You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Energy changes are a fundamental part of every chemical reaction. In this lesson you will learn the difference between exothermic and endothermic reactions, understand how energy is transferred to and from the surroundings, and explore everyday examples of both types. This topic is a core part of the Energy Changes chapter of the AQA GCSE Chemistry specification (Section 5).
During a chemical reaction, existing bonds in the reactants are broken and new bonds are formed to make the products. Breaking bonds requires energy (it is an endothermic process), while forming bonds releases energy (it is an exothermic process).
The overall energy change of a reaction depends on the balance between the energy needed to break bonds and the energy released when new bonds form.
| Process | Energy Change |
|---|---|
| Breaking bonds | Energy is taken in (endothermic) |
| Forming bonds | Energy is released (exothermic) |
Exam Tip: A common mistake is to say that breaking bonds releases energy. Remember: breaking bonds ALWAYS requires energy. It is only when NEW bonds form that energy is released.
An exothermic reaction is one that transfers energy to the surroundings. The surroundings usually get hotter, so there is a measurable temperature increase.
In an exothermic reaction, the energy released by forming new bonds in the products is greater than the energy needed to break bonds in the reactants.
| Reaction | Example |
|---|---|
| Combustion | Burning methane in a Bunsen burner |
| Neutralisation | Hydrochloric acid + sodium hydroxide |
| Oxidation | Rusting of iron, respiration in cells |
| Many displacement reactions | Magnesium reacting with copper sulfate solution |
graph LR
A[Reactants] -->|Bonds broken energy taken in| B[Transition State]
B -->|New bonds formed energy released| C[Products]
C -->|MORE energy released than taken in| D[Surroundings get HOTTER]
style D fill:#ff9999,stroke:#cc0000
Exam Tip: When a question asks you to identify an exothermic reaction, look for clues such as a temperature rise, flames, or the word "combustion." Neutralisation reactions are also always exothermic — this is a favourite exam question.
An endothermic reaction is one that takes in energy from the surroundings. The surroundings usually get cooler, so there is a measurable temperature decrease.
In an endothermic reaction, the energy needed to break bonds in the reactants is greater than the energy released when new bonds form in the products.
| Reaction | Example |
|---|---|
| Thermal decomposition | Heating calcium carbonate to produce calcium oxide and carbon dioxide |
| Citric acid + sodium hydrogencarbonate | A classic endothermic reaction in solution |
| Photosynthesis | Plants absorb light energy to convert CO2 and water into glucose and oxygen |
| Dissolving ammonium nitrate in water | The solution becomes noticeably cold |
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy transfer | To the surroundings | From the surroundings |
| Temperature change | Surroundings get hotter (temperature rises) | Surroundings get cooler (temperature falls) |
| Bond energy balance | Energy released by forming bonds > energy needed to break bonds | Energy needed to break bonds > energy released by forming bonds |
| Examples | Combustion, neutralisation, oxidation | Thermal decomposition, photosynthesis, dissolving ammonium nitrate |
| Everyday use | Hand warmers, self-heating cans | Cold packs |
Exam Tip: The terms "exo-" and "endo-" can help you remember. "Exo" means out (energy goes OUT to surroundings). "Endo" means in (energy goes IN from surroundings). Write these prefixes on your revision notes.
Some reactions are reversible, meaning they can go in both directions. When a reversible reaction is exothermic in one direction, it is endothermic in the reverse direction, and the energy transferred is the same in both cases.
Example — hydrated and anhydrous copper sulfate:
graph LR
A["Hydrated copper sulfate (blue)"] -->|"Heat (endothermic)"| B["Anhydrous copper sulfate (white) + Water"]
B -->|"Add water (exothermic)"| A
This principle applies to all reversible reactions: the energy change in the forward direction is equal and opposite to the energy change in the reverse direction.
You can detect whether a reaction is exothermic or endothermic by measuring the temperature change using a thermometer or a temperature probe in a polystyrene cup (calorimeter).
| Observation | Conclusion |
|---|---|
| Temperature of solution rises | Reaction is exothermic |
| Temperature of solution falls | Reaction is endothermic |
| No temperature change | No reaction, or energy changes balance out |
In all chemical reactions, energy is conserved. This means that the total energy of the reactants equals the total energy of the products plus the energy transferred to or from the surroundings. Energy cannot be created or destroyed — it can only be transferred between stores.
Exam Tip: AQA may ask you to explain energy changes in terms of bond breaking and bond forming. Always structure your answer: (1) breaking bonds takes in energy, (2) forming bonds releases energy, (3) state which is greater to determine whether the reaction is exothermic or endothermic overall.
