Calculations Involving the Equilibrium Constant (AQA A-Level Chemistry): Revision Notes

Example: How to Calculate $K_c$

Step 1: Write the Balanced Equation

Start with the balanced equation for the reversible reaction. This helps determine the correct expression for $K_c$

Example: For the reaction

The equilibrium expression for $K_c$ is:

Step 2: Calculate Moles at Equilibrium

Use the information provided to calculate the moles of each species at equilibrium.

In the example, if 2.00 moles of $PCl_5$ are placed in a 20 $dm^3$ container and at equilibrium, 1.20 moles of $Cl_2$ are present:

Moles of $PCl_5$ at equilibrium = Initial moles - Moles of $Cl_2$

Moles of $PCl_3$ at equilibrium = 1.20 (since 1 mole of $PCl_3$ is produced for every mole of $Cl_2$).

Step 3: Calculate Concentrations

Convert the moles into concentrations by dividing the number of moles by the volume of the container in $dm^3$

For $PCl_5$, $PCl_3$, and $Cl_2$:

Step 4: Substitute into the K_c Expression

Substitute the concentrations into the expression for $K_c$: