The equilibrium constant

Vforward = kforward x [I2] x [H2]

Vbackward = kbackward x [HI]2

We know that at equilibrium, the forward and the backward reaction occur in the same rate:

Vforward = Vbackward


kforward x [I2] x [H2] = kbackward x [HI]2

kforwardbackward (1K)

Att.: the quotient of the two constants gives one new constant; we may write:

K is called: "the equilibrium constant"