Energy transfer during exothermic and endothermic reactions — AQA GCSE Combined Science
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Energy transfer during exothermic and endothermic reactions explained
Chemical reactions rearrange atoms but never create or destroy energy.
Read the full explanation
The total energy of the universe before and after a reaction is identical, so any energy transferred to the surroundings must have come from the reacting system. In an exothermic reaction, the products have less energy than the reactants; the difference is transferred to the surroundings, often as heat, so the surroundings warm up. For example, burning methane transfers energy to the surroundings, and the products carbon dioxide and water have less chemical energy than methane and oxygen. Conversely, in an endothermic reaction, products have more energy than reactants because energy is taken in from the surroundings. Energy diagrams show reactant and product energy levels, with the vertical difference representing the energy transferred.
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.
An exothermic reaction transfers energy from the reacting system to the surroundings, causing the surroundings to warm up. The products have less chemical energy than the reactants, and the difference is released, often as heat. A familiar example is the combustion of fuels such as methane, where the surroundings become hot. Another is the reaction of an acid with an alkali, which warms the mixture. In contrast, endothermic reactions absorb energy from the surroundings, so the surroundings cool. To identify an exothermic reaction, measure the temperature change of the surroundings: an increase shows energy is being transferred out of the system. Energy level diagrams show products at a lower energy level than reactants, with the energy gap equal to the energy transferred.
Exothermic reactions include combustion, many oxidation reactions and neutralisation.
Exothermic reactions transfer energy to the surroundings, causing the surroundings to warm up and the temperature to rise. Combustion is burning a fuel in oxygen, for example methane + oxygen → carbon dioxide + water, which releases energy. Many oxidation reactions also release energy, such as iron reacting with oxygen to form iron oxide during rusting, or respiration in cells. Neutralisation occurs when an acid reacts with an alkali or base to form a salt and water, for example hydrochloric acid + sodium hydroxide → sodium chloride + water; the temperature rise confirms energy is released. In all these cases, the overall energy change is negative and the reaction mixture warms up. Measuring this temperature increase is a standard way to identify an exothermic process.
Everyday uses of exothermic reactions include self-heating cans and hand warmers.
Exothermic reactions release energy to the surroundings, and this energy transfer can be put to practical use. A self-heating can contains a compartment holding substances that react exothermically when mixed, for example calcium oxide reacting with water to form calcium hydroxide, so the drink warms without a cooker or microwave. A hand warmer often uses the oxidation of iron in air; when the sealed packet is opened, air reaches the iron and the exothermic oxidation releases heat for a period. In both cases the design controls when the reaction starts and how much heat is released, and the temperature rise of the surroundings shows energy transfer. The same principle explains why some reactions are chosen for heating purposes: they release useful energy safely and at a suitable rate.
An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.
An endothermic reaction absorbs energy from its surroundings, so the surroundings lose thermal energy and their temperature falls. Energy is transferred from the surroundings to the reacting system, which stores it in the products; the products therefore have more energy than the reactants. A practical way to observe this is to add citric acid to sodium hydrogencarbonate in a polystyrene cup with a thermometer: the thermometer reading drops as the reaction proceeds. The temperature decrease is evidence that energy has been taken in, not released. In an energy level diagram the products are drawn higher than the reactants, and the energy difference is the energy absorbed. The temperature change depends on amounts and concentrations, so a small change may be hard to detect without a sensitive thermometer or insulation.
Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate. Some sports injury packs are based on endothermic reactions.
Many endothermic reactions are familiar from the laboratory and everyday life. Thermal decomposition means breaking a compound down using heat; for example, heating calcium carbonate strongly forms calcium oxide and carbon dioxide, and energy must be supplied continuously for the decomposition to continue. Another example is the reaction between citric acid and sodium hydrogencarbonate, which fizzes and cools as it takes in energy from the surroundings. This cooling effect is used in some sports injury packs: when the pack is activated, the reaction absorbs energy from the injured area, providing a cold compress. The temperature drop can be measured with a thermometer in an insulated cup, and the reaction is recognised as endothermic because the surroundings cool.
