Reaction profiles — AQA GCSE Combined Science

Reaction profiles are graphical diagrams showing the relative energies of reactants and products, activation energy, and overall energy change during a chemical reaction. In AQA GCSE Combined Science, mastering these profiles for exothermic and endothermic reactions is critical for explaining energy transfers and the effects of catalysts.

Reaction profiles are energy diagrams that plot the progress of a chemical reaction against energy changes. They visually demonstrate the energy barrier that must be overcome for particles to react, as well as the net energy released or absorbed when old bonds break and new bonds form.

In AQA GCSE Combined Science, reaction profiles link directly to collision theory, bond energy calculations, and catalysts. Understanding how to interpret and construct these graphs allows students to explain macroscopic temperature changes using microscopic chemical principles.

• →Exothermic reaction profiles show products at a lower energy level than reactants because energy is released to the surroundings.
• →Endothermic reaction profiles show products at a higher energy level than reactants because energy is taken in from the surroundings.
• →Activation energy (Ea) is the minimum energy colliding particles must possess to react, represented from the reactant line to the peak of the curve.
• →The overall energy change is the vertical difference between the energy level of the reactants and the energy level of the products.
• →Catalysts provide an alternative reaction pathway requiring lower activation energy without changing the energy levels of reactants or products.

• States that reacting particles must collide with each other.
• Explains that collisions must have sufficient energy to be successful.
• Defines activation energy as the minimum energy required for a reaction to occur.
• Identifies activation energy on a reaction profile as the energy difference between reactants and the peak of the curve.
• Links increased temperature to a greater proportion of particles having energy greater than or equal to the activation energy.
• Explains that catalysts lower activation energy by providing an alternative reaction pathway.
• A reaction profile plots energy vertically against progress of reaction horizontally, with the curve beginning at the reactant energy level and finishing at the product energy level.
• The overall energy change is the vertical difference between the reactant level and the product level; it is negative for an exothermic reaction and positive for an endothermic reaction.
• The activation energy is the vertical distance from the reactant energy level to the highest point of the curve, representing the minimum energy particles need to react.
• The peak of the curve represents the transition state or activated complex, where bonds in the reactants are being broken and new bonds are forming.
• Adding a catalyst lowers the peak of the curve, reducing the activation energy, while leaving the reactant and product energy levels unchanged.
• On a labelled profile, arrows or brackets should clearly identify which gap represents activation energy and which represents the overall energy change.
• Axes correctly labelled: progress of reaction on the horizontal axis and energy on the vertical axis, with no numerical scale required.
• Reactant and product energies shown as short horizontal lines at different heights, lower products for exothermic and higher products for endothermic.
• A single curved line connects the reactant level to the product level via a maximum, showing energy changing continuously as the reaction proceeds.
• Activation energy labelled as the energy difference from the reactant level to the peak of the curve.
• Overall energy change labelled as the energy difference between reactant and product levels, with the correct sign convention.
• Exothermic profile shows products at lower energy than reactants; endothermic profile shows products at higher energy than reactants.
• Compare the energy level of the reactants with the energy level of the products on the profile.
• State that a lower product energy level means energy is released to the surroundings, so the reaction is exothermic.
• State that a higher product energy level means energy is absorbed from the surroundings, so the reaction is endothermic.
• Link the direction of the overall energy change to the sign of ΔH: negative for exothermic and positive for endothermic.
• Recognise that activation energy is the energy needed to reach the peak and is separate from the overall energy change.
• Activation energy is the minimum energy that particles need for a collision to result in reaction.
• It is represented on a reaction profile as the energy difference between the reactants and the highest point of the curve.
• Particles must collide with at least this energy so that bonds in the reactants can begin to break.
• A reaction with a higher activation energy needs more energetic collisions, so fewer collisions are successful and the rate is lower at the same temperature.
• Activation energy is not the overall energy change of the reaction; the overall change is the difference between reactants and products.
• Catalysts lower the activation energy by providing an alternative pathway, which increases the proportion of successful collisions.

