The Equilibrium Constant (DP IB Chemistry: SL): Revision Note

The equilibrium constant

• The size of the equilibrium constant, K, tells us how the equilibrium mixture is made up with respect to reactants and products

$\mathit{K}\mathbf{=}\frac{{\left[\mathrm{products}\right]}_{\mathbf{eqm}}}{{\left[\mathrm{reactants}\right]}_{\mathbf{eqm}}}$

Interpreting the magnitude of K

• If K << 1

  • The reaction hardly proceeds

  • The equilibrium lies far to the left

  • The mixture contains mostly reactants

• If K<1:

  • The reaction favours the reactants

  • The equilibrium lies to the left-hand side

• If K=1:

  • There are significant amounts of both reactants and products

  • The equilibrium is balanced between the two sides

• If K>1:

  • The reaction favours the products

  • The equilibrium lies to the right-hand side

• If K >> 1:

  • The reaction goes almost to completion

  • The equilibrium lies far to the right with mostly products present

• K is a constant at a specified temperature

• Since temperature can affect the position of equilibrium, it follows that K is dependent on temperature

The relationship between K values for reactions that are the reverse of each other

• The equilibrium constant expression is dependent on a specific reaction

• For example, take the reaction between nitrogen and hydrogen to make ammonia:

N₂ (g) + 3H₂ (g)  ⇌  2NH₃ (g)

• The equilibrium constant expression for this reaction is:

$K=\frac{{\left[{\mathrm{NH}}_3\right]}^2}{\left[{\mathrm{N}}_2\right]{\left[{\mathrm{H}}_2\right]}^3}$

• If we reverse the equation:

2NH₃ (g₎ ⇌ N₂ (g) + 3H₂ (g)

• The equilibrium constant expression for the reverse of this reaction, K', is:

$K\text{'}=\frac{\left[{\mathrm{N}}_2\right]{\left[{\mathrm{H}}_2\right]}^3}{{\left[{\mathrm{NH}}_3\right]}^2}$

• What is the relationship between these two K values? At the same temperature, K'  becomes the reciprocal of the original K value:

$K\text{'}=\frac{1}{K}$   or    $K\text{'}=K^{-1}$