distinguish between exothermic and endothermic reactions on the basis of the temperature change of the surroundings
You distinguish the two reaction types by observing what happens to the temperature of the surroundings. In an exothermic reaction, energy is transferred from the reacting system to the surroundings, so the surroundings warm up and the thermometer reading rises. In an endothermic reaction, energy is transferred from the surroundings to the system, so the surroundings cool down and the thermometer reading falls. For example, dissolving ammonium nitrate in water lowers the temperature, so it is endothermic; burning magnesium raises the temperature, so it is exothermic. The key skill is to link the direction of temperature change to the direction of energy transfer, not simply to memorise examples.
evaluate uses and applications of exothermic and endothermic reactions given appropriate information.
Exothermic reactions transfer energy to the surroundings, so the surroundings warm up; endothermic reactions take in energy from the surroundings, so they cool down. To evaluate a use, weigh the useful energy change against practical factors such as safety, cost, control and whether the effect suits the purpose. For example, a self-heating can uses an exothermic reaction to warm food, but the reaction must be safe, controllable and give enough energy. A sports injury pack uses an endothermic reaction to cool tissue, but it must not become dangerously cold. Evaluation means judging how well the reaction fits the job, not simply naming it as exothermic or endothermic.
Required practical activity 10: investigate the variables that affect temperature changes in reacting solutions such as, eg acid plus metals, acid plus carbonates, neutralisations, displacement of metals.
This practical explores how temperature changes when solutions react. You choose a reaction such as hydrochloric acid with magnesium, sodium carbonate or sodium hydroxide, or copper sulfate with zinc. Measure initial temperature, mix reactants, stir, and record the highest or lowest temperature. Calculate temperature change ΔT = final − initial. Variables include concentration, volume, particle size, metal identity and reaction type. Control variables to make comparisons valid. Exothermic reactions release energy, raising temperature; endothermic reactions absorb energy, lowering it. Plot bar charts or line graphs to identify trends. Assessed on planning, safe technique, accurate measurement, recording and interpreting results.
Your focus
- State that energy is conserved in chemical reactions and that the total energy of the universe remains constant.
- Explain, using the idea of energy conservation, why products have less energy than reactants when energy is transferred to the surroundings.
- Interpret simple energy level diagrams to identify whether a reaction is exothermic or endothermic and the energy transferred.
Show all 27 objectives
- Define an exothermic reaction as one that transfers energy to the surroundings, causing a temperature increase.
- Describe how to identify an exothermic reaction from a temperature change in the surroundings.
- Give examples of exothermic reactions and explain the energy transfer involved.
- Identify combustion, many oxidation reactions and neutralisation as exothermic reactions.
- Describe the energy transfer to the surroundings in an exothermic reaction using a temperature rise.
- Interpret experimental temperature data to classify a reaction as exothermic.
- Describe how self-heating cans and hand warmers use exothermic reactions.
- Explain that energy is transferred to the surroundings in these devices, causing a temperature rise.
- Evaluate the suitability of an exothermic reaction for an everyday heating device using given information.
- Define an endothermic reaction in terms of energy transfer from the surroundings.
- Relate a decrease in the temperature of the surroundings to energy being taken in.
- Represent an endothermic change on an energy level diagram with products higher than reactants.
- Identify thermal decomposition and the citric acid–sodium hydrogencarbonate reaction as endothermic.
- Explain how an endothermic reaction can be used in a sports injury pack to cool the body.
- Describe the temperature change of the surroundings as evidence for an endothermic reaction.
- Define exothermic and endothermic reactions in terms of energy transfer to or from the surroundings.
- Interpret temperature change data to classify a reaction correctly.
- Explain a classification using the direction of energy transfer rather than temperature alone.
- Classify given reactions as exothermic or endothermic using temperature or energy-transfer information.
- Explain how the direction of energy transfer produces a warming or cooling effect in a named application.
- Evaluate the suitability of a reaction for a stated use by weighing benefits against limitations and reaching a justified conclusion.
- Plan a safe procedure to measure temperature change in a reacting solution.
- Record and process temperature data to calculate ΔT.
