Two chemists in the name of Cato Guldberg and Peter Waage observed that there is a change in the mass of a substance participating in a reversible reaction produced a shift in equilibrium. This principle is called the law of mass action. Chemists studied the following type of reversible reaction:
They experimentally observed that the ratio of the product concentration raised to the powers of c and d to the reactant concentration raised to the powers of a and b, always got a constant value. This is called the equilibrium constant, keq. Equilibrium constant (Keq) expression of the above reaction is written below:
where [A], [B], [C], [D] are the molar concentrations of reactants and products while a, b, c, and d are the coefficients in the reactants and products. The above equation is the mathematical form of the law of mass action. The relationship that is applied to every reversible reaction is called law of chemical equilibrium.
Let us have an example of writing the chemical equilibrium expression of some reversible reaction,
In writing the equilibrium constant expression, keq, the concentration of the product raise to certain exponent is written as the numerator while the concentration of the reactant raise to a certain power is the denominator. The exponent is taken from the coefficient of the reactants and products. Therefore the chemical equilibrium constant expression of the equation above is:
To calculate the equilibrium constant, the concentration of the products are multiplied if there are two or more products and it is being divided to the product of the concentration of the reactants. When the Keq value is greater than 1, it means that the equilibrium lies to the right and if the Keq value is less than 1 it means that the equilibrium lies to the left.
Let us try some more examples, write the equilibrium constant expression of the following:
Answer:
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