Question

Can changes in volume be negative in chemical reactions or ever?

Answer 1

Of course. A change in volume is NOT the same as a volume.

If a change in volume `V` is negative, then it follows that `DeltaV < 0`. That is a definition of compression, i.e. the volume of something gets smaller.

If a change in volume `V` is positive, then it follows that `DeltaV > 0`. That is a definition of expansion, i.e. the volume of something gets larger.

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Suppose for a generic substitution reaction, we have the following hypothetical volumes:

`V_("reactants") = "5 cm"^3`
`V_("transition state") = V^‡ = "3 cm"^3`

and in this reaction, suppose that the first order reactants involve one molecule and another different molecule. In this example, the transition state is made from the combination of the two molecules.

For this reaction, a specific type of change in volume, known as the volume of activation, is labeled `DeltaV^‡` and is negative.

Could you rationalize why `DeltaV^‡ < 0`? Does that make sense? (This is not a hard question. Imagine what is going on and determine why the volume decreased.)

SIDENOTE: This is a general description of an associative interchange reaction (very similar to `"S"_N2`), where the nucleophile is strong enough that it is more important than breaking the bond between the molecule and a leaving group.