Exam Tip: A 6-mark question on exothermic vs endothermic reactions is very common. Make sure you can clearly define both terms, give examples of each, explain the role of bond breaking and forming, and link temperature changes to the type of reaction.
A student mixes 50 cm³ of sodium hydroxide with 50 cm³ of hydrochloric acid in a polystyrene cup. The starting temperature of both solutions was 20.0 °C. After mixing, the maximum temperature reached was 26.8 °C.
Step 1 — state the temperature change. ΔT = 26.8 − 20.0 = +6.8 °C. The solution has warmed.
Step 2 — decide the direction of energy transfer. Because the temperature of the mixture has risen, energy has been transferred from the chemical store of the reactants to the thermal store of the surroundings (the solution and, eventually, the cup and air).
Step 3 — state the sign of the enthalpy change. For an exothermic reaction ΔH is negative. We would record this as ΔH < 0.
Step 4 — link it back to bonds. More energy was released when new O–H bonds formed in water molecules than was needed to break the H–Cl and Na–OH bonds in the reactants. This net release is what appears as a temperature rise.
Common mistake: "The reaction is hot, so it must be exothermic." Temperature rise of the surroundings is the correct evidence, not how "hot" the reactants feel. A reaction started at 50 °C that cools to 30 °C is still endothermic, even though the final mixture is warmer than room temperature.
| Feature | Exothermic profile | Endothermic profile |
|---|---|---|
| Position of products on y-axis | Below reactants | Above reactants |
| Direction of ΔH arrow | Downwards from reactants to products | Upwards from reactants to products |
| Sign of ΔH | Negative | Positive |
| Activation energy location | From reactants up to peak | From reactants up to peak |
| Typical classroom example | Neutralisation (HCl + NaOH) | Citric acid + NaHCO₃ |
Common mistake: Students often draw the activation energy arrow from the bottom of the graph to the peak. The activation energy is the height from the reactants line to the peak — not from the x-axis.
Although full cells and batteries are covered later in this topic, it is useful at this stage to see how energy changes link to everyday devices.
| Feature | Non-rechargeable cell | Rechargeable cell |
|---|---|---|
| Overall reaction | Irreversible exothermic discharge | Reversible — exothermic one way, endothermic when recharging |
| Energy flow during use | Chemical store → electrical store (exothermic) | Chemical store → electrical store (exothermic) |
| Energy flow during charging | Not possible | Electrical store → chemical store (endothermic) |
This shows that the same chemistry you are learning for neutralisation applies to batteries: an exothermic direction and an endothermic reverse direction, differing only by sign.
Exam-style question (4 marks): A student dissolves ammonium nitrate in water. The temperature of the solution falls from 21 °C to 8 °C. State whether the process is exothermic or endothermic and explain the energy change in terms of bonds broken and bonds formed.
Grade 4–5 answer: The reaction is endothermic because the temperature went down. Energy was taken in from the surroundings. Breaking bonds needs energy and making bonds gives out energy.
Grade 8–9 answer: The process is endothermic: energy is transferred from the thermal store of the water (surroundings) to the chemical store of the system, which is why the solution cools from 21 °C to 8 °C. When ammonium nitrate dissolves, the ionic lattice must be broken apart and the ion–water interactions formed. The energy needed to break the ionic bonds in NH₄NO₃ is greater than the energy released when new ion–dipole attractions form with water molecules, so the overall change has a positive ΔH. The fall in temperature of the surroundings is direct evidence that the net energy flow is into the system — a hallmark of an endothermic change.
A student carries out three separate reactions in polystyrene cups, starting from 20.0 °C in each case:
| Reaction | Final temperature (°C) | ΔT (°C) | Type |
|---|---|---|---|
| A: HCl + NaOH | 27.5 | +7.5 | Exothermic |
| B: NH₄Cl + water | 15.0 | −5.0 | Endothermic |
| C: Mg + HCl | 34.8 | +14.8 | Exothermic |
Reaction C has the largest positive temperature change, so it releases the most energy per unit volume. Reaction B is the only endothermic example — the surroundings have cooled because the system has absorbed energy. The sign convention helps you see this at a glance: positive ΔT indicates exothermic, negative ΔT indicates endothermic. Matching the sign of ΔT (observed) to the sign of ΔH (calculated) is a routine exam skill.
Common mistake: Writing "the reaction took energy from itself." Reactions transfer energy between the system and the surroundings. In an endothermic reaction the surroundings lose energy to the system, not the system to itself.
AQA alignment: This content is aligned with AQA GCSE Chemistry (8462) specification section 5.5 Energy changes — specifically 5.5.1.1 Exothermic and endothermic reactions. Assessed on Paper 1.