- Interpret results to classify reactions as exothermic or endothermic and identify variables that affect ΔT.
Energy transfer during exothermic and endothermic reactions exam tips
Marking Points
- States that energy is conserved in chemical reactions: the total energy of the universe remains constant.
- Explains that if energy is transferred to the surroundings, the products must have less energy than the reactants.
- Uses the idea that the energy transferred equals the difference in energy between reactants and products.
- Applies conservation of energy to a named reaction, such as combustion of methane, identifying that the surroundings gain energy while products have lower energy.
- Interprets an energy level diagram to show that the products are at a lower energy level than the reactants for an exothermic change.
- Distinguishes between energy stored in chemical bonds and energy transferred to the surroundings, avoiding the misconception that energy is used up.
- Defines an exothermic reaction as one that transfers energy to the surroundings.
- States that the temperature of the surroundings increases during an exothermic reaction.
- Links the temperature increase to energy transfer from the reaction to the surroundings.
- Gives a named example of an exothermic reaction, such as combustion of methane or neutralisation of an acid with an alkali.
- Explains that in an exothermic reaction the products have less energy than the reactants, and the energy difference is transferred to the surroundings.
- Interprets an energy level diagram for an exothermic reaction, identifying products at a lower energy level than reactants.
- States that exothermic reactions transfer energy to the surroundings, causing a temperature rise.
- Identifies combustion as burning in oxygen, giving examples such as methane + oxygen → carbon dioxide + water.
- Describes oxidation as a reaction with oxygen, for example iron + oxygen → iron oxide, and notes that many oxidation reactions are exothermic.
- Explains neutralisation as acid + alkali/base → salt + water, for example hydrochloric acid + sodium hydroxide → sodium chloride + water, and links the temperature rise to energy release.
- Interprets temperature change data from a simple experiment, such as measuring the temperature rise when sodium hydroxide solution is added to hydrochloric acid.
- States that self-heating cans and hand warmers use exothermic reactions to release energy to the surroundings.
- Describes a self-heating can as having a compartment where reactants mix and react exothermically, warming the drink.
- Describes a hand warmer as using oxidation of iron in air, releasing heat when the packet is opened.
- Links the everyday use to the temperature rise of the surroundings and to the idea that energy is transferred, not created.
- Compares the design features that control the reaction, such as a seal, a compartment or a permeable wrapper, so heat is released when needed.
- Evaluates a simple claim about a hand warmer or self-heating can using temperature change data or a described observation.
- Endothermic reactions transfer energy from the surroundings to the reacting system.
- The surroundings lose thermal energy, so their temperature decreases.
- The products have more stored energy than the reactants.
- Energy is absorbed, so the temperature change of the surroundings is negative.
- A thermometer reading that falls during a reaction is evidence for an endothermic change.
- An energy level diagram for an endothermic reaction shows products higher than reactants.
- Thermal decomposition is an endothermic process because it requires energy input to break down a compound.
- Heating calcium carbonate to form calcium oxide and carbon dioxide is an example of thermal decomposition.
- Citric acid reacting with sodium hydrogencarbonate takes in energy and cools the surroundings.
- Sports injury packs can use endothermic reactions to produce a cooling effect on the body.
- The temperature of the surroundings falls during these reactions, which is evidence of energy absorption.
- Energy must be supplied continuously for a thermal decomposition to keep going.
- States that an exothermic reaction transfers energy to the surroundings and causes a temperature rise in the surroundings.
- States that an endothermic reaction takes in energy from the surroundings and causes a temperature fall in the surroundings.
- Uses temperature change data from a described experiment to classify a named reaction correctly as exothermic or endothermic.
- Explains the classification in terms of energy transfer direction rather than only quoting the temperature value.
- Applies the distinction to unfamiliar examples, such as a self-heating hand warmer or an instant cold pack, using the observed temperature change.
- Correctly classify a described reaction as exothermic or endothermic from temperature change or energy transfer information.
- Link the direction of energy transfer to the intended effect, such as warming or cooling.
- Judge suitability using practical criteria such as safety, cost, controllability, reversibility and energy released or absorbed per unit.
