Your revision route
What you’ll learn
- Distinguish exothermic and endothermic reactions from temperature data and evaluate an application using supplied evidence.
- Draw and read energy profiles, locating the activation barrier and comparing reactant and product energy levels.
- Higher Tier: calculate reaction energy changes from supplied bond energies.
- Plan the temperature-change practical, identify variables, interpret supplied results and evaluate limitations.
- Separate Chemistry: interpret relative-reactivity and voltage data, then evaluate a cell using supplied evidence.
- Separate Chemistry: make a conditional, evidence-led judgement between a rechargeable cell and a hydrogen fuel cell.
- Separate Chemistry Higher Tier: write hydrogen fuel-cell half equations when the electrolyte is specified.
Build the big picture
Key ideas
The thermometer watches the surroundings
When the reaction is the main cause of the temperature change, warming shows energy leaving the system; cooling shows energy entering it.
- During an exothermic change, energy leaves the reacting chemicals and nearby matter normally warms.
- During an endothermic change, nearby matter supplies energy and normally cools.
- Combustion, many oxidations and neutralisation are exothermic; thermal decomposition is commonly endothermic.
- Hand warmers and self-heating food cans exploit exothermic chemistry; certain cold injury packs exploit endothermic chemistry.
The punchline: Name the surroundings' temperature change and then state the energy-transfer direction.
Every successful reaction must climb an energy hill
Even an exothermic reaction needs an initial push: reacting particles must collide with at least the activation energy.
- A reaction profile plots relative energy vertically against progress of reaction horizontally.
- Activation energy is the energy rise from the reactant level to the peak.
- Products below reactants show an exothermic reaction; products above reactants show an endothermic reaction.
- The overall energy change is the vertical difference between reactant and product levels.
The punchline: Label reactants, products, activation energy and overall change on every profile.
Follow energy across the boundary
The system is the reacting chemicals; the surroundings include the solution, container, thermometer and nearby air.
- ExothermicEnergy transfers from the reacting system to the surroundings, which usually warm up.
- EndothermicEnergy transfers from the surroundings into the reacting system, so the surroundings usually cool.
- ConservationThe products' energy differs from the reactants' by the same amount transferred across the boundary.
Higher Tier: bonds keep a two-column account
Bond breaking requires energy; bond making releases it. The difference determines whether the reaction as written is exothermic or endothermic, not whether it proceeds.
- Add supplied bond energies for all bonds broken in the reactants.
- Add supplied bond energies for all bonds formed in the products.
- Energy change = energy required to break bonds − energy released when bonds form.
- More energy released than required gives an exothermic result; the reverse gives an endothermic result.
The punchline: Count bonds from the balanced structures before touching the calculator.
Calorimetry catches only part of the energy story
The thermometer records the solution, while some energy slips into the cup, air and apparatus.
- Measure a starting temperature, mix measured reactants and record the highest or lowest temperature reached.
- Change one variable, such as reactant concentration or volume, while controlling the others.
- An insulated cup and lid reduce energy transfer to the surroundings; repeats improve confidence.
- For a fair comparison, calculate temperature change from final extreme temperature minus starting temperature.
The punchline: Use insulation, consistent timing and repeats; identify heat loss as a limitation.
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Read the surroundings
What does the thermometer reveal?
Select each reaction. Track the energy direction and the surroundings' response, noting when an external heater masks that response.
Combustion transfers energy from the reacting chemicals to the surroundings.
1 of 4 states explored
A temperature change can classify energy transfer only when external heating does not dominate. Continued heating alone is not evidence that a reaction is endothermic.
Match chemistry to behaviour
Separate Chemistry: which energy source fits each description?
Pair each electrochemical source with the feature that distinguishes it.
Separate Chemistry: focus on reactant supply and whether an external current can reverse the cell chemistry.
Separate Chemistry: different metals create electrical tension
A voltage can arise when electrodes with different reactivities share an electrolyte and are linked through a circuit.
- A basic cell places two unlike electrodes in the same electrolyte and links them electrically.
- Changing either the pair of electrode materials or the electrolyte can alter the cell's voltage; relative-reactivity data can support predictions.
- A battery combines at least two cells in a series arrangement, so their voltages add.
- An alkaline battery cannot be recharged and fails once a required reactant is exhausted. In a rechargeable cell, supplied electrical energy drives the cell chemistry backwards.
The punchline: Do not call one cell a battery: a battery contains two or more cells.
