Reaction Pathway Diagram

Summary: A reaction pathway diagram (also called an energy profile diagram or enthalpy level diagram) is a graphical representation that tracks the enthalpy change of a chemical system as reactants transform into products, showing the relative energies of reactants and products, the overall enthalpy change (ΔH), and the activation energy barrier (Ea) that must be overcome for the reaction to proceed. Tags: reaction-pathway-diagram energy-profile-diagram enthalpy-level-diagram enthalpy-change activation-energy exothermic endothermic catalyst igcse-chemistry Created: 2026-07-18

A reaction pathway diagram is a graph that plots the enthalpy (energy content) of a chemical system on the vertical axis against the progress of reaction or reaction coordinate on the horizontal axis, providing a visual account of the energy changes that accompany a chemical transformation from start to finish. The diagram captures three essential features: the relative enthalpy levels of the reactants and products, the overall enthalpy change ΔH (the vertical difference between reactants and products), and the activation energy Ea (the height of the energy barrier, or “hump,” that must be surmounted for bonds in the reactants to begin breaking so that new bonds can form). In an exothermic reaction, the products sit at a lower enthalpy than the reactants and ΔH is represented by a downward arrow, indicating energy has been transferred from the chemical system to the surroundings. In an endothermic reaction, the products sit at a higher enthalpy than the reactants and ΔH is shown by an upward arrow, indicating energy has been absorbed by the chemical system from the surroundings. The activation energy Ea is always measured from the reactant level to the peak of the hump, and the peak itself corresponds to the transition state or activated complex where existing bonds are partially broken and new bonds are partially formed. At IGCSE level, candidates must be able to sketch and label these diagrams for both exothermic and endothermic reactions, and must also understand that adding a catalyst provides an alternative reaction pathway with a lower activation energy — a smaller hump — without altering the enthalpy levels of the reactants or products, meaning ΔH remains unchanged.

Axes and Key Features

The reaction pathway diagram consists of two labelled axes:

The curve itself represents the changing enthalpy of the system as reactants approach each other, collide, pass through the transition state, and finally separate as products.

Exothermic Reaction Pathway Diagram

In an exothermic reaction, energy is released from the system to the surroundings (the surroundings heat up). On the diagram:

• The reactants are drawn at a higher enthalpy level than the products.
• The products are drawn at a lower enthalpy level than the reactants.
• ΔH (enthalpy change) is the vertical distance between reactants and products, drawn as an arrow pointing downward from reactants to products. It carries a negative sign (ΔH < 0).
• The activation energy Ea is measured upward from the reactant level to the peak of the hump.

Energy ↑
        \
         \  Ea
Reactants —\
            \
             —————— Products
              ← ΔH ↓

            Progress of reaction →

The downward arrow for ΔH shows energy has left the system: the products are more energetically stable than the reactants because stronger bonds have formed in the products and/or energy has been released.

For a detailed treatment of exothermic processes, see Exothermic Reaction.

Endothermic Reaction Pathway Diagram

In an endothermic reaction, energy is absorbed by the system from the surroundings (the surroundings cool down). On the diagram:

• The reactants are drawn at a lower enthalpy level than the products.
• The products are drawn at a higher enthalpy level than the reactants.
• ΔH (enthalpy change) is drawn as an arrow pointing upward from reactants to products. It carries a positive sign (ΔH > 0).
• The activation energy Ea is still measured upward from the reactant level to the peak of the hump.

Energy ↑
              —————— Products
             /  ↑ ΔH
            /
           /  Ea
Reactants —/

            Progress of reaction →

Note a critical point: even endothermic reactions have an activation energy barrier. The fact that a reaction absorbs energy overall does not mean it occurs spontaneously without needing an initial energy input. For example, the thermal decomposition of calcium carbonate (CaCO₃ → CaO + CO₂, ΔH = +178 kJ mol⁻¹) requires continuous heating not only to supply the activation energy but also to sustain the endothermic enthalpy change.

For a detailed treatment, see Endothermic Reaction.

Labelling Requirements for IGCSE

Cambridge IGCSE Chemistry 0620 expects candidates to label the following four features clearly on any drawn reaction pathway diagram:

1. Reactants — written at the start of the curve on the left, levelled with the reactant enthalpy.
2. Products — written at the end of the curve on the right, levelled with the product enthalpy.
3. ΔH — a vertical double-headed arrow between the reactant and product levels, labelled “ΔH”. The direction indicates sign: downward for exothermic (negative), upward for endothermic (positive).
4. Ea (or Activation energy) — a vertical arrow from the reactant level up to the peak of the curve, labelled “Ea” or “activation energy”.

Some mark schemes also accept a label for the transition state at the peak of the curve, though this is not mandatory at IGCSE.

