Chemistry: Enthalpy Changes

Overview of Enthalpy Changes

Enthalpy is a measure of heat energy in a system. It is represented as H.
Enthalpy change refers to the amount of energy absorbed or released by a reaction at constant pressure.
It’s symbolised as ΔH, where ‘Δ’ indicates change.
When energy is absorbed, the process is endothermic; when energy is released, the process is exothermic.

Types of Enthalpy Changes

Enthalpy change of reaction (ΔHr) is the enthalpy change when the number of moles of reactants react in a chemical equation under standard conditions.
Enthalpy change of formation (ΔHf) is the enthalpy change when one mole of a compound is formed from its elements in their standard states.
Enthalpy change of combustion (ΔHc) is the enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions.
Enthalpy change of neutralisation (ΔHn) is the enthalpy change when an acid and a base react to produce one mole of water under standard conditions.

Measuring Enthalpy Changes

Enthalpy changes are typically determined experimentally using a calorimeter.
Using the formula q = mcΔT, where q represents heat, m is mass, c is specific heat, and ΔT represents change in temperature.
The outcome of measurement is usually reported in joules (J) or kilojoules (kJ).

Hess’s Law

Hess’s Law states that the total enthalpy change for a reaction is independent of the route by which the chemical change occurs.
This is particularly useful when studying reactions that occur in stages, as it allows for calculation of overall enthalpy changes.

Factors Affecting Enthalpy Change

Catalysts can alter the speed of a reaction but do not affect the overall enthalpy change.
Pressure and Temperature: Changes in pressure and temperature can also affect the observed enthalpy change.

Significance of Enthalpy Changes

Understanding enthalpy changes is crucial in various sectors including energy production, chemical synthesis and environmental science.
For example, combustion reactions, central in energy production are assessed based on their enthalpy changes.

Remember the meanings of the types of enthalpy change and recall how to measure enthalpy change in experiments. Understand the application of Hess’s Law and recognise the factors that affect enthalpy change. All these concepts are crucial in understanding the importance of enthalpy changes in real-world applications.