Question

Why does pKa affect equilibrium?

Answer 1

Because `pK_a` is a MEASUREMENT of the equilibrium?

Explanation:

For the generalized reaction.....

`A+BrightleftharpoonsC+D`

Now there is a rate forward....

`"Rate forward"=k_f[A][B]`, and a `"rate backward"=k_r[C][D]`.

And equilibrium does NOT specify cessation of chemical change; it does specify EQUALITY of FORWARD or REVERSE rates.

And so.......

`"Rate forward"=k_f[A][B]-="rate backward"=k_r[C][D]`.

And thus the quotient `([C][D])/([A][B])=k_r/k_f=K_"eq"`

And `K_"eq"` is otherwise known as the thermodynamic equilibrium constant.

`pK_a` or `pK_"eq"` is simply the logarithm of `K_a`, `K_"eq"`..

`pK_a=-log_10K_a`