- Compare two possible reactions or applications and justify a choice with evidence from the supplied information.
- Recognise that a reaction may be unsuitable even when the energy change is in the right direction.
- Use the supplied data rather than assumed values, and state when information is insufficient for a firm judgement.
- Selects a suitable reaction and identifies independent, dependent and control variables.
- Uses a thermometer or temperature probe correctly, reading to the nearest appropriate precision and allowing equilibration.
- Measures initial temperature before mixing and records maximum or minimum temperature after reaction.
- Controls variables such as volume, concentration, starting temperature and particle size to ensure valid comparison.
- Calculates temperature change ΔT = final temperature − initial temperature, including sign.
- Plots appropriate graph (bar chart for categoric data, line graph for continuous data) and describes trend.
- Evaluates risks and suggests improvements, such as insulation or repeated trials.
Examiner Tips
- 💡When asked about energy changes, always refer to the total energy of the universe remaining constant and link this to the energy difference between reactants and products.
- 💡Use energy level diagrams to support your answer: label reactants higher than products for an exothermic reaction and state that the gap equals the energy transferred.
- 💡Avoid phrases like 'energy is lost'; instead write 'energy is transferred to the surroundings' to show correct scientific vocabulary.
- 💡If a question gives a temperature change, state whether the surroundings gain or lose energy and relate this to the relative energy of products and reactants.
- 💡When describing an exothermic reaction, always mention that energy is transferred to the surroundings and that this causes a temperature increase.
- 💡Use a named example, such as burning methane or neutralisation, to support your explanation.
- 💡If given temperature data, state that a rise in temperature indicates an exothermic reaction because energy is transferred to the surroundings.
- 💡Avoid saying 'heat is given out' without specifying that it is transferred to the surroundings; use precise language.
- 💡When asked to classify a reaction, look for a temperature rise in the surroundings as evidence of exothermic behaviour.
- 💡Give the general word equation for neutralisation and one named example with the salt formed, rather than only naming the reaction type.
- 💡If a question gives temperature data, calculate or describe the temperature change and link it to energy transfer to the surroundings.
- 💡Name the reaction type and the energy transfer when explaining a self-heating can or hand warmer, rather than only saying it gets hot.
- 💡Use the words 'exothermic' and 'surroundings' in your answer to show the direction of energy transfer.
- 💡If a question asks for a comparison, give one similarity and one difference between the two devices, such as the trigger mechanism or the reactants used.
- 💡State clearly that energy is taken in from the surroundings and that the surroundings' temperature decreases.
- 💡When describing a practical, name the thermometer reading falling as the observation that supports an endothermic change.
- 💡If asked to compare, contrast endothermic with exothermic by the direction of energy transfer and the sign of the temperature change.
- 💡Give named examples when asked, such as thermal decomposition of calcium carbonate or citric acid with sodium hydrogencarbonate.
- 💡Link the everyday use of a sports injury pack to the endothermic cooling effect rather than just naming the pack.
- 💡Use the temperature change of the surroundings as evidence when explaining why a reaction is endothermic.
- 💡Use the phrase energy transferred to the surroundings for exothermic and energy taken in from the surroundings for endothermic.
- 💡When given data, quote the starting and finishing temperatures and state the direction of change before naming the process.
- 💡Link each classification to a real example, such as combustion for exothermic or thermal decomposition for endothermic, to show understanding.
- 💡Underline the purpose in the question, such as warming food or cooling an injury, then judge the reaction against that purpose.
- 💡Use the supplied information as evidence; quote or refer to the data rather than relying on memory.
- 💡Structure an evaluation as a benefit, a limitation and a justified overall judgement.
- 💡Check that every claim about temperature change matches the direction of energy transfer you have identified.
- 💡State the independent variable, dependent variable and at least two control variables for your chosen reaction.
- 💡Describe how you ensure accurate temperature measurement, such as stirring and reading at eye level.
- 💡When calculating temperature change, show the subtraction clearly and include the unit °C.
- 💡Link your conclusion to exothermic or endothermic behaviour using the sign of ΔT.
