What you'll learn
This revision guide covers everything you need to know about energy changes in chemical reactions for AQA GCSE Chemistry. You'll learn how to distinguish between exothermic and endothermic reactions, measure energy changes, interpret reaction profile diagrams, and understand the concept of activation energy. These topics are essential for both Foundation and Higher tier papers.
Key terms and definitions
Exothermic reaction — a reaction that transfers energy to the surroundings, usually by heating, causing the temperature of the surroundings to increase.
Endothermic reaction — a reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease.
Activation energy — the minimum amount of energy that particles must have to react when they collide.
Reaction profile — a diagram showing the relative energies of reactants and products during the course of a reaction, including the activation energy.
Bond energy — the energy required to break one mole of a particular covalent bond.
Energy level diagram — another term for reaction profile, showing the energy changes during a chemical reaction.
Overall energy change — the difference in energy between the products and reactants, represented by ΔH (delta H).
Calorimetry — the experimental technique used to measure energy changes in chemical reactions.
Core concepts
Exothermic reactions
Exothermic reactions release energy to the surroundings. The energy released comes from the formation of new chemical bonds in the products.
Key characteristics:
- Temperature of surroundings increases
- Energy is transferred from the reaction system to the surroundings
- Products have less energy than reactants
- ΔH is negative (shown with a minus sign)
Common examples you must know:
- Combustion — burning fuels like methane, propane or wood releases large amounts of energy
- Neutralisation — acid + alkali reactions always release energy (e.g., HCl + NaOH)
- Oxidation — many oxidation reactions release energy, including rusting (though this happens slowly)
- Respiration — the breakdown of glucose in cells releases energy for biological processes
Practical applications:
- Self-heating cans for coffee or soup use exothermic reactions (calcium oxide + water)
- Hand warmers contain iron powder that oxidises exothermically when exposed to air
- Rocket fuel combustion provides thrust for spacecraft
When you perform an exothermic reaction in the laboratory and measure the temperature with a thermometer, you will observe the temperature rising.
Endothermic reactions
Endothermic reactions absorb energy from the surroundings. The energy is used to break chemical bonds in the reactants.
Key characteristics:
- Temperature of surroundings decreases
- Energy is transferred from the surroundings to the reaction system
- Products have more energy than reactants
- ΔH is positive (shown with a plus sign)
Common examples you must know:
- Thermal decomposition — heating calcium carbonate to produce calcium oxide and carbon dioxide
- Electrolysis — breaking down compounds using electricity requires continuous energy input
- Dissolving ammonium nitrate — this process absorbs energy and is used in instant cold packs
- Photosynthesis — plants absorb light energy to convert carbon dioxide and water into glucose
Practical applications:
- Sports injury packs use endothermic dissolving of ammonium nitrate to provide instant cooling
- Sherbet sweets contain citric acid and sodium hydrogencarbonate which react endothermically in your mouth, creating a cooling sensation
When you perform an endothermic reaction in the laboratory and measure the temperature with a thermometer, you will observe the temperature falling.
Reaction profiles
Reaction profiles (also called energy level diagrams) show how energy changes during a chemical reaction.
Exothermic reaction profile:
- Reactants start at a higher energy level
- Products finish at a lower energy level
- The difference represents energy released
- There is an energy 'hump' showing activation energy
- The peak of the curve represents the transition state or activated complex
Endothermic reaction profile:
- Reactants start at a lower energy level
- Products finish at a higher energy level
- The difference represents energy absorbed
- There is still an activation energy 'hump'
- Products store more energy than reactants
Key features to identify:
- Activation energy — always measured from the reactants up to the peak of the curve
- Overall energy change — measured from reactants to products (vertically)
- Progress of reaction — shown on the x-axis
- Energy — shown on the y-axis
You must be able to label these features on blank reaction profiles in the exam. The activation energy is required even for exothermic reactions because particles need sufficient energy to break existing bonds before new ones can form.
Measuring energy changes
You need to know how to measure temperature changes in simple calorimetry experiments.
Basic method:
- Measure a fixed volume of solution (e.g., 25 cm³ of acid) using a measuring cylinder
- Record the initial temperature using a thermometer
- Add the second reagent (e.g., alkali or metal)
- Stir the mixture with the thermometer
- Record the maximum or minimum temperature reached
- Calculate the temperature change: ΔT = final temperature − initial temperature
Equipment required:
- Polystyrene cup (provides insulation to reduce heat loss)
- Thermometer (to ±0.5°C accuracy)
- Measuring cylinder (for volumes)
- Lid (to reduce heat loss through evaporation)
Common reactions to investigate:
- Neutralisation: acid + alkali
- Displacement: reactive metal + metal salt solution (e.g., zinc + copper sulfate)
- Dissolving: ionic substances in water
Improving accuracy:
- Use a lid to reduce energy transfer to surroundings
- Use a polystyrene cup rather than a glass beaker (better insulator)
- Increase the volume of solution used
- Take repeat measurements and calculate a mean
- Use a more sensitive thermometer or digital temperature probe
Bond breaking and bond making
Understanding why reactions are exothermic or endothermic requires knowledge of bond energies.
Bond breaking:
- Requires energy input
- Is an endothermic process
- The stronger the bond, the more energy required to break it
Bond making:
- Releases energy
- Is an exothermic process
- The stronger the bond formed, the more energy is released
Overall energy change calculation:
For any reaction:
- Energy in = energy required to break all bonds in reactants
- Energy out = energy released when all bonds in products are formed
- Overall energy change = energy in − energy out
If more energy is released making bonds than is needed to break bonds:
- The reaction is exothermic
- Energy out > energy in
- ΔH is negative
If more energy is needed to break bonds than is released making them:
- The reaction is endothermic
- Energy in > energy out
- ΔH is positive
Important note: At GCSE level, you may need to use bond energy values to calculate overall energy changes. You will be given the bond energies in a data table.
