Reaction Profiles

SPEC 4.5.1.2 • PAPER 1 • ⭐ PRIORITY

Drawing them precisely. Reading them confidently. The four examiner alerts that catch most students out.

What Reaction Profiles Show

A reaction profile is an energy diagram that captures three things in one picture: where reactants and products sit on the energy scale, the energy barrier between them, and the overall change.

1

Reactant & product energies

2

Activation energy (Eₐ)

3

Overall energy change

"Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. The minimum amount of energy that particles must have to react is called the activation energy. Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction... [drawn] with a curved line to show the energy as the reaction proceeds." — AQA Specification 4.5.1.2

Get The Axes Right

Axis labels are marked separately. Get them wrong and you lose marks before you've even drawn the curve.

Y-axis

Energy

Sometimes labelled "Energy / kJ" but plain "Energy" is accepted at GCSE.

X-axis

Progress of reaction

"Reaction pathway" is also accepted. NOT "time".

Examiner Alert

Common axis errors that score 0:

❌ Y-axis labelled "activation energy" — wrong. The Y-axis is energy in general; activation energy is just the gap between two points on it.

❌ Y-axis labelled "temperature" — wrong. Reaction profiles are about energy, not temperature.

❌ X-axis labelled "time" — wrong. The x-axis represents how far along the reaction is, not the clock time.

Source: P1 Jun 2022 Q02.5 mark scheme — credit only "energy" (Y) and "(overall) energy change" terminology.

Activation Energy — The Barrier

Definition · Mark scheme

Activation energy (Eₐ) is the minimum amount of energy that reacting particles must have to react.

On a reaction profile, it is the gap between the reactant energy level and the peak (top) of the curve — never the height of the peak from the x-axis.

Why a barrier exists

For a reaction to happen, particles must collide with enough energy to break existing bonds in the reactants. Without that minimum energy, collisions just bounce off — no reaction. The activation energy is the cost of admission to the reaction.

Connecting forward to Module 6

Activation energy reappears in Lesson 6.03 (Collision Theory) and 6.04 (Catalysts) — where catalysts work by lowering Eₐ, providing an alternative pathway with a smaller barrier. Same concept, used to explain rates.

Exothermic Profile

Reactants HIGH, products LOW. The system loses energy to the surroundings, so products sit lower on the energy scale.

Reading the profile

Reactants climb the activation energy barrier to reach the peak. The curve then falls to the product level — and crucially, the products end up lower than the reactants started. The overall energy change arrow points downward: net energy has been transferred to the surroundings.

Endothermic Profile

Reactants LOW, products HIGH. The system absorbs energy from the surroundings, so products sit higher than reactants.

Reading the profile

Reactants still climb an activation energy barrier to reach the peak. After the peak, the curve falls — but only partway, ending above where the reactants started. The overall energy change arrow points upward: net energy has been absorbed from the surroundings. Note: the size of Eₐ depends on the specific reaction, not on whether it's endothermic or exothermic.

Side-by-Side: Exo vs Endo

FeatureExothermicEndothermic

Reactants levelHIGH (left side)LOW (left side)

Products levelLOW (below reactants)HIGH (above reactants)

Energy change arrowPoints DOWN (reactants → products)Points UP (reactants → products)

SurroundingsWarm up (energy released to them)Cool down (energy absorbed from them)

Activation energyAlways present (gap to peak)Always present (gap to peak)

The single test

When asked "How does the profile show this is exothermic?", the mark-scheme answer is simple: "The (level of) products is below the (level of) reactants" — or equivalently "energy decreases overall" / "energy is transferred to the surroundings". (Source: P1 Jun 2022 Q02.6 MS.)

Alert 1 — Eₐ Is The GAP

Examiner Alert · Most-missed mark

Students who write the words "activation energy" at the top of the curve score 0 for that mark. Activation energy is not a place — it is a difference between two energy levels.

Source: P1 Nov 2021 Q08.3 + P1 Jun 2023 Q04.3 mark schemes — both require "activation energy labelled from reactant energy to top of curve".

