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# Equilibrium Constant

## Equilibrium Constant at Chemical Equilibrium

The equilibrium constant helps you predict whether a reaction will occur.

Steve McAlister, Getty Images

The equilibrium constant is calculated from the expression for chemical equilibrium. Learn what the equilibrium constant, how to calculate the equilibrium constant and what it means if the equilibrium constant is very large or very small.

### Calculating the Equilibrium Constant

For the following chemical reaction:

aA(g) + bB(g) ↔ cC(g) + dD(g)

The equilibrium constant Kc is:

Kc = [C]c[D]d / [A]a[B]b

where:

[A], [B], [C], [D] etc. are the molar concentrations of A, B, C, D
a, b, c, d etc. are the coefficients in the balanced chemical equation

### Significance of the Equilibrium Constant

For any given temperature, there is only one value for the equilibrium constant. Kconly changes if the temperature at which the reaction occurs changes.

If the value for Kc is very large, then the equilibrium favors the reaction to the right and there are more products than reactants. The reaction may be said to be "complete" or "quantitative."

If the value for the equilibrium constant is small, then the equilibrium favors the reaction to the left and there are more reactants than products. If the value of Kc approaches zero the reaction may be considered not to occur.

If the values for the equilibrium constant for the forward and reverse reaction are nearly the same then the reaction is about as likely to proceed in one direction and the other and the amounts of reactants and products will be nearly equal. This type of reaction is considered to be reversible.

Anne Marie Helmenstine, Ph.D.