6. Combining Reactions

Reactions can be combined (like mathematical equations). How can we calculate the overall equilibrium constant?

• Adding/substracting chemical reactions

1	2 A B
2	B + C D
Sum	2 A + C D

1	2 A + C D
2	2 A B
Difference	B + C D

The equilibrium constant of the reaction obtained by adding two chemical equilibria, is the product of the original equilibrium constants.
The equilibrium constant of the reaction obtained by substracting two chemical equilibria, is the quotient of the original equilibrium constants.

• Multiplying/division chemical reactions by a coefficient

1	2 A B
2	4 A 2 B

1	4 A 2 B
2	2 A B

The equilibrium constant of the reaction obtained by multiplying a chemical equilibrium by a coefficient n, is the n^th power of the original equilibrium constants.
The equilibrium constant of the reaction obtained by dividing a chemical equilibrium by a coefficient n, is the n^th root of the original equilibrium constants.

• Reversing a chemical reaction

1	2 A B
2	B 2 A

The equilibrium constant of the reaction obtained by reversing a chemical equilibrium is the reciprocal value of the original equilibrium constant.

Summary

Equilibria	K
addition	multiplication
substraction	division
multiplication by n	nth power
division by n	nth root
reversion	reciprocal value