What are K values?

Question

What exactly are K values? Like Kp, Ka, Kc, Kw...

What is their relevance to equilibrium reactions?

Answer 1

They are not rate constants.

They are the equillibrium constants. When a reaction reaches the state of equilibrium then there are some mathematical expresions which are equal to a constant.

e.g. for aA +bB <=> cC+ dD

The most common one is the constant for concentrations

Kc=[A]^a *[B]^b / ([C]^c *[D]^d)
However this can also be derived from the fact that at equilibrium the forward and reverse reactions have equal rates, therefore

kfor*[A]^a *[B]^b = krev *[C]^c *[D]^d and

krev/kfor= =[A]^a *[B]^b / ([C]^c *[D]^d)= Kc
and so Kc is also equal to the ratio of the rate constants of the two reactions.

Kp is similar expression for gases and instead of concentrations you have partial pressures.

Ka is the Kc for the dissociation of an acid

Kw is the Kc for the auto-dissociation of water. For reactions in water (or any aqueous solutions), the molarity (a unit of concentration) of water, [H2O], is practically constant and is omitted from the equilibrium constant expression by convention.
That's why Kw=[H+][OH-]

Answer 2

They are rate constants.

The value of K becomes the value of the rate of the reaction when all the concentration/pressure of the reacting components are one.