Common Mistakes
- Error: saying energy is 'used up' or 'lost' in a reaction. Correction: energy is conserved; it is transferred to the surroundings, not destroyed.
- Error: thinking products always have more energy than reactants. Correction: in an exothermic reaction, products have less energy than reactants by the amount transferred.
- Error: confusing energy conservation with mass conservation. Correction: mass is also conserved, but energy conservation refers to total energy before and after the reaction.
- Error: assuming the surroundings always cool down. Correction: if energy is transferred to the surroundings, the surroundings warm up; cooling occurs when energy is taken in from the surroundings.
- Error: saying the temperature of the reaction mixture itself increases rather than the surroundings. Correction: the surroundings increase in temperature because energy is transferred to them.
- Error: confusing exothermic with endothermic. Correction: exothermic transfers energy to the surroundings (temperature rises); endothermic takes in energy (temperature falls).
- Error: thinking energy is created in an exothermic reaction. Correction: energy is conserved; it is transferred from the reactants to the surroundings.
- Error: assuming all reactions that release gas are exothermic. Correction: exothermicity depends on energy transfer, not gas production.
- Thinking that all oxidation reactions are exothermic: the error is assuming every oxidation releases heat; correction is to note that while many are exothermic, some oxidation reactions, such as the reaction between nitrogen and oxygen to form nitrogen monoxide, are endothermic.
- Confusing neutralisation with combustion: the error is mixing up reaction types; correction is to remember that neutralisation forms a salt and water from an acid and a base, while combustion needs oxygen and a fuel and produces oxides.
- Writing that exothermic reactions 'create' energy: the error is violating the conservation of energy; correction is to state that energy is transferred to the surroundings, not created.
- Saying the can or warmer 'makes' energy; correction: it transfers energy from chemical stores to the surroundings, and total energy is conserved.
- Confusing a hand warmer with an endothermic cold pack; correction: hand warmers release heat, while cold packs absorb heat from the surroundings.
- Assuming the reaction continues forever; correction: the reactants are used up, so heat release stops when the reaction finishes or the oxidising agent is exhausted.
- Saying energy is 'used up' or 'destroyed' rather than transferred and stored in the products; correct this by describing energy transfer from surroundings to the system.
- Confusing endothermic with exothermic and predicting a temperature rise; correct this by linking 'endo' to energy entering the system and a fall in surroundings temperature.
- Believing the reacting mixture itself gets colder because it 'loses' energy; correct this by stating the surroundings lose energy while the system gains it.
- Thinking all decompositions release energy; correct this by noting that thermal decomposition requires heating and is endothermic.
- Assuming a fizzing reaction must be exothermic; correct this by testing the temperature change, as citric acid and sodium hydrogencarbonate cool the surroundings.
- Confusing the purpose of a sports injury pack with an exothermic hand warmer; correct this by linking injury packs to cooling and hand warmers to warming.
- Confusing the temperature change of the reaction mixture with the temperature change of the surroundings: the classification is based on the surroundings, so state which temperature is being measured.
- Thinking that an endothermic reaction always feels cold because energy is destroyed: energy is absorbed from the surroundings, not destroyed.
- Reversing the labels: exothermic means energy out and temperature up; endothermic means energy in and temperature down.
- Saying that exothermic reactions 'give out heat' but not linking this to warming the surroundings; correct by stating that energy is transferred to the surroundings, raising their temperature.
- Assuming any exothermic reaction is automatically a good hand warmer; correct by checking safety, cost, controllability and how much energy is released.
- Treating 'endothermic' as meaning no energy transfer occurs; correct by explaining that energy is taken in from the surroundings, so they cool.
- Error: reading the thermometer immediately after mixing. Correction: wait until the temperature stabilises or reaches its highest/lowest value.
- Error: using different volumes or concentrations when comparing reactions. Correction: keep all variables except the independent variable constant.
- Error: confusing exothermic and endothermic temperature changes. Correction: exothermic increases temperature; endothermic decreases temperature.
- Error: ignoring heat loss to surroundings. Correction: use a polystyrene cup or lid, or note this as a source of error.