Required practical: temperature changes
The AQA specification includes investigating the variables that affect temperature changes in chemical reactions as a required practical.
Variables you might investigate:
- Volume of reactants
- Concentration of solutions
- Type of acid or alkali used
- Type of metal used in displacement reactions
Control variables to keep constant:
- Volume of the second reagent
- Starting temperature
- Type of container
- Time allowed for reaction
Safety considerations:
- Wear eye protection (especially with acids and alkalis)
- Avoid contact with corrosive substances
- Clean up spills immediately
- Wash hands after practical work
Analysis:
- Plot graphs of temperature change against the independent variable
- Identify patterns and trends
- Explain results using collision theory and energy changes
- Evaluate the method and suggest improvements
Worked examples
Example 1: Identifying reaction types (Foundation/Higher)
Question: A student mixes citric acid and sodium hydrogencarbonate. The temperature decreases from 21°C to 15°C.
(a) State whether this is an exothermic or endothermic reaction. [1 mark] (b) Explain what has happened to cause the temperature change. [2 marks]
Answer: (a) Endothermic [1]
(b) Energy has been absorbed from the surroundings [1] to break bonds in the reactants / for the reaction to occur [1]
Examiner note: For part (b), you must explain the energy transfer, not just describe what happened to the temperature. The mark scheme requires reference to energy being taken in or absorbed.
Example 2: Reaction profile interpretation (Higher tier)
Question: The diagram shows the reaction profile for the decomposition of hydrogen peroxide.
[Diagram shows: Reactants at 50 kJ, peak at 100 kJ, products at 30 kJ]
(a) Is this reaction exothermic or endothermic? Explain your answer. [2 marks] (b) What is the activation energy for this reaction? [1 mark] (c) Suggest why hydrogen peroxide decomposes much faster when manganese dioxide is added. [2 marks]
Answer: (a) Exothermic [1] because the products have less energy than the reactants / energy is released [1]
(b) 50 kJ (or 50 kJ/mol or 50 kJ mol⁻¹) [1] [Calculated as: 100 − 50 = 50 kJ]
(c) Manganese dioxide acts as a catalyst [1] which provides an alternative pathway with lower activation energy / allows more particles to have sufficient energy to react [1]
Examiner note: Always measure activation energy from the reactants to the peak, not from the products. Read values carefully from the y-axis.
Example 3: Bond energy calculations (Higher tier only)
Question: Hydrogen reacts with chlorine to form hydrogen chloride: H₂ + Cl₂ → 2HCl
Use the bond energies in the table to calculate the energy change for this reaction.
| Bond | Bond energy (kJ/mol) |
|---|---|
| H–H | 436 |
| Cl–Cl | 242 |
| H–Cl | 431 |
[4 marks]
Answer:
Bonds broken: 1 × H–H = 436 kJ 1 × Cl–Cl = 242 kJ Total energy in = 678 kJ [1]
Bonds formed: 2 × H–Cl = 2 × 431 = 862 kJ [1]
Energy change = energy in − energy out [1] = 678 − 862 = −184 kJ/mol [1]
Examiner note: Remember that 2HCl means two H–Cl bonds are formed. Always show your working clearly. The negative sign indicates an exothermic reaction.
Common mistakes and how to avoid them
Confusing the definitions — Remember: exothermic reactions give out heat (the surroundings get hotter), endothermic reactions take in heat (the surroundings get cooler). Think "exo = exit" and "endo = enter".
Measuring activation energy incorrectly — Always measure from the energy level of the reactants up to the peak, never from the products or from zero. Draw a horizontal line from the reactants to help you.
Wrong signs for energy changes — Exothermic reactions have negative ΔH values (energy is lost from the system). Endothermic reactions have positive ΔH values (energy is gained by the system).
Forgetting to count all bonds — In bond energy calculations, if you have 2HCl, you must count two H–Cl bonds. Check the balancing numbers carefully in the equation.
Confusing bond breaking with bond making — Breaking bonds always requires energy (endothermic process). Making bonds always releases energy (exothermic process). The overall reaction type depends on which releases more energy.
Not using polystyrene cups in calculations — When describing improvements to calorimetry experiments, explain that polystyrene is a better insulator than glass, so less heat is transferred to the surroundings, making results more accurate.
Exam technique for "Energy changes: exothermic and endothermic reactions"
Command words matter — "State" requires a simple answer with no explanation (1 mark). "Explain" requires you to give reasons using scientific principles (usually 2+ marks). "Calculate" requires numerical working and units.
Reaction profile questions — Practice sketching and labeling diagrams. You must be able to: (1) draw the correct relative positions of reactants and products, (2) show activation energy clearly, (3) label all axes and features requested. Use a ruler for the axes.
Practical questions — When asked to describe improvements or evaluate methods, focus on: reducing heat loss, improving accuracy of measurements, controlling variables, and repeating for reliability. Link improvements to their effect on results.
Show all working in calculations — Even if you make an arithmetic error, you can still gain method marks if your approach is correct. Always include units in your final answer. For bond energy calculations, set out your work clearly with "bonds broken" and "bonds formed" sections.
Quick revision summary
Energy changes occur in all chemical reactions. Exothermic reactions release energy, increasing the temperature of surroundings; examples include combustion and neutralisation. Endothermic reactions absorb energy, decreasing the temperature of surroundings; examples include thermal decomposition and photosynthesis. Reaction profiles show activation energy (minimum energy needed to react) and overall energy change. Breaking bonds requires energy (endothermic); making bonds releases energy (exothermic). Temperature changes can be measured using simple calorimetry with polystyrene cups and thermometers.