The Marriage May Be Bad, But the Divorce Still Costs Money

by James

in Chemical Bonds, Thermodynamics and Kinetics

Common misconception: energy is released through the breaking of bonds.

Not correct.

Energy is released through the FORMATION of chemical bonds.  Formation of a new bond between two atoms results in a net lowering of energy of the atoms involved, corresponding with a release of energy (heat) to the environment.  If there wasn’t some kind of attraction to begin with, the chemical bond would never have formed in the first place.

It costs energy to break a bond.  Even if the bond is weak.

For instance, the oxygen-oxygen bond of hydrogen peroxide is one of the weaker bonds you’ll encounter, with a bond dissociation energy of 142 kJ/mol.

As you might be aware, hydrogen peroxide is useful for the de-skunkification of dogs: the hydrogen peroxide oxidizes the stinky thiols to odorless sulfoxides. [Note – despite what you might have heard, the tomato juice method is pure B.S.]. The peroxide-thiol reaction is extremely favorable, resulting in the formation of a strong S-O bond (bond strength 522 kJ/mol).

Here’s the reaction:

1-bondbreaking

Notice that even though we’re breaking a weak bond like the oxygen-oxygen bond of HO-OH to form a strong bond like S-O, it still costs energy to break the O-O bond.

“Expenses”:  breakage of O–O bond. 142 kJ/mol

“Income”:   formation of S-O bond 522 kJ/mol

Energy profit/loss =  expenses – income -380 kJ/mol (negative denotes exothermic)

So energy is released by the overall process, but breaking the weak bond in the first place still consumes energy.

The marriage of two oxygen atoms in a peroxide molecule may be a relatively unhappy one, but they still have to pay expenses to get a divorce.

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{ 3 comments… read them below or add one }

cdrouin

I love your posts. I might not leave comments often enough but I read every single one of yours !!! Keep going, the chemical world turns with a better spin since you started posting !!!!

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James

Thanks!

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Anon

I love this blog! I didn’t actually think that chem could sound interesting but it does in the way that you write it. Thank you so much!

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