5.3 Measuring Energy Changes

Energy is absorbed when bonds are broken, and released when bonds are formed. You can then use bond enthalpy values to calculate ∆H in a reaction. 

*Bond enthalpy values are averages taken across a range of environments

∆H= ∑ Bonds Broken - ∑ Bonds Made

Example:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

 4 C-H Bonds

 2 O=O Bonds

Which yields C(g) + 4H(g) + 4O(g)

This then forms:

2 C=O Bonds

4 O-H Bonds

So

∆H= [4(C-H) + 2(O=O)] - [2(C=O) + 4(O-H)]