Activation Energy and the Energy Barrier

The activation energy (Ea) is the minimum energy that colliding reactant particles must possess for a successful reaction to occur — that is, for bonds in the reactants to break and new bonds to form in the products. On a reaction pathway diagram, Ea is the vertical distance from the enthalpy level of the reactants to the top of the energy hump.

The hump exists because bond breaking is endothermic: energy must be supplied to weaken and eventually break existing bonds before the energy-releasing step of new bond formation can take hold. Even in a strongly exothermic reaction (e.g., combustion of hydrogen: 2H₂ + O₂ → 2H₂O, ΔH = −572 kJ mol⁻¹), a spark or flame is needed to initiate the reaction — this spark supplies the activation energy.

After the transition state is reached, bonds reform and the system’s enthalpy drops (for exothermic reactions) or continues to rise (for endothermic reactions) to the product level.

For further detail, see Activation Energy.

Effect of a Catalyst

A catalyst is a substance that increases the rate of a chemical reaction without being chemically consumed or permanently changed at the end of the reaction. On a reaction pathway diagram, a catalyst’s effect is visualised as a lower, alternative pathway that reduces the activation energy:

• The catalysed pathway has a smaller hump — meaning a lower Ea.
• The enthalpy levels of the reactants and products are unchanged.
• Consequently, ΔH is unchanged by the addition of a catalyst.

Energy ↑
        \
         \  Ea (uncatalysed)
          \      _
           \   _/  \  ← Ea (catalysed) is smaller
Reactants —\ /      —————— Products
                ← ΔH (unchanged)

            Progress of reaction →

A catalyst achieves this by providing a different mechanistic route — often by orienting reactant molecules on a surface (heterogeneous catalysis) or by forming a temporary intermediate that breaks down in a later step (homogeneous catalysis). Regardless of mechanism, the catalyst emerges chemically unchanged at the end and can be reused.

This distinction is a staple of IGCSE examination: a catalyst lowers Ea but does not affect ΔH.

For a full discussion, see Catalyst.

Interpreting Reaction Pathway Diagrams from Data

IGCSE exam questions frequently present a table of enthalpy data and ask the candidate to draw or label the corresponding reaction pathway diagram. Key points for interpretation:

• If ΔH is negative (e.g., ΔH = −184 kJ mol⁻¹), the diagram must be exothermic: the product level is below the reactant level.
• If ΔH is positive (e.g., ΔH = +90 kJ mol⁻¹), the diagram must be endothermic: the product level is above the reactant level.
• The magnitude of Ea is independent of ΔH. A reaction can have a large negative ΔH yet a very high Ea (e.g., the combustion of diamond is thermodynamically favourable but kinetically hindered by a large activation barrier).
• When comparing two reactions, a larger Ea means a higher hump and, all else being equal, a slower reaction at a given temperature.

Relationship to Bond Enthalpies

Reaction pathway diagrams are intimately connected to bond enthalpies. During a chemical reaction:

1. Bonds in the reactants are broken — this is endothermic (energy is absorbed; ΔH = +ve for bond breaking).
2. New bonds are formed in the products — this is exothermic (energy is released; ΔH = −ve for bond making).

The overall ΔH of a reaction is the balance between these two processes:

ΔH = Σ (bond enthalpies of bonds broken) − Σ (bond enthalpies of bonds formed)

The activation energy hump reflects the initial investment of energy required for step 1 (bond breaking) to begin in earnest, before the energy released from step 2 (bond making) can be realised. For a more detailed treatment, see Enthalpy Change and Bond Enthalpy.

Comparison: Exothermic vs. Endothermic Diagrams

Related Pages

• Exothermic Reaction — reactions that release energy to the surroundings
• Endothermic Reaction — reactions that absorb energy from the surroundings
• Enthalpy Change — definition, calculation, and sign conventions for ΔH
• Activation Energy — the energy barrier and its relationship to reaction rate
• Catalyst — how catalysts lower Ea and provide alternative pathways
• Bond Enthalpy — using bond enthalpies to calculate ΔH
• Collision Theory — activation energy in the context of successful particle collisions
• Reaction Rate — factors affecting rate, including temperature and catalysts
• Haber Process — industrial example using an iron catalyst to lower Ea
• Energy Changes in Reactions — overview of thermochemistry for IGCSE

Sources

• Cambridge IGCSE Chemistry 0620 Syllabus, Topic 5.1: Exothermic and Endothermic Reactions
• Gallagher, R. & Ingram, P., Complete Chemistry for Cambridge IGCSE, 3rd Edition, Oxford University Press, 2016
• Harwood, R. & Lodge, I., Cambridge IGCSE Chemistry Coursebook, 5th Edition, Cambridge University Press, 2021
• Clegg, A. et al., Cambridge IGCSE Chemistry Study and Revision Guide, Hodder Education, 2017
• Cambridge IGCSE Chemistry 0620 Learner Guide and past paper mark schemes (2021–2025)

Common Misconceptions