep 18, 2019 12:21 am

Enthalpy changes

Postby Matt Sanruk 2H » Sun Jan 26, 2020 10:27 pm

So just to make sure, but the bond enthalpy is reactants minus products while the last method is products minus reactants, right?

Ashley Wang 4G
Posts: 103
Joined: Wed Sep 11, 2019 12:16 am

Re: Enthalpy changes

Postby Ashley Wang 4G » Sun Jan 26, 2020 10:33 pm

I don't think bond enthalpy is reactants minus products! It's the sum of all the bond enthalpies of reactants and products, but it just so happens that these values are negative for the products because energy is released when bonds form (while bond enthalpy is defined to be E required to break a bond).
So while it may appear that it's the reactants minus the products, it's actually the sum of all the bond enthalpies needed to break bonds and form new ones.

Hope this clarifies things!

Ariana Iranmahboub1G
Posts: 114
Joined: Fri Aug 09, 2019 12:17 am

Re: Enthalpy changes

Postby Ariana Iranmahboub1G » Sun Jan 26, 2020 10:37 pm

Somewhat because essentially for the enthalpy method, it is reactants minus products because all the products have negative delta H since bond are formed and all the reactants have positive delta H since bonds are broken. And he standard enthalpy of formation method, the last method, subtracts the standard enthalpy of formation of all products by the standard enthalpy of formation of all reactant.

Sofia Barker 2C
Posts: 101
Joined: Wed Sep 18, 2019 12:21 am

Re: Enthalpy changes

Postby Sofia Barker 2C » Sun Jan 26, 2020 11:44 pm

I think it would be less confusing if you thought of method 2 (calculating delta H rxn through bond enthalpies) as adding up all the bond enthalpy values for the reactants and products. All bonds broken have positive bond enthalpy values because it takes energy to break those bonds, while all bonds formed have negative bond enthalpy values because heat is released when the bonds form. Bonds in reactants almost always break (but not always!) so those values will be positive, and bonds in products always form, so those individual bond enthalpies will be negative.

Matt Sanruk 2H
Posts: 131
Joined: Wed Sep 18, 2019 12:21 am

Re: Enthalpy changes

Postby Matt Sanruk 2H » Thu Jan 30, 2020 12:07 am

Basically the method with Delta H^0 is the one we want to use?


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