HookWhy 50 Dreamliners were grounded by a chemistry problem
On 16 January 2013, regulators grounded every Boeing 787 Dreamliner in the world — all fifty then in service — after lithium-ion batteries on two aircraft overheated, one of them catching fire on the tarmac at Boston. The batteries had entered thermal runaway: a chemical reaction releasing heat faster than the heat could escape, which raised the temperature, which sped the reaction, which released still more heat. It took Boeing three months and a redesigned steel containment box to get the fleet flying again. A battery is nothing more than a chemical reaction sealed in a can, and every reaction moves energy — the Dreamliner's fault was a reaction pouring it out far too fast.
Chemical reactions either release energy to their surroundings or absorb it from them, and that single split — exothermic versus endothermic — sits underneath hand warmers, cold packs, explosives, fuels and every cell and battery you own. This section explains where the energy actually comes from (the answer is bonds: breaking them costs energy, making them pays it back), how to read the energy change off a reaction profile, how to calculate it from bond energies, and how batteries and hydrogen fuel cells turn it into electricity. Get the direction of energy flow right and the rest of C5 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 rusting, 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.
An endothermic reaction takes energy in from the surroundings, so the temperature falls. Thermal decomposition is the classic case — calcium carbonate will not break into calcium oxide and carbon dioxide unless you keep heating it — as is the reaction of citric acid with sodium hydrogencarbonate. Instant cold packs for sports injuries use an endothermic change: ammonium nitrate dissolving in water pulls heat out of the pack and out of the bruise.
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 heat. 'Exothermic' describes the direction of that transfer, not a temperature that lasts forever; the warmth of a hand warmer fades as the reaction finishes.
MechanismReaction profiles and activation energy
A reaction profile plots energy up the side against the progress of the reaction along the bottom, reactants on the left and products on the right. In an exothermic reaction the products sit lower than the reactants, and the 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 is a hump, and its height above the reactants is the activation energy — the minimum energy that colliding particles must have for a reaction to start, enough to begin 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 then it burns. That is exactly why fuels are safe to store and only dangerous once lit. On the diagram, the activation energy is the reactants-to-peak height, while the overall energy change is the reactants-to-products height — two different measurements students often confuse.
DataBond energies — the arithmetic of where the energy goes (Higher tier)
Here is the reason reactions release or absorb energy at all. Breaking a chemical bond always needs energy (it is endothermic); 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 the discipline is fixed: total the bonds broken first, total the bonds made second, then subtract the second from the first.
Calculate the energy change for \(\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}\), given bond energies of 436 kJ/mol for H–H, 242 kJ/mol for Cl–Cl and 431 kJ/mol for H–Cl.
Bonds broken (in the reactants): one H–H and one Cl–Cl, \(436 + 242 = 678\ \text{kJ/mol}\) — energy in, so positive.
Bonds made (in the products): two H–Cl bonds, \(2 \times 431 = 862\ \text{kJ/mol}\) — energy out.
Overall: \(\Delta H = 678 - 862 = -184\ \text{kJ/mol}\).
The negative sign says the reaction is exothermic, releasing 184 kJ for every mole of the equation. Subtract the wrong way and you would report +184 and declare an exothermic reaction endothermic — so the sign, not just the number, is what earns the final mark.
CaseMeasuring a temperature change (Required practical 4)
Required practical 4 investigates what changes the temperature rise or fall of a reaction in solution — for example neutralising an acid with an alkali, displacing a metal (zinc added to copper sulfate), or dissolving a salt. The independent variable is often the concentration of one reactant; the dependent variable is the temperature change; and the control that makes the comparison fair is keeping the total volume the same each time.
The method matters as much as the result. Measure a fixed volume of the first solution into a polystyrene cup with a lid — the insulation is the whole point, because it stops heat leaking to the room and spoiling the reading — record the starting temperature, add the second reactant, stir, and record the maximum temperature for an exothermic reaction or the minimum for an endothermic one. A typical pattern: as the acid concentration rises the temperature change gets bigger, until the alkali becomes the limiting reactant and the graph plateaus. That plateau is a neat, real way to find the proportions in which the two solutions actually react.
CaseCells, batteries and hydrogen fuel cells
A cell turns chemical energy directly into electrical energy. Its voltage depends on the metals chosen — the further apart they sit in the reactivity series, the larger the voltage — as well as on the electrolyte and the conditions. A battery is simply two or more cells joined in series, their voltages adding up.
A non-rechargeable cell, such as an alkaline AA, runs its reaction one way only; once a reactant is used up it is flat for good. A rechargeable cell — the lithium-ion in a phone, the lead-acid in a car, the cells in the Dreamliner — runs a reversible reaction, and forcing a current through it backwards restores the original reactants so it can be used again.
A hydrogen fuel cell is different again: hydrogen and oxygen combine to form water, but the energy comes out as electricity rather than as the heat of a flame, and the only product is water, \(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\). At the negative electrode hydrogen is oxidised, \(2\text{H}_2 \rightarrow 4\text{H}^+ + 4e^-\); at the positive electrode oxygen is reduced, \(\text{O}_2 + 4\text{H}^+ + 4e^- \rightarrow 2\text{H}_2\text{O}\). The evaluation examiners reward: a fuel cell gives only water at the point of use and never runs flat the way a battery does, but hydrogen is difficult and bulky to store, and it is often made using electricity from fossil fuels — so how 'clean' the cell really is depends entirely on where the hydrogen came from.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
State the direction of energy flow first and in words: name the reaction exothermic or endothermic, say whether energy is transferred to or from the surroundings, and say whether the surroundings warm or cool. That sentence alone often carries the first mark.
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, higher for an endothermic one.
For bond-energy calculations, write the method 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 4, mention the polystyrene cup and lid (to reduce heat loss) and a fixed total volume (a fair control) whenever you are asked why the method is valid. And when you evaluate a hydrogen fuel cell, weigh 'only water at the point of use, never runs flat' against 'hydrogen is hard to store and is often made using fossil-fuel electricity'.