Entropy | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial provides a comprehensive overview of entropy in the context of A-level Chemistry. It outlines key concepts related to entropy, how to calculate entropy changes, and how to predict these changes in various reactions. Understanding entropy is crucial for grasping why certain chemical reactions occur spontaneously.

Step 1: Understanding Spontaneous Reactions

• Reactions that occur without external assistance are termed spontaneous.
• Example: Magnesium burns in oxygen without additional energy input.
• Not all spontaneous reactions result in a decrease of total energy; endothermic reactions can also be spontaneous.

Step 2: Defining Entropy

• Entropy is a measure of disorder or the number of ways energy and particles can be arranged.
• An increase in entropy signifies a greater number of possible configurations.
• The second law of thermodynamics states that total entropy in a closed system always increases.

Step 3: Calculating Entropy Changes

Entropy Change of the System

• Every substance has an associated entropy value, measured in J K^-1 mol^-1.
• To determine the entropy change (ΔS) for a reaction:
  • Look up standard entropy values for reactants and products.
  • Use the formula:

    ΔS = Σ(S_products) - Σ(S_reactants)
    

Entropy Change of the Surroundings

• The surroundings' entropy changes only via heat transfer.
• The heat absorbed by the surroundings equals the negative of the enthalpy change (ΔH) of the system.
• The formula for calculating the entropy change of the surroundings is:

  ΔS_surroundings = -ΔH / T
  

  where T is the temperature in Kelvin.

Step 4: Total Entropy Change

• The total entropy change (ΔStotal) of the universe is the sum of the system and surroundings.
• Only reactions with a positive ΔStotal can occur:

  ΔStotal = ΔS_system + ΔS_surroundings
  

Step 5: Predicting Entropy Changes

• Entropy changes can often be predicted based on qualitative observations:
  • Change of State: Transition from solid to liquid or gas usually increases entropy.
  • Change in Number of Moles: An increase in the number of moles typically leads to higher entropy.
  • Dissolving Ionic Solids:
    • Breaking down the lattice and the hydration of ions usually increases the system's entropy.

Conclusion

Understanding entropy is essential for predicting the behavior of chemical reactions. Key takeaways include recognizing spontaneous reactions, calculating entropy changes for systems and surroundings, and predicting entropy changes based on qualitative analysis. As a next step, practice calculating entropy changes using different chemical reactions to reinforce these concepts.