We can combine equilibrium reactions whose Kc values are known to obtain Kc for the overall reaction

**Combining rules **

- If a reaction is reversed, invert the value of Kc.
- If each of the coefficients in an equation is multiplied by the same factor (2, 3 etc), raise Kc to the same power (2, 3 etc).
- If each coefficient in an equation is divided by the same factor (2, 3, etc), take the corresponding root of Kc (i.e., square root, cube root, etc).
- When the individual equations are combined/added together, take the product of the equilibrium constants to obtain the overall Kc.

**Equilibrium constant for the sum of reactions **

For example, nitrogen and oxygen can combine to form either NO (g) or N_{2}O (g) according to the following equilibria.

(1) N_{2} (g) + O_{2} (g) ↔ 2NO (g) K_{c1} = 4.1 x 10^{-31}

(2) N_{2} (g) + ½ O_{2} (g) ↔ N_{2}O (g) K_{c1} = 2.4 x 10^{-18}

Using these two equations, we can obtain Kc for the formation of NO (g) from N_{2}O (g):

(3) N_{2}O (g) + ½ O_{2} (g) ↔ 2NO (g)