Separate Chemistry: a fuel cell is fed rather than recharged
A hydrogen fuel cell keeps working while hydrogen and oxygen arrive; it does not store its whole reactant supply inside.
- The cell converts electrochemical oxidation of the incoming fuel into an electrical potential difference.
- The overall hydrogen fuel-cell reaction combines hydrogen and oxygen to form water.
- Evaluation should include fuel manufacture, energy efficiency, storage, safety, refuelling, emissions and material use.
- Separate Chemistry HT: write electrode half equations only for the stated acidic or alkaline electrolyte.
The punchline: Compare complete life cycles, not just the substance leaving the device.
Higher Tier: the bond-energy ledger
Every reaction pays to break reactant bonds and receives energy when product bonds form.
- List bonds brokenCount each relevant bond in the reactants and multiply by its supplied bond energy.
- List bonds formedCount each relevant bond in the products and multiply by its supplied bond energy.
- SubtractEnergy change = total energy to break bonds − total energy released forming bonds.
- Interpret signA negative result is exothermic; a positive result is endothermic under this convention.
Two reaction profiles, the same three landmarks
Both plots have relative energy on the vertical axis and progress of reaction on the horizontal axis. Each curved path rises from reactants to a peak, then falls to products.
Separate Chemistry: stored reactants or supplied fuel?
Both technologies turn chemical change into a voltage, but one stores its reactants while the other receives them from outside.
- Non-rechargeable cellElectrical output ends once the limiting stored chemical has been consumed.
- Rechargeable cellA charging current drives the cell chemistry backwards.
- Hydrogen fuel cellA continuing feed of hydrogen and oxygen sustains operation; the net chemical product is water.
Judge the job, not the label
Evaluation starts with the task and supplied data. A justified answer weighs relevant benefits and costs, then states when its choice would change.
- For a hand warmer or cold pack, compare temperature change, useful duration, mass, control, cost and safety against the stated need.
- Separate Chemistry: under the same conditions, a larger reactivity difference between electrodes generally supports a larger voltage; use the given data.
- Separate Chemistry: evaluate cells using voltage, lifetime, recharge cycles, cost, resource use, waste and safety—not a single favourable number.
- Separate Chemistry: hydrogen fuel cells may suit rapid refuelling or continuous operation; rechargeable cells may suit available charging and repeated use. Decide from the scenario.
The punchline: Make the judgement conditional: cite the decisive data, acknowledge a trade-off and state what changed evidence could reverse the choice.
Words worth knowing
Key definitions
- exothermic reaction
- A reaction in which energy is transferred overall from the reacting system to the surroundings.
- endothermic reaction
- A reaction in which energy is transferred overall from the surroundings to the reacting system.
- activation energy
- The collision-energy threshold that reacting particles must reach.
- reaction profile
- A diagram showing relative energy as a reaction progresses.
- Separate Chemistry: cell
- A chemical source of potential difference using electrodes and an electrolyte.
- Separate Chemistry: battery
- An electrical source made by wiring multiple cells in series.
- Separate Chemistry: fuel cell
- An electrochemical power source that operates while fuel and oxygen or air are fed into it.
Calculate with confidence
Equations
Higher Tier: reaction energy change
energy change = Σ(bonds broken) − Σ(bonds formed)
Overall energy change equals energy required to break reactant bonds minus energy released forming product bonds.
| Symbol | Meaning | Unit |
|---|---|---|
| Σ broken | total supplied bond energies for bonds broken | kJ/mol |
| Σ formed | total supplied bond energies for bonds formed | kJ/mol |
Exam tip: A negative answer is exothermic under this sign convention; include every bond from the balanced reaction.
Separate Chemistry: hydrogen fuel-cell overall reaction
2H₂ + O₂ → 2H₂O
Hydrogen is oxidised and oxygen is reduced; the overall chemical product is water.
Exam tip: Separate Chemistry: balance both hydrogen and oxygen atoms; half equations depend on the stated electrolyte.
Separate Chemistry HT: acidic fuel-cell electrodes
anode: 2H₂ → 4H⁺ + 4e⁻; cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O
In an acidic electrolyte, hydrogen is oxidised at the anode and oxygen is reduced at the cathode.
Exam tip: Check both atoms and charge, then confirm that cancelling H⁺ and e⁻ gives the overall reaction.
Separate Chemistry HT: alkaline fuel-cell electrodes
anode: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻; cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻
In an alkaline electrolyte, hydroxide ions appear in the balanced electrode half equations.
