Applications of Equilibrium Constants

Applications of Equilibrium Constants

Before we examine how equilibrium constants are used, we need to fix the key features that govern them. These features are not just facts to memorise — they are the logical foundation on which every prediction about chemical equilibrium rests.

Important Features of Equilibrium Constants

1. The equilibrium constant expression is valid only at equilibrium.

This means you cannot plug in concentrations measured at an arbitrary time and call the result $K_c$. The expression $\frac{[\text{products}]^{\text{coefficients}}}{[\text{reactants}]^{\text{coefficients}}}$ gives $K_c$ only when the concentrations have stopped changing — that is, when the system has reached dynamic equilibrium.

2. The value of $K_c$ is independent of the initial concentrations of reactants and products.

Whether you start with pure reactants, pure products, or a mixture, at a given temperature the equilibrium constant is the same number. This is a direct consequence of the law of mass action: the ratio of concentrations at equilibrium is fixed by thermodynamics, not by how you got there.

3. $K_c$ depends only on temperature.

For a particular reaction written as a balanced equation, there is exactly one value of $K_c$ at a given temperature. Change the temperature, and $K_c$ changes. This is why every equilibrium constant must be quoted with its temperature.

4. The equilibrium constant for the reverse reaction is the reciprocal of the constant for the forward reaction.

If a forward reaction has $K_c$, then the reverse reaction has $K_c' = 1/K_c$. This follows directly from the expression: reversing the reaction swaps numerator and denominator.

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Where the Applications Are Developed

The three applications above each get their own section: …