Thermodynamic Terms (AQA A-Level Chemistry): Revision Notes

5.1.1 Thermodynamic Terms

This section introduces key thermodynamic terms used to describe energy changes in chemical reactions. Understanding these terms is crucial for analysing reaction energetics, calculating enthalpy changes, and understanding factors that influence reaction feasibility.

Standard Conditions

• Standard Conditions: 100 kPa pressure and a temperature of 298 K (25°C).
• Standard States: The most stable physical form of an element or compound under standard conditions.

Enthalpy ($ΔH$)

Enthalpy is a measure of the total energy in a thermodynamic system, often reflecting heat changes at constant pressure.

Types of Enthalpy Changes

Enthalpy Change of Formation ($ΔfH⁰$)

The enthalpy change when 1 mole of a compound forms from its elements in their standard states, under standard conditions.

Enthalpy Change of Combustion ($ΔcH⁰$)

The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions, with all substances in their standard states.

Bond Dissociation Enthalpy ($ΔBDEH⁰$)

The enthalpy change when 1 mole of covalent bonds is broken in the gaseous state under standard conditions.

Enthalpy Change of Atomisation ($ΔatH⁰$)

The enthalpy change for forming 1 mole of gaseous atoms from an element or compound in its standard state.

Ionisation Enthalpy

First Ionisation Enthalpy ($Δie1H⁰$)

The enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms to form gaseous ions with a +1 charge.

Second Ionisation Enthalpy ($Δie2H⁰$)

The enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous ions with a +1 charge to form gaseous ions with a +2 charge.

Electron Affinity ($ΔEAH⁰$)

First Electron Affinity ($ΔEA1H⁰$)

The enthalpy change when 1 mole of gaseous atoms gains 1 mole of electrons to form ions with a -1 charge.

• Note: First electron affinities are typically negative (exothermic).

Second Electron Affinity ($ΔEA2H⁰$)

The enthalpy change when 1 mole of gaseous ions with a -1 charge gains 1 mole of electrons to form ions with a -2 charge.

• Note: Second electron affinities are positive (endothermic), as the incoming electron is repelled by the negative ion.

Lattice Enthalpy ($ΔLH⁰$)

Enthalpy of Lattice Dissociation ($ΔLHD⁰$)

The enthalpy change when 1 mole of an ionic compound is separated into its constituent gaseous ions.

Enthalpy of Lattice Formation ($ΔLHF⁰$)

The enthalpy change when 1 mole of an ionic compound forms from its gaseous ions.

Enthalpy of Hydration ($ΔhydH⁰$)

The enthalpy change when 1 mole of gaseous ions is converted into 1 mole of aqueous ions.