Re: How does the bond energy relate to the reactivity of a compound?

Date: Mon Nov 13 21:53:42 2000
Posted By: Nigel Barker, Head of Science, International School of Lusaka
Area of science: Chemistry
ID: 972741987.Ch

Message:

You are confusing energy needed to break a bond with energy possessed by a molecule. Look at the diagram to see what I mean.

Two molecules of A-B possesses some quantity of energy "high"
molecules A-A and B-B together possess less energy (a total of "low")
So in the reaction
2AB --> A2 + B2
the products are more stable than the reactants. (I like to think about little kids at break time - the ones with high energy are running around, and those with low energy are standing still. The high energy kids are less stable because they are more likely to fall down)

Back to our reaction. The reaction is exothermic, because the total energy of the system drops. That means the bonds in AB are weaker than those in the products, and the difference is given out to the surroundings. Less energy is needes to break two AB bonds than an AA and a BB bond. This can also be visulaised on the diagram. You can see that to get up to the individual atoms you have to input energy to either AB or the products, but you have to give more to the products (from low to very high, compared with just high to very high for reactancts). So bond breaking is always endothermic (as you know from bond enthalpy). The bond with the biggest bond enthalpy is the strongest, and comes therefore from a more stable molecule (in the case of these small one-bond molecules anyway)

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