Bond Enthalpies.

But first, just to recap.

Enthalpy of Combustion; The enthalpy change it takes to burn one mole of compound in oxygen under standard conditions, all products/reactants in their standard states.

Enthalpy of Formation; The enthalpy change it takes to form one mole of compund from its constituent elements, under standard conditions, all products/reactants in standard states.

(I can finally remember these without looking. YEA! :D)

So yea, bond enthalpies.

Energy has to be put in to break a bond, endothermic. (takes in heat.) Bond Dissasociation energy is the value of energy required to break a covalent bond, with all species in a gassious state. And in reverse, the same amount of energy would be given out, exothermically. (When it is formed)

Each bond may have slightly different enthalpies in different molecules. Becuase of this, bond enthalpies are normally averages, to make things a bit of accurate. However, using them for calulations, answers won't be entirely accurate, as I'm sure you can tell, but they are quicker to use.

We can use bond enthalpies to work out the enthalpy changes in reactions. We do this by counting up the types of bonds, and how many there are, and filling in their average values. All the bonds have to break, in the reactants, so add these up to get the energy that must be put in. The energy given out is the added up reactants. The difference between these two values is the enthalpy change. Also, if more energy is given out then put in, it is exothermic and needs a '-' sign at the start.

Although, a way to shorten it down is to only calculate with the bonds that actually take part in the reaction, as this will really cut down calculation times.

In a lot of past exam questions I've seen recently there's a load of stuff about comparing the average bond enthapies to thermodynamic type things. This would be that, in a thermodynamic cycle, all the calculated (delta)Hf/c values are used of the actual compounds, whereas with bond enthalpies they are average, so thermos are more accurate.