✗ Loses the mark

Writing "Eₐ" or "activation energy" as a label on the peak with no arrow

Drawing an arrow from the x-axis to the peak (this is too tall)

Drawing only the upward part of the arrow without anchoring at reactant level

✓ Wins the mark

A clear vertical arrow spanning from the reactant energy level to the top of the curve

Labelled "Eₐ" or "activation energy" next to the arrow

Anchored on a dashed reference line at reactant level

Alert 2 — Use Dashed Reference Lines

Examiner Alert

Examiner reports describe arrows that "didn't clearly start at the reactant energy level" or "didn't reach the peak" — losing marks because the reference points were ambiguous. The fix is simple: draw dashed horizontal lines at each energy level you're going to anchor an arrow to.

Source: P1 Nov 2021 Q08.3 examiner report.

The technique

Draw a dashed horizontal line at the reactant energy level — extending across most of the chart.

Draw a dashed horizontal line at the product energy level — also across most of the chart.

Draw your Eₐ arrow as a vertical line: bottom anchored on the reactant dashed line, top reaching the peak. Add arrowheads at both ends.

Draw your energy change arrow as a vertical line: from reactant dashed line to product dashed line. Add arrowheads at both ends.

Use a ruler

Examiners specifically credit cleanly-drawn diagrams. Freehand arrows that wobble between two close energy levels look ambiguous to a marker. A ruler turns "vaguely near" into "clearly at".

Alert 3 — Use A CURVED Line

Spec-Mandated Drawing Requirement

The AQA specification 4.5.1.2 explicitly requires students to draw profiles "with a curved line to show the energy as the reaction proceeds". A profile drawn as straight line segments — reactant level, then a sharp triangular peak, then product level — does not match the spec wording for the shape mark.

Source: AQA GCSE Chemistry 8462 specification, section 4.5.1.2 (verbatim).

✗ Loses the shape mark

Triangular peak — straight up, sharp point, straight down

Step-function "platforms" with vertical jumps between levels

Visibly wobbly freehand curve

✓ Wins the shape mark

Smooth, continuously curved line from reactant level to peak to product level

Rounded peak (transition state region)

Reactant and product levels can be drawn flat with a ruler; the connecting curve through the peak must be curved

Watch out for products labelling

A note on labels: AQA mark schemes do not award a separate mark for labelling "Products" on the curve (Nov 2021 Q08.3 MS explicitly says "ignore labelling of products"). The shape mark requires the product line to be at the right energy level — not labelled with a word. So you don't need to write "Products" — but labelling everything is still good practice for clarity.

Common Errors — Reaction Profiles

What students doCorrect version

Y-axis labelled "activation energy" or "temperature"Y-axis labelled " Energy "

X-axis labelled "time"X-axis labelled " Progress of reaction "

"Eₐ" written at the peak (no arrow)Arrow from reactant level to peak , labelled "Eₐ"

Eₐ arrow drawn from x-axis to peakEₐ arrow drawn from reactant level to peak (not from zero)

Endothermic drawn with products below reactantsEndothermic: products above reactants

Profile drawn as straight lines / triangular peakSpec requires a curved line (smooth curve through the peak)

Freehand wobbly arrowsRuler-drawn arrows with dashed reference lines

Practice — Labels X and Y

Figure 4 below shows a reaction profile for the reaction of sodium carbonate with hydrochloric acid. Label X points to the Y-axis. Label Y points to the vertical arrow from reactant level to product level.

What do labels X and Y represent on Figure 4?2 MARKS

Source: P1 Jun 2022 Q02.5 (verbatim)

X

(X) energy

Y

(Y) (overall) energy change

📝

Mark scheme: exact phrasing — "energy" for X (the y-axis); "(overall) energy change" for Y (the vertical arrow). Note: just "energy change" is enough; the word "overall" is bracketed in the MS, meaning optional. The MS for X requires the word "energy" itself — describing what the axis represents, not just its unit.

