Revise Enthalpy change, ΔH for Cambridge 9701 (5.1)

Every substance contains a certain amount of chemical energy stored in its bonds, known as its enthalpy (symbol H). We cannot measure the absolute enthalpy of a substance directly. However, we can measure the enthalpy change (symbol ΔH) that occurs during a chemical reaction. Enthalpy change is defined as the heat energy transferred at constant pressure. The units of ΔH are kilojoules per mole (kJ mol⁻¹).

ΔH = H_{products} - H_{reactants}

Chemical reactions involve the breaking of old bonds and the forming of new ones. The overall energy balance determines whether a reaction is exothermic or endothermic. In an exothermic reaction, more energy is released when forming new bonds in the products than is required to break the bonds in the reactants. This excess energy is released as heat into the surroundings, which get hotter. In an endothermic reaction, more energy is required to break the bonds in the reactants than is released when forming bonds in the products. This energy deficit is absorbed from the surroundings, which get colder.

The amount of heat transferred during a reaction can depend on the temperature, pressure, and concentrations of substances involved. To allow for fair comparisons between different reactions, chemists have agreed upon a set of standard conditions. The enthalpy change measured under these conditions is called the standard enthalpy change, given the symbol ΔH⦵ (pronounced 'delta H standard').

Calorimetry is the experimental technique used to measure the heat energy transferred in a chemical or physical process. A simple calorimeter can be made from a polystyrene cup with a lid, which insulates the system to minimise heat loss. By measuring the temperature change of a known mass of a substance (usually water), we can calculate the heat energy change, q.

q = mcΔT

Here, 'q' is the heat energy change in Joules (J), 'm' is the mass of the substance whose temperature changes (in grams, g), 'c' is the specific heat capacity of that substance (in J g⁻¹ K⁻¹ or J g⁻¹ °C⁻¹), and 'ΔT' is the temperature change (in K or °C). For water, c is 4.18 J g⁻¹ K⁻¹. To find the molar enthalpy change, ΔH, you must then divide q by the number of moles of the limiting reactant and apply the correct sign.