The Equilibrium Constant, Kc - Part 1

This lesson covers: 

1. What the equilibrium constant (K_c) is
2. How K_c indicates the position of equilibrium
3. How to write an expression for K_c
4. Calculations involving K_c

Introducing the equilibrium constant

When a reversible reaction reaches a state of dynamic equilibrium, we can calculate a value called the equilibrium constant (K_c) using the molar concentrations of the reactants and products at equilibrium.

K_c gives us a quantitative measure of where the equilibrium lies - whether there are more products or more reactants present at equilibrium.

• A large K_c value indicates the equilibrium position favours the products.
• A small K_c value indicates the equilibrium position favours the reactants.

A K_c value of 1 indicates that the reaction is at equilibrium, and the concentrations of reactants and products are equal when raised to their respective stoichiometric coefficients.

Writing an expression for K_c

For the general equilibrium reaction:

aA + bB ⇌ dD + eE

The equilibrium constant K_c is given by:

Kc ​=AaBbDdEe​

Where the lower case letters represent the coefficients in the balanced chemical equation.

For example, for the reaction:

H_2(g) + I_2(g) ⇌ 2HI_(g)

The K_c expression would be:

Kc ​=[H2​][I2​][HI]2​

Calculating values for K_c

If we know the equilibrium concentrations of all reactants and products, we can substitute them into the K_c expression to calculate a value for K_c.

Worked example 1 - Determining the equilibrium constant (K_c)

For the reaction H_2(g) + I_2(g) ⇌ 2HI_(g) at 580 K, the equilibrium concentrations are:

[HI] = 0.60 mol dm⁻³

[H₂] = 0.20 mol dm⁻³

[I₂] = 0.20 mol dm⁻³

Determine the equilibrium constant (K_c).

Step 1: Write the equilibrium constant (K_c) expression

Kc​=[H2​][I2​][HI]2​

Step 2: Substitution and correct evaluation

Kc​=0.20×0.20(0.60)2​=9