Practice — Exothermic Identification

The reaction of sodium carbonate with hydrochloric acid is exothermic. The reaction profile in Figure 4 shows reactants on the left at a higher level, a curve rising to a peak, then falling to products on the right at a lower level.

How does the reaction profile show that the reaction is exothermic? Use Figure 4.1 MARK

Source: P1 Jun 2022 Q02.6 (verbatim)

MP1

"The (level of) products is below the (level of) reactants" — this is the primary mark-scheme answer.

ALT

Also accepted: "the energy decreases (overall)" OR "energy is transferred to the surroundings".

⚠️

Ignored / not credited: references to bond breaking or bond making. The question is about the profile shape, not the molecular-level explanation.

📝

Phrasing tip: the word "level" appears in brackets in the mark scheme — it's optional but using it makes the answer cleaner. Always describe what the GRAPH shows, not what you remember about exothermic reactions.

Practice — Complete the Profile

The reaction between hydrogen and chlorine is exothermic. Figure 4 shows part of a reaction profile for this reaction (the rising curve from reactants is drawn, but the rest is not). Below: a sketch of what the question looks like — the curve has been started but not completed; no labels added yet.

Complete the reaction profile in Figure 4. You should:

label the activation energy

label the overall energy change.3 MARKS

Source: P1 Jun 2023 Q04.3 (verbatim)

MP1

Profile completed with product energy below reactant energy (because reaction is exothermic).

MP2

Activation energy labelled from reactant energy to top of curve — vertical arrow with both endpoints clearly anchored.

MP3

Overall energy change labelled from reactant energy to product energy — vertical arrow from reactant level down to product level.

📝

Mark scheme accepts: arrows without arrowheads ("ignore arrow heads"). What matters is the start and end points are clearly at the right energy levels — use dashed reference lines.

Visualised solution

Drawing Checklist

Run through this list every time you draw a profile. Real exam questions tend to be 3 marks (shape + Ea arrow + energy change arrow), but the full checklist below covers everything that could be tested.

The 7-point check

1

Y-axis labelled "Energy" ✓

2

X-axis labelled "Progress of reaction" (or "reaction pathway") ✓

3

Reactants line at correct level (HIGH for exo, LOW for endo) ✓

4

Products line at correct level (LOW for exo, HIGH for endo) ✓

5

Curve drawn as a smooth CURVED line through the peak (spec 4.5.1.2) ✓

6

Eₐ arrow from reactant level to peak, with dashed reference line ✓

7

Overall energy change arrow from reactant level to product level ✓

Use a ruler — but keep the curve curved

Reactant and product energy levels: ruler-drawn horizontal lines. Dashed reference lines: ruler-drawn. Arrows: ruler-drawn. The connecting line through the peak: smooth curve, freehand — drawn carefully but not in straight segments. Pencil first, then ink.

Checkpoint — Lesson 5.02

You should now be able to:

Concepts

Define activation energy as "the minimum energy needed for a reaction to occur"

Explain that activation energy is the gap between reactant level and the peak — never the height of the peak

Identify a reaction as exothermic or endothermic from a given profile

Drawing

Draw exothermic profiles (reactants HIGH, products LOW) and endothermic profiles (reactants LOW, products HIGH)

Use a smooth curved line through the peak (spec-mandated)

Label both axes correctly: "Energy" (y), "Progress of reaction" (x)

Anchor arrows on dashed reference lines for clear start/end positions

Label Eₐ and overall energy change clearly — these are the two arrow marks

Examiner-proof answers

Identify exothermic from a profile: "products level is below reactants level"

Use the verbatim mark-scheme phrases: "energy" for the y-axis; "(overall) energy change" for the energy difference arrow

Avoid the classic trap: never write "Eₐ" as a label on the peak — always draw an arrow showing the gap

Next up

Lesson 5.03 — The Energy Change of Reactions [HT only]. We turn the qualitative profile into a quantitative calculation: bond energies are used to compute the actual energy change in kJ/mol. Three labelled steps, every time. (Foundation tier students can skip ahead to 5.04.)