HookHow a soldier eats a hot meal with no fire, no stove and no electricity
On a mountainside, on a submarine, or pinned down in a trench, you often cannot light a fire — but you can still eat a hot meal. The flameless ration heater tucked inside a military food pack is a flat bag of powdered magnesium and iron; add a splash of water and, within a couple of minutes, it climbs to nearly 100°C and steams the food beside it. There is no flame, no fuel and no electricity. All the heat comes out of a single chemical reaction — the magnesium being oxidised — pouring energy into its surroundings. Turn the same idea inside out and you get the instant cold pack a physiotherapist cracks over a sprained ankle: ammonium nitrate dissolving in water, a change that pulls heat in and drops to near freezing in seconds.
That single split — reactions that give heat out versus reactions that take heat in — is the whole of C5. Chemists call them exothermic and endothermic, and the split sits underneath hand warmers, cold packs, fuels, explosives and every reaction in your body. This section explains where the energy actually comes from (the answer is bonds — breaking them costs energy and making them pays it back), how to read the energy change off a reaction profile, and how to calculate it from bond energies. Get the direction of the energy flow right and the rest of the topic follows.
ModelExothermic and endothermic — which way does the heat flow?
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings rises. Combustion, oxidation such as the rusting of iron or the magnesium in that ration heater, and neutralisation are all exothermic — and so are most of the reactions you meet day to day. We put the effect to work in self-heating cans of coffee and in disposable hand warmers, where iron slowly oxidising keeps your gloves warm for hours.
An endothermic reaction takes energy in from the surroundings, so the temperature falls. Thermal decomposition is the classic example — calcium carbonate will not break down into calcium oxide and carbon dioxide unless you keep pouring heat in — as is the reaction between citric acid and sodium hydrogencarbonate. Instant cold packs for sports injuries use an endothermic change: ammonium nitrate dissolving in water draws heat out of the pack and out of the swelling.
Energy is always conserved. It is neither created nor destroyed, only shifted between the chemicals — where it is stored in bonds — and the surroundings, where you feel it as a change in temperature. 'Exothermic' names the direction of that transfer, not a heat that lasts forever: the warmth of a hand warmer fades as the reaction runs out of reactants.
CaseMeasuring a temperature change (Required practical 10)
Required practical 10 investigates the variables that affect the temperature change of reacting solutions — for example neutralising an acid with an alkali, displacing a metal from a salt solution, or dissolving a salt. In the neutralisation version, the independent variable is the volume of alkali added, the dependent variable is the temperature change, and the control variables that keep it fair are the concentration of each solution and a fixed starting temperature.
The method is where the marks hide. Measure a fixed volume of acid into a polystyrene cup with a lid — the insulation is the whole point, because it stops heat leaking to the room and ruining the reading — record the starting temperature, add the alkali in measured steps, stir, and record the temperature after each addition. Heat loss to the surroundings is the largest source of error, cut down by the lid and the insulating cup; not stirring, and a thermometer that only resolves to \(1\)°C, are the next two. The result is a pattern you can read like a graph.
You add sodium hydroxide in \(5\ \text{cm}^3\) steps to a fixed \(25\ \text{cm}^3\) of hydrochloric acid and record the temperature rise from the start. The readings climb — \(2.6\), \(4.9\), \(7.1\) and \(9.0\)°C after \(5\), \(10\), \(15\) and \(20\ \text{cm}^3\) — reach a maximum of about \(9.4\)°C at \(25\ \text{cm}^3\), then fall away to \(7.8\) and \(6.5\)°C by \(30\) and \(35\ \text{cm}^3\).
The temperature climbs as more acid is neutralised, because neutralisation is exothermic, and peaks where the acid is exactly used up. Beyond that peak, adding more cool alkali only dilutes the warm mixture with no further reaction, so the temperature falls. That maximum therefore locates the volumes in which the two solutions react exactly — a real, quantitative result read straight off the data.
MechanismReaction profiles and activation energy
A reaction profile plots energy up the side against the progress of the reaction along the bottom, with reactants on the left and products on the right. In an exothermic reaction the products sit lower than the reactants, and that drop is the energy released to the surroundings. In an endothermic reaction the products sit higher, and the climb is the energy taken in.
Between reactants and products there is a hump, and its height above the reactants is the activation energy — the minimum energy that colliding particles must have for the reaction to get going, enough to start breaking bonds. Even a strongly exothermic reaction needs that initial push: methane will sit in the air indefinitely until a spark supplies the activation energy, and only then does it burn. That is precisely why fuels are safe to store and dangerous only once lit. On the diagram, two measurements are easy to confuse and examiners test the difference: the activation energy is the height from the reactants up to the peak, while the overall energy change is the height from the reactants across to the products.
DataBond energies — the arithmetic of where the energy goes (Higher tier)
Here is the reason a reaction gives out or takes in energy at all. Breaking a chemical bond always needs energy — it is endothermic — and making a chemical bond always releases energy — it is exothermic. The overall energy change is simply the balance of the two: \(\Delta H = \Sigma(\text{bonds broken}) - \Sigma(\text{bonds made})\).
If making the new bonds releases more energy than breaking the old ones absorbed, \(\Delta H\) comes out negative and the reaction is exothermic. If breaking costs more than making pays back, \(\Delta H\) is positive and the reaction is endothermic. The sign is the entire point of the calculation, and it flips if you subtract the wrong way round, so fix the discipline: total the bonds broken first, total the bonds made second, then subtract the second from the first.
Calculate the energy change for the complete combustion of methane, \(\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}\), given bond energies in kJ/mol of C–H \(= 413\), O=O \(= 498\), C=O \(= 805\) and O–H \(= 464\).
Bonds broken (in the reactants): four C–H bonds and two O=O bonds, \((4\times413) + (2\times498) = 1652 + 996 = 2648\ \text{kJ/mol}\) — energy in, so positive.
Bonds made (in the products): two C=O bonds in the carbon dioxide and four O–H bonds in the two water molecules, \((2\times805) + (4\times464) = 1610 + 1856 = 3466\ \text{kJ/mol}\) — energy out.
Overall: \(\Delta H = 2648 - 3466 = -818\ \text{kJ/mol}\).
The negative sign says the reaction is exothermic, releasing \(818\) kJ for every mole of methane burned — which is why methane is a fuel. Subtract the wrong way and you would report \(+818\) and call a fire endothermic, so the sign, not just the number, is what earns the final mark.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
State the direction of energy flow first and in words, because that sentence often carries the first AO1 mark: name the reaction exothermic or endothermic, say whether energy is transferred to or from the surroundings, and say whether the surroundings warm or cool.
On reaction profiles, label all four features the examiner is checking for — reactants, products, the activation energy (reactants up to the peak) and the overall energy change (reactants across to products) — and make sure the products are drawn lower than the reactants for an exothermic reaction and higher for an endothermic one. Muddling the activation energy with the overall change is the commonest drawing error.
For bond-energy calculations, write the method explicitly as 'bonds broken minus bonds made', show both totals, keep the sign all the way to the end and quote the answer in kJ/mol; a correct method with the sign reversed still reaches the wrong conclusion and loses the interpretation mark. In Required practical 10, whenever you are asked why the method is valid, name the polystyrene cup and lid (to reduce heat loss to the surroundings) and the fixed total volume and concentrations (the fair controls) — these are the AO2 points the mark scheme lists.