Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 13.9 Extent of reaction from K-Study Notes - New Syllabus

13.9 Using Equilibrium Constants to Predict Extent of Reaction

The value of the equilibrium constant (\( \mathrm{K_c} \) or \( \mathrm{K_p} \)) indicates how far a reaction proceeds before reaching equilibrium. It provides quantitative information about whether reactants or products are favoured.

Key Idea

The magnitude of \( \mathrm{K} \) determines the extent to which a reaction proceeds.

Interpreting the Value of \( \mathrm{K} \)

\( \mathrm{K \gg 1} \) (very large):

• • Products are strongly favoured.
  • Reaction proceeds almost to completion.
  • Equilibrium lies far to the right.

\( \mathrm{K \approx 1} \):

• • Significant amounts of both reactants and products present.
  • Equilibrium lies in the middle.

\( \mathrm{K \ll 1} \) (very small):

• • Reactants are favoured.
  • Reaction proceeds very little.
  • Equilibrium lies far to the left.

Link to Equilibrium Expression

• \( \mathrm{K = \frac{[products]}{[reactants]}} \) (simplified view).
• Large \( \mathrm{K} \) → numerator (products) is large.
• Small \( \mathrm{K} \) → denominator (reactants) dominates.

Predicting Extent of Reaction

• Use the value of \( \mathrm{K} \) to predict composition at equilibrium.
• No need for full calculation in qualitative questions.
• Larger \( \mathrm{K} \) → greater yield of products.

Important Notes

• The value of \( \mathrm{K} \) does not indicate reaction rate.
• A reaction with large \( \mathrm{K} \) may still be slow (kinetic limitation).
• Temperature must be specified, as \( \mathrm{K} \) changes with temperature.

Key Features

• \( \mathrm{K} \) indicates extent of reaction.
• Large \( \mathrm{K} \) → products favoured.
• Small \( \mathrm{K} \) → reactants favoured.
• Independent of reaction rate.