Summary notes for AQA A-Level Chemistry Unit 3.1.8 - Thermodynamics (A-level only) by an Imperial College London MSci Chemistry graduate. Notes divided into the following sections: Types of Enthalpy Changes, Born-Haber Cycles, Gibbs Free-energy Change and Entropy Change
Thermodynamics
Types of Enthalpy Changes
Enthalpy change of formation (∆fHƟ): the enthalpy change when 1 mole of a compound is formed
from its constituent elements in their standard states under standard conditions.
• E.g. 2C(s) + 3H2(g) + ½O2(g) → C2H5OH(l)
• The enthalpy change of formation of an element is zero if the element is in its standard state.
Enthalpy change of combustion (∆cHƟ): the enthalpy change when 1 mole of a substance is
completely burned in O2 under standard conditions w/ all reactants + products in their standard states.
• E.g. CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Bond dissociation enthalpy (∆BDEHƟ): the enthalpy change when 1 mole of covalent bonds is broken
under standard conditions in the gaseous state.
• E.g. Cl2(g) → 2Cl(g)
Enthalpy change of atomisation (∆atHƟ): the enthalpy change for the formation of 1 mole of gaseous
atoms from the element/compound in its standard state.
• E.g. NaCl(s) → Na(g) + Cl(g)
First ionisation enthalpy (∆ie1HƟ): enthalpy change when 1 mole of electrons is removed from 1
mole of gaseous atoms to form one mole of gaseous ions w/ a charge of +1.
• E.g. Na(g) → Na+(g) + e-
Second ionisation enthalpy (∆ie2HƟ): enthalpy change when 1 mole of electrons is removed from 1
mole of gaseous ions w/ a charge of +1 to form one mole of gaseous ions w/ a charge of +2.
• E.g. Na+(g) → Na2+(g) + e-
First electron af nity (∆EA1HƟ): enthalpy change when 1 mole of gaseous atoms form 1 mole of
gaseous ions w/ a charge of -1.
• E.g. O(g) + e- → O-(g)
• Values of rst electron af nities are -ve as the process is exothermic.
- This is due to the net attraction between the nucleus of the atom and the incoming electron.
Second electron af nity (∆EA2HƟ): enthalpy change when 1 mole of gaseous ions w/ a charge of -1
form 1 mole of gaseous ions w/ a charge of -2.
• E.g. O-(g) + e- → O2-(g)
• Values of second electron af nities are +ve as the process is endothermic.
- This is due to the repulsion of the -ve ion for the -ve electron being added.
Enthalpy of lattice dissociation (∆LHƟ): the enthalpy change when 1 mole of an ionic compound is
separated into its constituent gaseous ions.
• E.g. NaCl(s) → Na+(g) + Cl-(g)
Enthalpy of lattice formation (∆LHƟ): the enthalpy change when 1 mole of an ionic compound is
formed from its constituent ions in the gaseous state.
• E.g. Na+(g) + Cl-(g) → NaCl(s)
Enthalpy of hydration (∆hydHƟ): the enthalpy change when 1 mole of gaseous ions is converted into
1 mole of aqueous ions.
• E.g. Na+(g) → Na+(aq)
Enthalpy of Solution
Enthalpy of solution (∆solHƟ): the enthalpy change when 1 mole of an ionic substance dissolves in
enough solvent to form an in nitely dilute solution.
• E.g. NaCl(s) → Na+(aq) + Cl-(aq)
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