A reaction pathway diagram shows the energy of the reacting chemicals as the reaction happens. Where the products end up tells you exothermic or endothermic; the hill in the middle tells you how much energy the particles need to get started.
On a reaction pathway diagram, if the products are lower in energy than the reactants the reaction is exothermic; if the products are higher it is endothermic. Extended: the enthalpy change ΔH is the difference in energy between reactants and products. The activation energy, Ea, is the minimum energy that colliding particles need in order to react; it is the height of the hill from the reactants to the top.
Fig. 1: reaction pathway diagrams for an exothermic and an endothermic reaction
Section 01
Reading the diagram.
Switch between exothermic and endothermic reactions in the simulator. Watch where the products sit compared with the reactants, and how the hill changes when you add a catalyst.
Axes: energy on the vertical axis, progress of the reaction on the horizontal axis.
Products lower than reactants: energy has been released to the surroundings, so the reaction is exothermic.
Products higher than reactants: energy has been taken in, so the reaction is endothermic.
The hill: every reaction, even an exothermic one, has to climb an energy barrier first. That is why a fuel needs a spark or a flame to start burning.
Stage 1 · Learn
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On a reaction pathway diagram, the products are at a lower energy than the reactants. The reaction is:
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What is shown on the vertical axis of a reaction pathway diagram?
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(Extended) Activation energy is:
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In an endothermic reaction pathway diagram, the products are:
Section 02
Labelling ΔH and Ea.
Extended. Exam questions ask you to draw the diagram and label both energy changes with arrows. The arrows must start and end at the right places.
Activation energy, Ea: an upward arrow from the reactants level to the top of the hill.
Enthalpy change, ΔH: an arrow from the reactants level to the products level. It points down for an exothermic reaction (ΔH negative) and up for an endothermic reaction (ΔH positive).
Label the levels with the reactants and products, ideally with their formulae, for example CH4 + 2O2 and CO2 + 2H2O.
Catalyst: provides a pathway with a lower activation energy, so the hill is lower; the reactant and product levels, and so ΔH, do not change.
Examiner trap
The activation energy arrow starts at the reactants, not at the bottom axis and not at the products. And ΔH is measured between the reactant and product levels, not from the top of the hill. Arrows drawn from the axis lose the mark.
Stage 2 · Exam
Exam-style questions
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(Extended) On a reaction pathway diagram, where does the activation energy arrow start and finish?
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(Extended) A reaction has reactants at 200 kJ, a peak at 350 kJ and products at 80 kJ on a pathway diagram. What are Ea and ΔH?
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(Extended) A catalyst is added to a reaction. How does the reaction pathway diagram change?
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Why does methane need a spark to start burning, even though combustion is exothermic?
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