Equilibrium Constant, Kc | A level Chemistry

Introduction

This tutorial provides a clear and concise explanation of the equilibrium constant (Kc) in physical chemistry, particularly for A Level Chemistry students. Understanding Kc is crucial for analyzing chemical reactions at equilibrium and predicting the effects of changes in concentration and temperature.

Step 1: Understand the Concept of Equilibrium

• Chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction.
• At equilibrium, the concentrations of reactants and products remain constant over time.
• The equilibrium constant (Kc) quantifies the ratio of products to reactants at equilibrium.

Step 2: Learn the Kc Expression

• The Kc expression is derived from the balanced chemical equation:

  • For a reaction: aA + bB ⇌ cC + dD

  • The Kc expression is written as:

    Kc = [C]^c [D]^d / [A]^a [B]^b
    

  • Square brackets indicate the concentrations of the substances at equilibrium.

  • Ensure that the coefficients from the balanced equation become exponents in the Kc expression.

Step 3: Calculate Kc

• Gather the equilibrium concentrations of the reactants and products.
• Substitute these values into the Kc expression.
• Perform the calculation to find the value of Kc.

Example Calculation

For the reaction:

2H2 + O2 ⇌ 2H2O

If at equilibrium, the concentrations are:

• [H2] = 0.5 M
• [O2] = 0.25 M
• [H2O] = 1.0 M

You would calculate Kc as follows:

Kc = [H2O]^2 / ([H2]^2 [O2])
Kc = (1.0)^2 / ((0.5)^2 * (0.25))
Kc = 1 / (0.25 * 0.25)
Kc = 16

Step 4: Analyze the Significance of Kc Values

• A Kc value much greater than 1 indicates a reaction that favors products at equilibrium.
• A Kc value much less than 1 indicates a reaction that favors reactants.
• A Kc value close to 1 suggests significant amounts of both reactants and products are present.

Step 5: Consider Factors Affecting Kc

• Kc is affected by temperature but remains constant at a constant temperature.
• Changes in concentration or pressure do not affect Kc; they only shift the position of equilibrium.

Conclusion

Understanding the equilibrium constant Kc is essential for predicting the behavior of chemical equilibria. By mastering the Kc expression, performing calculations, and interpreting Kc values, you can effectively analyze chemical reactions. As a next step, practice calculating Kc values from various chemical equations and explore how changes in conditions can affect equilibria.