Bond enthalpies to calculate ΔHrxn

[Reagan Feldman 1D]

I understand that tables of bond enthalpies give values in terms of breaking bonds in the gas phase. If there is a phase change in a reaction, would the enthalpy of said change be given to us in order to calculate the overall change in enthalpy of the reaction? If not how would we calculate it?

[Jaden Nguyen 2A]

I believe that the enthalpy you're speaking of is enthalpy of formation. This will typically be provided to be able to find the change in enthalpy of the total reaction, which is the enthalpy of formation of products - enthalpy of formation of reactants.

[Connor Davis 2K]

Yes. If there is a phase change, we also need to add the enthalpy of the phase change for liquids and solids, which means that the enthalpy of the phase change will have to be given in order to solve the problem. For example, Br2(l) --> 2Br(g): the change in enthalpy of the rxn = the change in enthalpy of vaporization + the change in bond enthalpy.

[Sujin Lim 1H]

To find the ΔHrxn using bond enthalpies, you find the amount of energy needed to break bonds on the reactant side, and add that value to the amount of energy released when new bonds are formed on the product side. First, you find the bond enthalpy values for the bonds that must be broken on the reactant side. Then, find which bonds were newly formed of the product side, and find the bond enthalpy values for those bonds. Then you must take the negative values for those bonds enthalpies for the new bonds formed because energy is always released when new bonds form, and therefore is always negative. Since energy is always required to break bonds, the bond enthalpy values are always reported as positive values. So the bond enthalpy values for the new bonds that are formed are the amount of energy required to break that same bond, so you would just take the negative value. In the end, you should have a sum of positive bond enthalpies on the reactant side and the negative bond enthalpy values for the product side, and that will give you the ΔHrxn.