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Reaction Pathway Diagrams

Chemistry: Cambridge International GCSE Supplement, topic 5.1.5

Reaction Pathway Diagrams

A reaction pathway diagram is a graph that shows energy changes during a chemical reaction, helping to identify whether energy is released or energy is absorbed.

What are reaction pathway diagrams?

What are reaction pathway diagrams?

  • A reaction pathway diagram is a graph that shows how energy changes during a chemical reaction.
  • The diagram has:
  • Y-axis: Energy level
  • X-axis: Progress of reaction
  • They compare the energy of reactants and products.
  • It helps to identify whether a reaction is exothermic or endothermic.
Exothermic reactions

Exothermic reactions

  • Exo- means out, so an exothermic reaction releases energy into the surroundings (usually as heat/light).
  • This means products have less energy than reactants.
  • The diagram shows:
  • A decrease in energy from the start to the end of the reaction
  • A downward arrow labelled “Energy released”
  • Examples:
  • Burning fuels (combustion), neutralisation, respiration
Endothermic reactions

Endothermic reactions

  • Endo- means in, so an endothermic reaction absorbs energy from the surroundings.
  • This means products have more energy than the reactants.
  • The diagram shows:
  • An increase in energy from the start to the end of the reaction
  • An upward arrow labelled “Energy absorbed”
  • Examples:
  • Melting ice, photosynthesis
Key parts of the diagram

Key parts of the diagram

  • Reactants: The starting substances in a reaction.
  • Their energy is shown at the beginning of the curve.
  • Products: The substances formed at the end of the reaction.
  • Their energy is shown at the end of the curve.
  • Activation Energy: The 'hump' or 'peak' of the curve.
  • Enthalpy Change (ΔH): The overall energy change in the reaction
  • Reaction pathway diagrams help us visualise energy changes.

Reaction Pathway Diagrams 2

Reaction pathway diagrams show activation energy, enthalpy change, and the balance between bond breaking and bond making to determine whether a reaction is exothermic or endothermic.

Activation energy (*Ea*)

Activation energy (Ea)

  • Activation energy (Ea) is the minimum energy that reacting particles must have for a reaction to occur.
  • This is shown as the 'hump' or 'peak' on the reaction pathway diagram.
  • This energy is required to break the initial bonds in the reactants before new bonds can form.
  • A high activation energy means a reaction is slower, as more energy is needed to start it.
Enthalpy change (Δ*H*)

Enthalpy change (ΔH)

  • ΔH is the difference in energy between the products and the reactants.
  • In an exothermic reaction:
  • The products have a lower energy level than the reactants.
  • The diagram shows a decrease in energy.
  • The enthalpy change (ΔH) is negative.
  • In an endothermic reaction:
  • The products have a higher energy level than the reactants.
  • The diagram shows an increase in energy.
  • The enthalpy change (ΔH) is positive.
Bond breaking & making

Bond breaking & making

  • Breaking bonds in the reactants requires energy → endothermic process.
  • Making bonds in the products releases energy → exothermic process.
Bond breaking & making 2

Bond breaking & making 2

  • The overall enthalpy change (ΔH) is calculated using bond energies:
  • ΔH = (Energy absorbed to break bonds) - (Energy released to make bonds)
  • If more energy is released than absorbed → exothermic (ΔH is negative).
  • If more energy is absorbed than released → endothermic (ΔH is positive).

Test yourself on Reaction Pathway Diagrams

  • What does a reaction pathway diagram show?

  • What does activation energy (Ea) represent in a reaction pathway diagram?

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