Chapter 6: Enthalpy Changes

Chemical processes are categorized as exothermic if they release heat, causing the temperature of the surroundings to rise, which is indicated by a negative enthalpy change value. Examples include combustion and respiration. Conversely, endothermic reactions absorb heat from the surroundings, resulting in a temperature decrease and a positive enthalpy change value. Energy transfers during reactions are fundamentally explained by the breaking and forming of chemical bonds: bond breaking is an endothermic process requiring energy input, while bond formation is an exothermic process releasing energy. The overall enthalpy change depends on whether the energy released during bond making is greater or lesser than the energy absorbed during bond breaking. These energy relationships, along with the minimum energy required for a reaction to occur, known as the activation energy (EA), are visualized using reaction pathway diagrams. To ensure fair comparison of results, enthalpy changes are often measured under standard conditions, defined as a pressure of 101 kPa, a temperature of 298 K, with all substances in their normal physical states. The chapter details various types of standard enthalpy changes, including the standard enthalpy change of reaction, formation (forming one mole of a compound from its elements), combustion (burning one mole in excess oxygen), and neutralisation (forming one mole of water from an acid and alkali). Experimental determination of these heat transfers is carried out using calorimetry, where the heat transferred (q) is calculated using the formula q=mcΔT, where m is the mass of the liquid, c is the specific heat capacity, and ΔT is the temperature change. This result is then scaled up to find the molar enthalpy change using the relationship ΔH=−mcΔT. For reactions that cannot be directly measured, Hess’s Law is applied; this law asserts that the total enthalpy change is independent of the route taken, allowing unknown values to be calculated indirectly through the construction of energy cycles from known data. Lastly, enthalpy changes can also be estimated using bond energies (or bond dissociation energies), which represent the energy required to break specific bonds. Because bond strength can vary based on the molecular environment, average bond energies derived from different molecules are commonly used for calculations.