Where does the energy come from (or go to) in a chemical reaction? It comes from the breaking or forming of chemical bonds. We can define a property called the bond energy which is equal to the DH when one mole of bonds is broken in the gaseous state.

For example, if the reaction

HF(g) -> H(g) + F(g)    DH = +565 kJ

then we can say that the HF bond energy is 565 kJ/mol. (Note that we have not formed H₂ or F₂- this would require forming H-H and F-F bonds.)

Bond energy is always a positive number: it always takes energy to break a chemical bond. Multiple bonds are stronger than single bonds: a C=C double bond has a bond energy of about 612 kJ/mol compared to 347 kJ/mole for a single C-C bond and 820 kJ/mol for a carbon carbon triple bond.

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