Question

Why does equilibrium exist?

is there a driving force behind it? eg the equilibrium of liquid water to water vapour in a bottle

Answer 1

Because some reactions are reversible.

Explanation:

Some reactions, like combustion reactions, are basically irreversible. This means that in this reaction:

`A + B -> C + D`

Virtually every single particle of `A` and every single particle of `B` becomes `C` and `D`.
Eventually, either `A` or `B` will run out and the reaction will stop.

However, some reactions are reversible. This means that, in a reversible reaction like this one:

`A + B rightleftharpoons C + D`

`C` and `D` will also turn back into `A` and `B`.

• Initially, `A` and `B` become `C` and `D` at a high rate.
• The more `A` and `B` become `C` and `D`, the more `C` and `D` there will be to form back `A` and `B`.
• `C` and `D` re-form `A` and `B`.
• The `A` and `B` that are formed go back to form `C` and `D` again.

Eventually, the rate of `C` and `D` turning back into `A` and `B` will be the same as the rate of `A` and `B` turning into `C` and `D`.
That's when equilibrium has been reached.

The "driving force" behind reaching this chemical equilibrium would be the tendency for the reactants to form products, and the products to form reactants in a reversible reaction.