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 analyzing 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.

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Example: For ethanol formation:

2C(s) + 3H_2(g) + \frac{1}{2}O_2(g) \rightarrow C_2H_5OH(l)

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.

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Example: Combustion of methane:

CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)

Bond Dissociation Enthalpy ( $ΔBDEH⁰$ )

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

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Example: Dissociation of chlorine gas:

Cl_2(g) \rightarrow 2Cl(g)

Enthalpy Change of Atomisation ( $ΔatH⁰$ )

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

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Example: Atomisation of sodium chloride:

NaCl(s) \rightarrow Na(g) + Cl(g)

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.

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Example:

Na(g) \rightarrow Na^+(g) + e^-

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.

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Example:

Na^+(g) \rightarrow Na^{2+}(g) + e^-

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).

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Example:

O(g) + e^- \rightarrow O^-(g)

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.

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Example:

O^-(g) + e^- \rightarrow O^{2-}(g)

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.

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Example:

NaCl(s) \rightarrow Na^+(g) + Cl^-(g)

Enthalpy of Lattice Formation ( $ΔLHF⁰$ )

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

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Example:

Na^+(g) + Cl^-(g) \rightarrow NaCl(s)

Enthalpy of Hydration ( $ΔhydH⁰$ )

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

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Example:

Na^+(g) \rightarrow Na^+(aq)

Thermodynamic Terms Simplified Revision Notes for A-Level AQA Chemistry

5.1.1 Thermodynamic Terms

Standard Conditions

Enthalpy ( $ΔH$ )

Types of Enthalpy Changes

Enthalpy Change of Formation ( $ΔfH⁰$ )

Enthalpy Change of Combustion ( $ΔcH⁰$ )

Bond Dissociation Enthalpy ( $ΔBDEH⁰$ )

Enthalpy Change of Atomisation ( $ΔatH⁰$ )

Ionisation Enthalpy

First Ionisation Enthalpy ( $Δie1H⁰$ )

Second Ionisation Enthalpy ( $Δie2H⁰$ )

Electron Affinity ( $ΔEAH⁰$ )

First Electron Affinity ( $ΔEA1H⁰$ )

Second Electron Affinity ( $ΔEA2H⁰$ )

Lattice Enthalpy ( $ΔLH⁰$ )

Enthalpy of Lattice Dissociation ( $ΔLHD⁰$ )

Enthalpy of Lattice Formation ( $ΔLHF⁰$ )

Enthalpy of Hydration ( $ΔhydH⁰$ )

500K+ Students Use These Powerful Tools to Master Thermodynamic Terms For their A-Level Exams.

Other Revision Notes related to Thermodynamic Terms you should explore

Thermodynamic Terms Simplified Revision Notes for A-Level AQA Chemistry

5.1.1 Thermodynamic Terms

Standard Conditions

Enthalpy (ΔHΔHΔH)

Types of Enthalpy Changes

Enthalpy Change of Formation (ΔfH0ΔfH⁰ΔfH0)

Enthalpy Change of Combustion (ΔcH0ΔcH⁰ΔcH0)

Bond Dissociation Enthalpy (ΔBDEH0ΔBDEH⁰ΔBDEH0)

Enthalpy Change of Atomisation (ΔatH0ΔatH⁰ΔatH0)

Ionisation Enthalpy

First Ionisation Enthalpy (Δie1H0Δie1H⁰Δie1H0)

Second Ionisation Enthalpy (Δie2H0Δie2H⁰Δie2H0)

Electron Affinity (ΔEAH0ΔEAH⁰ΔEAH0)

First Electron Affinity (ΔEA1H0ΔEA1H⁰ΔEA1H0)

Second Electron Affinity (ΔEA2H0ΔEA2H⁰ΔEA2H0)

Lattice Enthalpy (ΔLH0ΔLH⁰ΔLH0)

Enthalpy of Lattice Dissociation (ΔLHD0ΔLHD⁰ΔLHD0)

Enthalpy of Lattice Formation (ΔLHF0ΔLHF⁰ΔLHF0)

Enthalpy of Hydration (ΔhydH0ΔhydH⁰ΔhydH0)

500K+ Students Use These Powerful Tools to Master Thermodynamic Terms For their A-Level Exams.

Other Revision Notes related to Thermodynamic Terms you should explore

Enthalpy ( $ΔH$ )

Enthalpy Change of Formation ( $ΔfH⁰$ )

Enthalpy Change of Combustion ( $ΔcH⁰$ )

Bond Dissociation Enthalpy ( $ΔBDEH⁰$ )

Enthalpy Change of Atomisation ( $ΔatH⁰$ )

First Ionisation Enthalpy ( $Δie1H⁰$ )

Second Ionisation Enthalpy ( $Δie2H⁰$ )

Electron Affinity ( $ΔEAH⁰$ )

First Electron Affinity ( $ΔEA1H⁰$ )

Second Electron Affinity ( $ΔEA2H⁰$ )

Lattice Enthalpy ( $ΔLH⁰$ )

Enthalpy of Lattice Dissociation ( $ΔLHD⁰$ )

Enthalpy of Lattice Formation ( $ΔLHF⁰$ )

Enthalpy of Hydration ( $ΔhydH⁰$ )