Method 2: Using bond enthalpies

[Hai-Lin Yeh 1J]

Can someone explain how the method of using bond enthalpies work in a calculation?

[Fayez Kanj]

Hey!

So basically this method uses this formula:

Sum of the energies required to break all bonds of reactants - Sum of energies required to break all bonds of products. These values will have to be given to you

Alternatively, you could do the same for only the bonds that change.

This method will give you the enthalpy change of the reaction. However, this is also the least accurate measure of enthalpy change, since the bond enthalpy values for each bond is an average (ie, it takes different amounts of energies to break a C-H bond in methane vs ethane

Hope this helps :)

[Stevin1H]

For bond enthalpies, it would be the total energy to break the bonds of the reactants - the total energy released by products from bonds forming. Remember, to break a bond, energy is required (+) and to form a bond, energy is released (-). For bond enthalpies, you would draw the lewis structures of the molecules of the reactants and products to determine the bonds that occur in the reaction. By having the bonds, you can determine the amount of energy needed to break bonds or the energy released from forming bonds using a table that gives the bond enthalpies value. Having the amount of energy, you can calculate the overall (net) enthalpy change of the reaction.

[Nicolette_Canlian_2L]

Could you find the difference of the number of bonds broken and the total number of bonds formed? I remember learning it this way in high school, but I'm not too sure.