Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 6.3 Enthalpy level diagrams-Study Notes - New Syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 2 – 6.3 Enthalpy level diagrams- Study Notes- New syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 2 – 6.3 Enthalpy level diagrams- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.

Key Concepts:

6.3 be able to construct and interpret enthalpy level diagrams, showing exothermic and endothermic enthalpy changes

Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics

6.3 Enthalpy Level Diagrams

Enthalpy level diagrams (energy profile diagrams) are used to represent the energy changes that occur during a chemical reaction. They show the relative energies of reactants and products and the value of \( \Delta H \).

Key Features of Enthalpy Level Diagrams

  • Y-axis: Enthalpy (kJ mol\(^{-1}\))
  • X-axis: Reaction progress
  • Horizontal lines represent energy levels of reactants and products
  • Vertical arrow shows \( \Delta H \)

Exothermic Reaction Diagram

  • Reactants have higher energy than products.
  • Energy is released to surroundings.
  • \( \Delta H < 0 \) (negative).

Interpretation

  • Downward arrow shows decrease in enthalpy.
  • The difference represents energy released.

Endothermic Reaction Diagram

  • Products have higher energy than reactants.
  • Energy is absorbed from surroundings.
  • \( \Delta H > 0 \) (positive).

Interpretation

  • Upward arrow shows increase in enthalpy.
  • The difference represents energy absorbed.

How to Construct an Enthalpy Diagram

  • Step 1: Draw axes (enthalpy vs reaction progress).
  • Step 2: Draw horizontal line for reactants.
  • Step 3: Draw products at higher/lower level depending on reaction type.
  • Step 4: Add arrow showing \( \Delta H \).
  • Step 5: Label \( \Delta H \) with correct sign.

Key Points for Interpretation

  • Position of products relative to reactants determines sign of \( \Delta H \).
  • Larger vertical difference → larger energy change.
  • Direction of arrow indicates heat flow.

Summary

  • Exothermic: products lower, \( \Delta H < 0 \).
  • Endothermic: products higher, \( \Delta H > 0 \).
  • Diagrams visually represent energy changes.

Therefore, enthalpy level diagrams help in both understanding and predicting energy changes in reactions.

Example 1 :

An enthalpy diagram shows reactants at +50 kJ mol\(^{-1}\) and products at −150 kJ mol\(^{-1}\).

(a) Calculate \( \Delta H \).

(b) State whether the reaction is exothermic or endothermic and explain.

▶️ Answer/Explanation

(a) \( \Delta H = -150 – 50 = -200 \,\mathrm{kJ\,mol^{-1}} \)

(b) Negative \( \Delta H \) → exothermic.

Products are lower in energy than reactants.

Example 2:

Sketch an enthalpy level diagram for the reaction:

\( \mathrm{CaCO_3 \rightarrow CaO + CO_2} \quad \Delta H = +178 \,\mathrm{kJ\,mol^{-1}} \)

Explain the features of your diagram.

▶️ Answer/Explanation

Products drawn higher than reactants.

Upward arrow labelled \( \Delta H = +178 \,\mathrm{kJ\,mol^{-1}} \).

Indicates energy absorbed (endothermic).

Vertical difference represents energy change.

Scroll to Top