The Law of Mass Action for Related and Simultaneous Equilibria

Each time we write the mass action law, it is based on a specific balanced chemical equation in which the reaction is carried out as written left to right. Chemical reactions can be carried out in various alternative ways, including in reverse and in concert with other reactions. These variations lead to relationships among equilibrium expressions, which are best illustrated in a series of examples. 

In the first reaction, hydrogen and oxygen combine to form water vapor, whereas in the second, water vapor dissociates into hydrogen and oxygen. The equilibrium constants Ki and K2 are clearly the inverse of each other, so their product K1K2 = 1. This is true quite generally The equilibrium constant for a reverse reaction is the reciprocal of the equilibrium constant for the corresponding forward reaction. 

This is a perfectly satisfactory way to write the equation for the chemical reaction it says that 1 mol of hydrogen reacts with y mol of oxygen to yield 1 mol of water vapor. The corresponding equilibrium expression is 

When a balanced chemical equation is multiplied by a constant factor, the corresponding equilibrium constant is raised to a power equal to that factor. 

Bromine chloride, BrCI (top), and iodine bromide, IBr (bottom), are two examples of interhalogens, combinations of two or more halogen elements. 

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