Exam tip: Use alkaline half equations only when the question specifies alkaline conditions.
Follow it step by step
Processes to remember
How to draw a reaction profile
- Label the vertical axis relative energy and the horizontal axis progress of reaction.
- Draw and label the reactant energy level.
- Curve up to a peak, then down to the labelled product energy level.
- Add an activation-energy arrow from reactants to the peak.
- Add an overall-change arrow between reactants and products.
Exam tip: Product level below reactants is exothermic; above is endothermic.
Higher Tier: how to calculate from bond energies
- Use the balanced reaction and count every bond broken in the reactants.
- Multiply each count by its supplied bond energy and add the totals.
- Repeat for every bond formed in the products.
- Subtract total formed from total broken and interpret the sign.
Exam tip: Do not subtract individual bond pairs before checking their multiplicities.
How to evaluate an energy option from data
- Define the job: required temperature change, duration, voltage, range, operating time or refuelling and charging constraint.
- Quote two relevant values from the stimulus and compare them directly; do not replace evidence with a memorised slogan.
- For exothermic or endothermic applications, link the measured temperature change and duration to usefulness and safety.
- Separate Chemistry: for cells, compare voltage, usable life, rechargeability, resources, waste and safety under the stated conditions.
- Separate Chemistry: compare fuel manufacture and storage with charging source and infrastructure before choosing hydrogen or rechargeable.
- Conclude conditionally: choose for this scenario, name the decisive evidence and state one condition that would change the decision.
Exam tip: Evaluation marks come from applied evidence plus a qualified judgement, not from listing disconnected advantages and disadvantages.
See the thinking
Worked example
Worked example: hydrogen chloride from bond energies
Higher Tier: for H₂ + Cl₂ → 2HCl, bond energies are H–H 436, Cl–Cl 243 and H–Cl 431 kJ/mol. Calculate the energy change.
- Bonds broken: one H–H and one Cl–Cl, so 436 + 243 = 679 kJ/mol.
- Bonds formed: two H–Cl, so 2 × 431 = 862 kJ/mol released.
- Energy change = 679 − 862 = −183 kJ/mol.
- The negative result means more energy is released forming bonds than is required to break bonds.
Answer: The energy change is −183 kJ/mol, so the reaction is exothermic.
The balanced equation forms two H–Cl bonds. If only one were counted, the formed total would be 431 kJ/mol and 679 − 431 = +248 kJ/mol, incorrectly predicting an endothermic reaction.
Protect the marks
Common mistakes
Watch out: Calling a hot reaction endothermic because it contains energy.
Do this instead: When the reaction causes the temperature rise, the warming surroundings show exothermic transfer out of the reacting system.
Watch out: Saying bond breaking releases energy.
Do this instead: Breaking bonds requires energy; forming bonds releases energy.
Watch out: Drawing no activation-energy peak for an exothermic reaction.
Do this instead: Both exothermic and endothermic reactions need an initial activation-energy rise.
Watch out: Separate Chemistry: using one cell and calling it a battery.
Do this instead: Separate Chemistry: a battery combines multiple cells in a series arrangement.
Watch out: Separate Chemistry: claiming a hydrogen fuel cell creates no environmental impact.
Do this instead: Its device product is water, but evaluate hydrogen manufacture, storage and full life-cycle evidence.
Plan it like the exam
Required practicals
Investigate how acid concentration affects a neutralisation temperature change
Combined Science and separate Chemistry
Aim: Measure how hydrochloric acid concentration affects the maximum temperature rise when it reacts with sodium hydroxide solution.
Method
- Using its own clean, dry, labelled measuring cylinder, measure 25.0 cm³ of teacher-approved dilute hydrochloric acid into an insulated polystyrene cup supported in a beaker.
- Using a separate clean, dry, labelled measuring cylinder, measure 25.0 cm³ of fixed-concentration sodium hydroxide into a labelled beaker and fit a lid to the cup.
- Measure each reactant with a clean probe. Rinse it with distilled water and dry it between solutions; once both readings are steady at the same starting temperature, record them.
- Pour the sodium hydroxide into the acid, replace the lid, stir with the temperature probe and record at fixed intervals until the maximum passes.
- Calculate maximum temperature rise, rinse and dry the apparatus, then repeat with a fresh teacher-approved acid concentration.
- Repeat every concentration, calculate mean temperature rises and plot mean rise against hydrochloric acid concentration.
