1.8.4 Calculations Involving the Equilibrium Constant

Calculating the equilibrium constant $K_c$ involves determining the concentrations of reactants and products at equilibrium and using them in the expression for $K_c$ . These calculations are essential for predicting the position of equilibrium and understanding the extent of a reaction under specific conditions.

Units of $K_c$

The units of $K_c$ depend on the reaction's stoichiometry.

In this example:

K_c = \frac{(\text{mol dm}^{-3}) \times (\text{mol dm}^{-3})}{\text{mol dm}^{-3}} = \text{mol dm}^{-3}

Thus:

K_c = :highlight[0.09 \, \text{mol dm}^{-3}]

Worked Example Calculating $K_c$

infoNote

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

PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)

The equilibrium expression for $K_c$ is:

K_c = \frac{[PCl_3][Cl_2]}{[PCl_5]}

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 $Cl_2$

2.00 - 1.20 = :highlight[0.80 \text{ moles of } PCl_5]

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 $Cl_2$ :

[PCl_5] = \frac{0.80 \text{ mol}}{20 \text{ dm}^3} = :highlight[0.04 \text{ mol dm}^{-3}]

[PCl_3] = \frac{1.20 \text{ mol}}{20 \text{ dm}^3} = :highlight[0.06 \text{ mol dm}^{-3}]

[Cl_2] = \frac{1.20 \text{ mol}}{20 \text{ dm}^3} = :highlight[0.06 \text{ mol dm}^{-3}]

Step 4: Substitute into the K_c Expression

Substitute the concentrations into the expression for $K_c$ :

K_c = \frac{[PCl_3][Cl_2]}{[PCl_5]}

= \frac{0.06 \times 0.06}{0.04} = :highlight[0.09 \text{ mol dm}^{-3}]

Calculations Involving the Equilibrium Constant Simplified Revision Notes for A-Level AQA Chemistry

1.8.4 Calculations Involving the Equilibrium Constant

Units of $K_c$

Worked Example Calculating $K_c$

Example: How to Calculate $K_c$

500K+ Students Use These Powerful Tools to Master Calculations Involving the Equilibrium Constant For their A-Level Exams.

Other Revision Notes related to Calculations Involving the Equilibrium Constant you should explore

Calculations Involving the Equilibrium Constant Simplified Revision Notes for A-Level AQA Chemistry

1.8.4 Calculations Involving the Equilibrium Constant

Units of KcK_cKc​

Worked Example Calculating KcK_cKc​

Example: How to Calculate KcK_cKc​

500K+ Students Use These Powerful Tools to Master Calculations Involving the Equilibrium Constant For their A-Level Exams.

Other Revision Notes related to Calculations Involving the Equilibrium Constant you should explore

Units of $K_c$

Worked Example Calculating $K_c$

Example: How to Calculate $K_c$