Chemical reactions, Year 10: energy changes

The products have less, by the amount transferred

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

为什么Energy 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

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

为什么When 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

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

为什么A 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

提示Its prefix is the one found in 'exit' and 'exhale'.

为什么Combustion 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

提示The thermometer can only record what stays in the liquid.

为什么Some 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

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

为什么While 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

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

为什么Particles 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

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

为什么If 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

提示It measures the height of the barrier that has to be climbed.

为什么The 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

提示It compares where the chemicals start with where they finish.

为什么The 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

提示There is a minimum that every successful collision has to reach.

为什么A 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

提示Think about pulling apart two magnets that are stuck together.

为什么Energy 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

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

为什么Breaking 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

提示Compare the amount given out with the amount taken in.

为什么More 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

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

为什么The 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

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

为什么The 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.