Key terms: Thermochemistry

This glossary collects the terms you must be able to define for the Form 5 Thermochemistry chapter, each with a short, exam-ready definition. Learn them in pairs and groups, the exothermic/endothermic pair, the four heats of reaction, and the parts of the calculation, so the meanings reinforce one another. Every heat of reaction is defined per mole; keep that phrase in your definitions.

Core ideas

• Thermochemistry (Termokimia). The study of the heat changes that accompany chemical reactions, whether energy is released to or absorbed from the surroundings, and how much.
• Exothermic reaction (Tindak balas eksotermik). A reaction that releases heat to the surroundings, so the temperature of the mixture rises and the heat change ΔH is negative. Examples: combustion, neutralisation, metal with acid.
• Endothermic reaction (Tindak balas endotermik). A reaction that absorbs heat from the surroundings, so the temperature falls and the heat change ΔH is positive. Examples: thermal decomposition of carbonates, dissolving certain ammonium salts.
• Heat change / enthalpy change (Perubahan haba, ΔH). The heat energy taken in or given out during a reaction, measured in kJ mol⁻¹. It is negative for an exothermic reaction and positive for an endothermic one.
• Surroundings (Persekitaran). Everything around the reacting substances, usually the water or solution and the container, that gains or loses the heat of the reaction. The temperature of the surroundings is what you measure.

Bonds and energy

• Bond breaking (Pemutusan ikatan). The stage of a reaction that absorbs energy. Energy must be supplied to break the bonds in the reactants.
• Bond forming (Pembentukan ikatan). The stage of a reaction that releases energy as new bonds form in the products. A reaction is exothermic when bond forming releases more energy than bond breaking absorbs, and endothermic when it releases less.
• Activation energy (Tenaga pengaktifan). The minimum energy needed to start a reaction, shown as the height of the “hump” from the reactant level to the peak on an energy level diagram. It is not the same as ΔH.

Energy level diagram

• Energy level diagram (Gambar rajah aras tenaga). A diagram with energy on the vertical axis showing the energy of the reactants and of the products, with ΔH marked as the difference between the two levels. For an exothermic reaction the products are lower; for an endothermic reaction they are higher.
• Reactant level and product level (Aras bahan tindak balas dan aras hasil). The two horizontal lines on the diagram. The arrow between them, drawn from reactants to products, represents ΔH, downward and negative for exothermic, upward and positive for endothermic.

The four heats of reaction

• Heat of reaction (Haba tindak balas). The general term for the heat change when a reaction occurs, always quoted per mole of a named substance, in kJ mol⁻¹, with a sign.
• Heat of neutralisation (Haba peneutralan). The heat released when one mole of water is formed from the neutralisation of an acid by an alkali. It is higher for a strong acid and strong alkali (fully ionised) than for a weak acid or alkali (some heat is absorbed to ionise).
• Heat of displacement (Haba penyesaran). The heat released when one mole of a metal is displaced from its salt solution by a more electropositive metal, for example zinc displacing copper from copper(II) sulfate solution.
• Heat of precipitation (Haba pemendakan). The heat change when one mole of a precipitate is formed from its ions in solution.
• Heat of combustion (Haba pembakaran). The heat released when one mole of a substance is completely burnt in excess oxygen. In experiments a fuel heats water in a copper can.

The calculation

• Specific heat capacity (Muatan haba tentu). The heat energy needed to raise the temperature of 1 g of a substance by 1 °C. For aqueous solutions it is taken as 4.2 J g⁻¹ °C⁻¹, the value the question provides.
• Temperature change (Perubahan suhu, θ). The difference between the initial temperature and the highest temperature reached (exothermic) or the lowest (endothermic), in °C. It goes straight into Q = mcθ.
• The heat equation, Q = mcθ (Persamaan haba). The formula for the heat change: Q is the heat in joules, m is the mass of the solution or water in grams, c is the specific heat capacity, and θ is the temperature change. Because the density of a solution is taken as 1 g cm⁻³, a volume in cm³ gives the same number in grams.
• Per mole basis (Asas per mol). The rule that every heat of reaction is quoted for one mole of the named substance. After finding Q you divide by the number of moles, of water formed, metal displaced, precipitate formed, or fuel burnt.
• Complete combustion (Pembakaran lengkap). Burning in a plentiful supply of oxygen so that the fuel is fully oxidised, giving carbon dioxide and water for a hydrocarbon or alcohol. Incomplete combustion releases less heat and lowers an experimental heat of combustion.
• Calorimetry (Kalorimetri). The experimental measurement of heat changes, using apparatus such as a copper can or an insulated polystyrene cup with a thermometer, and applying Q = mcθ to the temperature change.

Using the glossary

Test yourself by writing each definition from memory and checking that the “per mole” phrase and the correct substance are present. Group the four heats of reaction together, because they share one method: Q = mcθ, then divide by the moles. A one-to-one teacher can check that your definitions carry the exact wording the marking scheme wants, which is where quick, easy marks are won in this chapter. Once the terms are secure, use them to read your working aloud, naming the surroundings, the temperature change and the per-mole basis as you go, and the calculations become far harder to get wrong.