Variables
- Independent
- concentration of dilute hydrochloric acid
- Dependent
- maximum temperature rise of the neutralisation mixture
- Controls
- 25.0 cm³ volume of each reactant
- concentration of sodium hydroxide solution
- same measured starting temperature for both reactants
- same insulated cup, lid, probe, stirring and reading intervals
Analysis: Compare mean maximum temperature rises without assuming a linear trend. A rise shows exothermic transfer; exchange with the cup and air means the measured maximum may be lower than the ideal value.
Safety
- Wear eye protection and use only teacher-approved dilute hydrochloric acid and sodium hydroxide concentrations and quantities.
- Avoid skin contact; rinse splashes or spills promptly with plenty of water and follow the teacher's local instructions.
- Keep the polystyrene cup supported in a beaker and carry the separate reactant beaker carefully before mixing.
Improvements
- Use a lid and insulation to reduce energy transfer to the surroundings.
- Use a temperature probe and data logger to capture the true maximum or minimum more reliably.
- Repeat each condition and use a mean after checking anomalies.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
A solution warms from 20 °C to 27 °C during a reaction. Is the reaction exothermic or endothermic?
Answer: Exothermic
The surroundings warmed, so energy transferred from the reacting system to them.
Pack A warms 12 °C for 15 min and is skin-safe; B warms 20 °C for 3 min and carries a burn warning. Which suits a 10-min hand warmer?
Answer: Pack A: its 15-minute duration covers the task and it is stated skin-safe, despite B's larger temperature rise.
The choice follows the 10-minute need and safety evidence. B could be preferred only for a short task needing a larger rise if its burn risk were controlled.
Means are 5.2, 8.1 and 7.9 °C at 0.5, 1.0 and 1.5 mol/dm³ HCl. Name the variables and give one conclusion.
Answer: Independent: HCl concentration. Dependent: mean maximum temperature rise. The rise increases to 1.0 mol/dm³, then changes little.
Use the numerical pattern; do not claim a linear trend. Fixed volumes, sodium hydroxide concentration and starting temperature make the comparison fair.
What does the peak above the reactants represent on a reaction profile?
Answer: The activation-energy barrier
Only collisions that reach the activation-energy threshold can lead to reaction.
Higher Tier: if breaking needs 500 kJ/mol and forming releases 650 kJ/mol, what is the change?
Answer: −150 kJ/mol, exothermic
500 − 650 = −150 kJ/mol.
Separate Chemistry: what is a battery?
Answer: Multiple cells wired in series.
The series voltages combine to give a greater total voltage.
Separate Chemistry: Mg/Cu gives 2.7 V, Zn/Cu 1.1 V and Cu/Cu 0.0 V in the same electrolyte. What does the data support?
Answer: A greater difference in electrode reactivity gives a greater voltage here; identical electrodes give no voltage.
The electrolyte is held constant, so electrode choice is the supported explanation. Say ‘in these data’ rather than claiming one exact voltage for every cell.
Separate Chemistry: what is the overall chemical product of a hydrogen fuel cell?
Answer: Water
Hydrogen is oxidised by oxygen: 2H₂ + O₂ → 2H₂O.
Separate Chemistry: a 160 km depot route has a 280 km rechargeable range and overnight charging; hydrogen gives 500 km but no local station. Choose.
Answer: Use the rechargeable cell for this scenario: its range covers the route and depot charging exists, whereas local hydrogen refuelling does not.
The judgement is conditional. Hydrogen's range and rapid refuelling could matter for longer, high-utilisation routes if low-impact fuel and stations were available.
Good questions, clear answers
Frequently asked questions
Does an exothermic reaction need activation energy?
Yes. Particles still need enough collision energy to begin rearranging bonds, even though energy leaves the reacting chemicals overall.
Why is measured temperature change smaller than the ideal change?
Energy transfers between the mixture, cup and air, and the thermometer may miss the true extreme. Insulation, a lid and rapid data logging reduce these limitations.
Higher Tier: are bond-energy calculations exact?
Supplied bond energies are mean values across compounds, so calculations estimate the energy change. They are Higher Tier content.
Separate Chemistry: how is a fuel cell different from a rechargeable cell?
A fuel cell receives fuel and oxygen continuously. A rechargeable cell stores its reactants and uses an external current to reverse its reactions during charging.
Separate Chemistry Higher Tier: are fuel-cell half equations always the same?
No. Separate Chemistry Higher Tier equations depend on whether the electrolyte is acidic or alkaline, so use the form specified in the question.
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