Equilibrium Constant: Kc | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial focuses on understanding the equilibrium constant (Kc) in chemical reactions, specifically within the context of A-level Chemistry. We will cover the concept of dynamic equilibrium, the decomposition of nitrogen tetroxide, the difference between homogeneous and heterogeneous reactions, and how to calculate the equilibrium constant.

Step 1: Understand Dynamic Equilibrium

Dynamic equilibrium occurs in reactions where the forward and reverse reactions happen at the same rate, resulting in constant concentrations of reactants and products.

• Key Points:
  • Not all reactions go to completion; some establish a balance between reactants and products.
  • Visualize dynamic equilibrium with a macroscopic example, like balls moving in both directions but with no overall change in quantity.
  • Remember that even at equilibrium, molecular activity continues.

Step 2: Explore the Decomposition of Nitrogen Tetroxide

Dinitrogen tetroxide (N2O4) decomposes into nitrogen dioxide (NO2), illustrating how equilibrium is established.

• Process Overview:
  • Start with a high concentration of N2O4 and no NO2.
  • As the reaction progresses, N2O4 concentration decreases, while NO2 concentration increases, resulting in a color change from colorless to brown.
  • At equilibrium, concentrations stabilize, and the mixture reaches its darkest brown color.
• Tip: Changes in conditions (like temperature or pressure) will shift the equilibrium position, as explained by Le Chatelier’s principle.

Step 3: Differentiate Between Homogeneous and Heterogeneous Reactions

Understanding the states of reactants and products is crucial for identifying reaction types.

• Homogeneous Reactions:

  • All reactants and products are in the same state (e.g., gases).
  • Example: Decomposition of N2O4.

• Heterogeneous Reactions:

  • Involve more than one state of matter (e.g., solids and gases).
  • Example: A reaction between a solid and a gas.

Step 4: Learn About the Equilibrium Constant Kc

The equilibrium constant (Kc) quantifies the ratio of concentrations of products to reactants at equilibrium.

• Kc Expression:

  • For the general reaction ( aA + bB \rightleftharpoons cC + dD ), Kc is expressed as: [ K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b} ]
  • Only include gases and aqueous species; do not include solids or pure liquids.

• Units of Kc:

  • Calculate units by substituting concentration units (mol/dm³) into the Kc expression.

Step 5: Calculate the Equilibrium Constant

Learn how to calculate Kc using a practical example.

• Example Reaction:

  • Reaction of SO2(g) and O2(g) in a sealed flask of volume 3 dm³.
  • Suppose at equilibrium, concentrations are given or can be determined.

• Steps for Calculation:

  1. Calculate concentrations of reactants and products.
  2. Substitute these values into the Kc expression.
  3. Determine the units based on the concentrations used.

  Example Calculation:

  • If at equilibrium you have:
    • [SO2] = x mol/dm³
    • [O2] = y mol/dm³
  • Then, calculate Kc as: [ K_c = \frac{[SO2]^x[O2]^y}{...} ]

Conclusion

In this tutorial, we have covered the concepts of dynamic equilibrium, the behavior of nitrogen tetroxide, and the distinction between homogeneous and heterogeneous reactions. Additionally, we learned how to express and calculate the equilibrium constant, Kc. Understanding these concepts is essential for mastering equilibrium in chemistry. For further study, practice calculating Kc with different reactions and explore how changes in conditions affect equilibrium.