Surroundings

In thermochemistry the surroundings are everything around the reacting substances, usually the water or solution and the container, that gains or loses the heat of the reaction. Malay: persekitaran · Chinese: 周围.

The reaction itself is called the system; everything outside it that can exchange heat with it is the surroundings. When an exothermic reaction releases heat, that heat passes into the surroundings and their temperature rises. When an endothermic reaction absorbs heat, it takes that heat from the surroundings and their temperature falls. Because you cannot put a thermometer inside the bonds, you measure the temperature of the surroundings instead, and read the reaction’s energy change from how that temperature moves.

Example. In a neutralisation experiment the acid and alkali form the system, while the water they are dissolved in, together with the polystyrene cup, are the surroundings. The heat released by the reaction warms this water, and it is the water’s mass and temperature rise that you put into Q = mcθ. That is why you take the mass of the solution, not the mass of the reacting ions, as m.

Confusion to avoid. Do not confuse the surroundings with the system. The chemicals that react are the system; the water and container that carry the heat away are the surroundings. Mixing them up leads to using the wrong mass in Q = mcθ, a common lost mark.

Naming the surroundings correctly is the key to every calculation in the chapter, because m in the heat equation is always the mass of the surroundings that changed temperature. Our teachers ask students to point out the surroundings before writing any working, so SPM Chemistry calculations start from the right quantity.