Reaction profiles

Chemistry · Reaction profiles

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In an exothermic reaction, the products are at a lower energy level than the reactants. Energy is transferred to the surroundings, so the temperature of the surroundings increases.

Switch between exothermic and endothermic. The vertical axis is energy — this is not a temperature graph.

Predict, then check

Read the energy levels, not the reaction's name.

A reaction profile finishes with the products at a lower energy level than the reactants. What does this tell you?

AQA Required Practical 4 investigates temperature changes. It is related to exothermic/endothermic reactions, but the reaction profile itself is an energy-level diagram, not a temperature plot.

What you need to know

• Exothermic: energy is transferred to the surroundings, so the surroundings' temperature increases; products are lower in energy than reactants.
• Endothermic: energy is taken in from the surroundings, so the surroundings' temperature decreases; products are higher in energy than reactants.
• A reaction profile shows reactant and product energy levels, a curved pathway, activation energy and the overall energy change between reactants and products.
• Activation energy is the minimum energy reacting particles must have for a reaction to occur; on the profile it is the barrier from the reactant level to the peak.
• Combustion, many oxidation reactions and neutralisation are exothermic examples; thermal decomposition and citric acid reacting with sodium hydrogencarbonate are endothermic examples.
• AQA Required Practical 4 is related because it measures temperature changes as evidence of energy transfer. The reaction profile itself still plots energy, not temperature.

The big picture

A reaction profile is an energy diagram, not a temperature graph. In an exothermic reaction the products are at a lower energy level than the reactants, energy is transferred to the surroundings and the surroundings' temperature increases. In an endothermic reaction the products are higher, energy is taken in from the surroundings and the surroundings' temperature decreases. The curved pathway rises to a peak: the activation energy is the minimum energy reacting particles need and is shown by the barrier from the reactant level to that peak.

⚠ Watch out

Reading the vertical axis as temperature, or drawing/measuring activation energy from zero or from the product level. The profile represents energy, and activation energy runs from the reactant level to the peak. Keep overall energy change qualitative on this page.

★ Exam tip

For AQA, keep the related RP4 temperature-change evidence separate from the reaction profile itself: the practical measures temperature change, while the profile represents relative energy levels, activation energy and overall energy change.

Memory hook

Products down → energy out. Products up → energy in. Peak → activation barrier.

Check yourself

A reaction profile shows the products above the reactants. Is the reaction exothermic or endothermic, what happens to energy and the surroundings' temperature, and where is the activation energy shown?