Thermodynamic Terms
Lajoy Tucker & Dr. Davinder Bhachu
Teachers
Born-Haber Cycles Explainer Video
Thermodynamic Terms
1. Enthalpy of formation
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions and .
Exothermic for most compounds, as bonds are formed.
Example:
2. Enthalpy of combustion
The enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
Exothermic due to formation of strong bonds in and .
Example:
3. Enthalpy of neutralisation
The enthalpy change when 1 mole of water is formed from the reaction of an acid and a base under standard conditions.
Exothermic due to formation of bonds in water.
Example:
For strong acids and bases, the value is typically
4. Ionisation enthalpy
First ionisation enthalpy
The enthalpy change when 1 mole of gaseous atoms loses 1 electron per atom to form 1 mole of gaseous ions.
Endothermic as energy is required to overcome nuclear attraction.
Example:
Second ionisation enthalpy
The enthalpy change when 1 mole of gaseous ions loses 1 electron per ion to form 1 mole of gaseous ions.
Endothermic – more energy is required than the first ionisation enthalpy because the electron is being removed from a positively charged ion, which has a stronger attraction to its remaining electrons.
Example:
5. Electron affinity
The enthalpy change when 1 mole of gaseous atoms gains electrons to form ions.
First EA is exothermic because energy is released when the atom gains an electron.
Second EA is endothermic because energy is needed to overcome the repulsion between the negatively charged ion and the incoming electron.
Examples:
6. Enthalpy of atomisation
The enthalpy change when 1 mole of gaseous atoms is produced from an element in its standard state.
Endothermic as bonds must be broken.
Example:
7. Hydration enthalpy
The enthalpy change when 1 mole of gaseous ions dissolves in water to form aqueous ions.
Exothermic as ion–dipole bonds form between ions and water.
Example:
More charge-dense ions (e.g. ) have more exothermic hydration enthalpies.
8. Enthalpy of solution
The enthalpy change when 1 mole of an ionic solid dissolves in water to give separated, hydrated ions.
Varies – depends on balance of lattice enthalpy (endothermic) and hydration enthalpies (exothermic).
Example:
9. Bond dissociation enthalpy
The enthalpy change when 1 mole of a specific covalent bond is broken in the gaseous state.
Endothermic as energy is required to break bonds.
Example:
This is sometimes given as a mean value across similar compounds.
10. Lattice enthalpy of formation
The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions.
Exothermic due to strong electrostatic attractions in the ionic lattice.
Example: More charge-dense ions result in more negative lattice enthalpies.
11. Lattice enthalpy of dissociation
The enthalpy change when 1 mole of an ionic lattice is broken into its gaseous ions.
Endothermic as strong ionic bonds must be overcome.
Example:
12. Enthalpy of vaporisation
The enthalpy change when 1 mole of a liquid is converted into a gas under standard conditions.
Endothermic due to overcoming intermolecular forces.
Example:
13. Enthalpy of fusion
The enthalpy change when 1 mole of a solid becomes a liquid under standard conditions.
Endothermic as particles must gain energy to overcome some forces.
Example:
Checkpoint Questions
1. Write an equation for the standard enthalpy of formation of aluminium oxide.
2. Write an equation for the standard enthalpy of combustion of methanol, CH₃OH(l).
3. Write an equation for the enthalpy of neutralisation between nitric acid and calcium hydroxide.
4. Write an equation for the first ionisation enthalpy of potassium.
5. Write an equation for the second ionisation enthalpy of magnesium.
6. Write an equation for the first electron affinity of sulfur.
7. Write an equation for the second electron affinity of sulfur.
8. Write an equation for the enthalpy of atomisation of chlorine.
9. Write an equation for the hydration enthalpy of lithium ions.
10. Write an equation for the enthalpy of solution of calcium nitrate.
11. Write an equation for the bond dissociation enthalpy of hydrogen chloride.
12. Write an equation for the lattice enthalpy of formation of barium fluoride.
13. Write an equation for the lattice enthalpy of dissociation of magnesium nitride.
14. Write an equation for the enthalpy of vaporisation of ethanol.
15. Write an equation for the enthalpy of fusion of lead.
16. Write an equation for the third ionisation enthalpy of aluminium.
17. Write an equation for the bond dissociation enthalpy of fluorine.
18. Write an equation for the enthalpy of neutralisation between and ammonia.
19. Write an equation for the standard enthalpy of formation of potassium carbonate.
20. Write an equation for the standard enthalpy of combustion of propane.
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