Chemical reactions, Year 10: energy changes

The products have less, by the amount transferred

HintEnergy cannot be made or lost, so whatever went out must be missing from somewhere.

The whyEnergy is conserved in every chemical reaction: the total before and after is the same. If the surroundings end up with more energy, the chemicals must end up with less by exactly that amount.

neutralisation

HintThe two reactants cancel each other out, leaving a salt and water.

The whyWhen an acid and an alkali react, the mixture warms up: energy is transferred to the surroundings. Burning fuels and many oxidations, such as the reaction inside a hand warmer, are exothermic too.

Endothermic

HintAsk which way the energy is moving if the surroundings, including the water and the thermometer, lose it.

The whyA reaction of this kind takes in energy from the surroundings, so the surroundings get colder. This reaction and thermal decompositions are the examples to know; some sports injury packs use the same kind of change to go cold.

An exothermic reaction

HintIts prefix is the one found in 'exit' and 'exhale'.

The whyCombustion is the everyday example: a burning fuel heats whatever is around it. Hand warmers and self-heating cans use reactions of this kind on purpose.

Polystyrene is a better insulator, so less energy escapes

HintThe thermometer can only record what stays in the liquid.

The whySome of what the reaction releases passes to the container and the air, so the measured temperature rise is smaller than it should be. A cup of this material and a lid cut these losses; standing the cup inside a beaker makes it steadier.

The acid is used up, so extra alkali only cools the mixture

HintOnce one reactant has gone, nothing more is released, and the liquid being poured in is at room temperature.

The whyWhile acid remains, each portion of alkali reacts and releases more energy, so the temperature climbs. After the acid is all neutralised no more energy is released, and the extra cooler solution shares out the energy already there. The peak shows the volume that just neutralises the acid.

The activation energy

HintThink of the push needed to get a ball over the top of a hill before it can roll down the far side.

The whyParticles react only if they collide with at least this much. On a reaction profile it is the height of the hump, measured from the reactants up to the peak.

Exothermic

HintThe chemicals finish with less stored than they started with; decide where the difference has gone.

The whyIf the products are at a lower energy level than the reactants, the difference has been transferred to the surroundings. In the opposite kind of profile the products line is higher than the reactants line.

At the top of the curve

HintIt measures the height of the barrier that has to be climbed.

The whyThe line rises from the reactants to a peak and then falls to the products. The activation energy is the vertical distance from the reactants level up to that peak, in both exothermic and endothermic profiles.

products

HintIt compares where the chemicals start with where they finish.

The whyThe overall change ignores the hump in between: it depends only on the starting and finishing energy levels. If the finish is lower the reaction is exothermic; if it is higher the reaction is endothermic.

Enough energy

HintThere is a minimum that every successful collision has to reach.

The whyA collision leads to reaction only if the particles have at least the activation energy between them. Collisions with less than this leave the particles unchanged, which is why not every collision counts.

It takes energy in

HintThink about pulling apart two magnets that are stuck together.

The whyEnergy must always be supplied to break a bond, and energy is always released when a bond forms. Whether a whole reaction is exothermic or endothermic depends on which of these two totals is larger.

184

HintAdd up what must be put in to break the reactants' bonds, add up what comes out when the products' bonds form, and compare.

The whyBreaking one H–H and one Cl–Cl bond takes 436 + 242 = 678 kJ/mol. Forming two H–Cl bonds releases 2 × 431 = 862 kJ/mol. More is released than is taken in, so 862 − 678 = 184 kJ/mol is released overall and the reaction is exothermic.

Exothermic

HintCompare the amount given out with the amount taken in.

The whyMore energy is released in making the new bonds than is needed to break the old ones, so the surplus, 1856 − 1370 = 486 kJ/mol, goes to the surroundings. If the breaking total were the larger, the reaction would be the other kind.

break

HintIt is the step that has to be paid for before any new substance can be made.

The whyThe overall change is the difference between the two totals. Written this way, a negative answer means more energy came out than went in (exothermic) and a positive answer means the reaction is endothermic.

4

HintMultiply the bonds in one molecule by the number in front of its formula.

The whyThe equation shows two molecules of water, and each has two O–H bonds, so 2 × 2 = 4 bonds form. In a bond energy calculation the O–H bond energy must therefore be multiplied by four.