• 💡Use the phrase 'minimum energy' when defining activation energy.
• 💡On a reaction profile, label the activation energy as the vertical distance from reactants to the highest point of the curve.
• 💡When explaining rate increases with temperature, refer to the proportion of particles with energy greater than or equal to the activation energy.
• 💡For catalyst questions, state that the activation energy is lowered, not that the particles gain energy.
• 💡Label both axes and both energy gaps on any profile you draw, using a bracket or double-headed arrow for each gap so the examiner can see exactly what you mean.
• 💡State the sign of the overall energy change explicitly: negative for exothermic and positive for endothermic, linking the sign to whether products are lower or higher than reactants.
• 💡When asked to compare two profiles, quote the direction of the difference, for example that the catalysed curve has a lower peak but the same start and end levels.
• 💡Sketch both profiles side by side and annotate each with reactant, product, activation energy and overall energy change before writing your final answer.
• 💡Use arrows with clear arrowheads for activation energy and overall energy change so the examiner can see exactly which gap you are labelling.
• 💡Check the sign of the overall energy change: exothermic reactions release energy so ΔH is negative, endothermic reactions absorb energy so ΔH is positive.
• 💡Quote the reactant and product energy levels or describe their relative heights before naming the reaction type.
• 💡Use the phrase energy transferred to the surroundings for exothermic and energy taken in from the surroundings for endothermic.
• 💡If a profile is unlabelled, trace the curve from left to right and note whether the finishing level is below or above the starting level.
• 💡Label the activation energy on a reaction profile with a vertical arrow from the reactants' level to the peak, and label the overall energy change separately.
• 💡When explaining rate effects, state that a greater proportion of collisions has energy greater than or equal to the activation energy.
• 💡Use the phrase minimum energy needed for a reaction to occur, and avoid saying energy needed to start the reaction without qualification.
• 💡Always use a ruler to draw straight, horizontal lines for the reactant and product energy levels.
• 💡Ensure your activation energy arrow points clearly upwards from the reactant level to the peak of the curve.
• 💡Check that the overall energy change arrow connects the reactant line to the product line, with the arrowhead pointing down for exothermic and up for endothermic.

• Error: saying any collision causes a reaction. Correction: only collisions with energy greater than or equal to the activation energy are successful.
• Error: confusing activation energy with overall energy change. Correction: activation energy is the energy barrier from reactants to the peak; overall energy change is from reactants to products.
• Error: thinking a catalyst increases the energy of particles. Correction: a catalyst lowers the activation energy by providing an alternative pathway.
• Error: assuming exothermic reactions do not have an activation energy. Correction: all reactions require an initial input of activation energy to start breaking bonds.
• Measuring activation energy from the products' level to the peak instead of from the reactants' level to the peak; correct this by always starting the activation energy measurement at the reactant energy level.
• Confusing the overall energy change with the activation energy; correct this by remembering that overall energy change compares reactants with products, while activation energy compares reactants with the peak.
• Assuming a catalyst changes the overall energy change; correct this by stating that a catalyst lowers only the activation energy and leaves the reactant and product levels, and therefore the overall energy change, unchanged.
• Drawing the curve as a straight diagonal line: correct by using a smooth curve that rises to a peak and then falls to the product level.
• Labelling activation energy from the product level to the peak: correct by measuring activation energy from the reactant level to the peak.
• Showing products higher than reactants for an exothermic reaction: correct by placing the product line below the reactant line for exothermic reactions.
• Judging the reaction type from the height of the activation energy peak: correct by comparing reactant and product energy levels instead.
• Assuming a large activation energy means the reaction is endothermic: correct by explaining that activation energy affects rate, not the overall energy classification.
• Ignoring the sign of the overall energy change: correct by stating that exothermic reactions have a negative overall energy change and endothermic reactions have a positive one.
• Confusing activation energy with the overall energy change: the overall change is measured between reactants and products, while activation energy is measured from reactants to the peak.
• Thinking that exothermic reactions need no energy input: bonds must still be broken first, so an activation energy is always required.
• Believing that all collisions cause reaction: only collisions with energy greater than or equal to the activation energy are successful.
• Thinking that activation energy starts from the bottom of the graph rather than from the energy level of the reactants.
• Believing that adding a catalyst changes the overall energy change (delta H) or increases the yield of products.
• Drawing arrows without arrowheads or with double arrowheads when showing direction of energy change.

1. 1Day 1: Practice drawing blank exothermic and endothermic reaction profile curves from memory until the general shapes are clear.
2. 2Day 2: Add exact labels to both profiles: axes, reactants, products, activation energy (Ea), and overall energy change.
3. 3Day 3: Practice drawing the effect of a catalyst as a dashed lower curve on both types of profiles.
4. 4Day 4: Work through quantitative past paper questions that require calculating Ea and overall energy change from numerical values.

• 📋Diagram completion: Adding arrows and labels for activation energy and overall energy change to a pre-drawn curve.
• 📋Comparative evaluation: Comparing two reaction profiles to deduce which reaction is faster or releases more energy.
• 📋Catalyst application: Sketching an alternative reaction pathway onto an existing profile to show the effect of adding a catalyst.