Energy Level (Reaction Pathway) Diagrams

“Complete the energy level diagram” is worth 3 to 4 near-mechanical marks on Papers 3 and 4, and it is marked with a checklist: lines labelled, ΔH arrow pointing the right way, Ea arrow starting from the right place. Examiner reports show the Ea arrow drawn from the products line, or from the baseline, in a large share of scripts, a one-second pencil decision that costs the mark.

The anatomy of the diagram (Core)

A reaction pathway diagram plots energy (y-axis) against progress of reaction (x-axis). Four parts earn marks:

1. A horizontal line for the reactants, labelled with their formulae or “reactants”.
2. A horizontal line for the products, at a different height.
3. A curve rising from reactants over a hump and down to products.
4. Labelled arrows for ΔH and (where asked) activation energy.

For an exothermic reaction the products line sits below the reactants line: energy has been transferred to the surroundings. For an endothermic reaction the products line sits above the reactants. If the relative heights are wrong, every arrow that follows is wrong too: fix the lines first. The link between temperature change and which diagram applies is covered in Exothermic and Endothermic Reactions.

The two arrows

ΔH is drawn vertically between the two lines, from the reactants level to the products level:

• Exothermic: arrow points downwards; ΔH is negative (S).
• Endothermic: arrow points upwards; ΔH is positive (S).

Activation energy (Ea) is the minimum energy that colliding particles must possess in order to react. On the diagram it is the vertical arrow from the reactants line to the peak of the hump. It always points upwards, in both exothermic and endothermic diagrams: every reaction needs an energy input to start, even one that releases energy overall. That is why petrol does not ignite until a spark supplies the activation energy.

Arrow	Starts at	Ends at	Direction
ΔH (exothermic)	Reactants line	Products line	Down
ΔH (endothermic)	Reactants line	Products line	Up
Ea (always)	Reactants line	Top of hump	Up

Catalysts on the diagram

A catalyst increases the rate of reaction by providing an alternative pathway with a lower activation energy. Drawn: a second, lower hump, usually dashed, between the same two lines. Three facts are checked when this appears:

• Ea is lower with the catalyst.
• ΔH is unchanged: reactants and products lines do not move.
• The catalyst is not used up.

The kinetics side of this (why a lower Ea means more successful collisions per second) is Supplement material handled in Collision Theory, and questions regularly bridge the two: draw the diagram here, explain the rate effect there.

Reading ΔH off the axes (Supplement)

Extended diagrams sometimes carry a numerical energy scale. ΔH is the difference between the products and reactants levels, with sign attached: products at 50 kJ and reactants at 200 kJ gives ΔH = −150 kJ/mol. Ea is read from the reactants line to the peak. The underlying arithmetic (bond breaking in, bond making out) is the subject of Bond Energy Calculations, and the diagram is simply that calculation drawn as a picture.

Worked exam question

Methane burns in oxygen: CH4 + 2O2 → CO2 + 2H2O. The reaction is exothermic. (a) Draw a labelled reaction pathway diagram for this reaction. Label the reactants, the products and the enthalpy change ΔH. [3] (b) On your diagram, label the activation energy, Ea. [1] (c) Explain, in terms of the diagram, how a catalyst would change it. [2]

Model answer: (a) Reactants line (CH4 + 2O2) drawn above products line (CO2 + 2H2O) (1); curve over a hump connecting them (1); ΔH arrow drawn downwards from reactants level to products level, labelled (1). (b) Ea arrow from the reactants line up to the peak of the curve, labelled (1). (c) The catalysed pathway has a lower peak, so a lower activation energy (1); the positions of the reactants and products lines, and therefore ΔH, are unchanged (1).

Mark-by-mark: each labelled feature is one tick. In (a) the most common dropped mark is a ΔH arrow drawn pointing upwards or floating without endpoints. It must span the two lines. In (b) an Ea arrow rooted on the products side scores zero. In (c) “the reaction is faster” without mentioning Ea earns nothing; the question asked about the diagram.

The mistakes that cost marks

1. Ea drawn from the products line or from zero. It starts at the reactants line and ends at the peak, no exceptions.
2. ΔH arrow the wrong way for the reaction type. Exothermic: down to lower products. Match the arrow to the word in the question.
3. Moving the products line when adding a catalyst. A catalyst lowers the hump only; ΔH is untouched.
4. Unlabelled diagrams. Two lines and a curve with no labels can score zero even when the shape is perfect. Every mark is attached to a label.

How examiners want it phrased

Student wording	Mark-scheme wording
”The products end up lower"	"The products have less energy than the reactants; ΔH is negative"
"The hump is the energy needed"	"The activation energy is the minimum energy colliding particles need to react"
"A catalyst makes the hill smaller"	"A catalyst provides an alternative pathway with a lower activation energy"
"ΔH is the gap"	"ΔH is the energy difference between reactants and products”

Diagram marks reward drawing discipline more than chemistry insight: lines, labels, arrows with defined start and end points. The wider topic logic is on the Chemical Energetics pillar, and if your diagrams keep losing one mark in four, a free 1-hour trial lesson with a Chemistry specialist will mark a set of them against the real checklist with you.

Test yourself

Try all three with the page scrolled past. Each answer is hidden until you click it.

Q1 (2 marks). The thermal decomposition of calcium carbonate is endothermic. Describe two features of the reaction pathway diagram for this reaction.

Show answer

• the products line is drawn above the reactants line [1] • the ΔH arrow points upwards, from the reactants level to the products level [1] (a curve over a hump linking the two lines is also creditworthy)

Q2 (2 marks). (Extended) On a reaction pathway diagram with a numerical scale, the reactants are at 300 kJ, the peak of the curve is at 550 kJ and the products are at 100 kJ. Calculate the activation energy and the enthalpy change.

Show answer

• Ea = 550 − 300 = 250 kJ (reactants line to peak) [1] • ΔH = 100 − 300 = −200 kJ/mol (products minus reactants, sign included) [1]

Q3 (2 marks). Petrol burns in an exothermic reaction, yet petrol vapour and air do not react until a spark is supplied. Explain why.

Show answer

• colliding particles must have at least the activation energy, the minimum energy needed to react, [1] • the spark supplies the activation energy; at room temperature too few collisions have enough energy to react [1]