Thermochemistry

Thermochemistry is the branch of chemistry that studies the heat energy released or absorbed during chemical reactions, and how much of it there is. Malay: termokimia · Chinese: 热化学.

Every chemical reaction is accompanied by an energy change. Bonds in the reactants must be broken, which takes in energy, and bonds in the products are formed, which gives out energy. Thermochemistry is the study of the balance between these two, seen from outside the reaction as a rise or a fall in the temperature of the surroundings. When a reaction gives out more energy than it takes in, the surroundings warm up; when it takes in more than it gives out, the surroundings cool down. The whole chapter is built on measuring and explaining that temperature change and turning it into a heat change in kilojoules.

Example. When you add magnesium ribbon to dilute hydrochloric acid in a test tube, the tube becomes warm to the touch. Thermochemistry asks two questions about this: is heat given out or taken in (given out here), and how many kilojoules per mole are involved. Using Q = mcθ on the temperature rise, you can put a number to it.

Confusion to avoid. Do not confuse thermochemistry with the rate of a reaction. Thermochemistry is only about the energy change and its size; how fast the reaction happens is a separate topic. A very slow reaction can still release a large amount of heat overall, and a fast one can release very little.

Thermochemistry is the umbrella for the whole Form 5 chapter, so every later term, exothermic and endothermic reactions, the energy level diagram, and the four heats of reaction, is one part of it. Our teachers keep returning to this definition so that SPM Chemistry answers always name where the heat goes.