### The equilibrium constant

V_{forward} = *k*_{forward } *x* [I_{2}] *x* [H_{2}]

and

V_{backward} = *k*_{backward } *x* [HI]^{2}

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

V_{forward} = V_{backward}

Or:

*k*_{forward} *x* [I_{2}] *x* [H_{2}] = *k*_{backward} *x* [HI]^{2}

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

K is called: "the equilibrium constant"