4.3.1Unit 4. Rates, Equilibria and Further Organic Chemistry
Born-Haber Cycles – Key Terms — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 23 of 56 Unit quiz
Part 1 of 5Formation and Atomisation
A Born–Haber cycle uses a series of defined steps, so each term must be known precisely.
Standard enthalpy of formation
- The enthalpy change when one mole of a compound forms from its elements in their standard states under standard conditions.
Enthalpy of atomisation
- The enthalpy change when one mole of gaseous atoms forms from an element in its standard state, always endothermic.
Bond dissociation enthalpy
- The enthalpy change when one mole of a covalent bond is broken in gaseous molecules,
- so atomisation of Cl₂ uses half its bond enthalpy.
Sign
- Atomisation and bond breaking take in energy, so their enthalpy changes are positive.
Part 2 of 5Ionisation and Electron Affinity
Ionisation and Electron Affinity
First ionisation energy
- The enthalpy change when one mole of gaseous atoms loses one mole of electrons to form one mole of gaseous 1+ ions;
- always endothermic.
Second ionisation energy
- The enthalpy change to remove one mole of electrons from one mole of gaseous 1+ ions, and it is larger than the first.
First electron affinity
- The enthalpy change when one mole of gaseous atoms gains one mole of electrons to form 1− ions;
- usually exothermic.
Second electron affinity
- Adding an electron to a 1− ion is endothermic because of repulsion, as for O⁻ becoming O²⁻.
| Term | Typical sign |
|---|---|
| Atomisation | Positive |
| Ionisation energy | Positive |
| First electron affinity | Negative |
| Lattice energy of formation | Negative |
Part 3 of 5Lattice Energy
1
Lattice energy of formation is the enthalpy change when one mole of an ionic solid forms from its gaseous ions.
2
Because strong electrostatic attractions form, this process is always exothermic.
3
Some courses define lattice energy as the opposite process, the breaking of the lattice, and it would then have the opposite sign.
4
Always check the definition used in a question and the sign convention before calculating.
5
The size of lattice energy depends on the charges and radii of the ions.
Part 4 of 5Factors Affecting Lattice Energy
Ionic charge
- Larger charges give stronger attractions, so MgO has a much more exothermic lattice energy than NaCl.
Ionic radius
- Smaller ions can get closer together, so the attraction is stronger and lattice energy is more exothermic.
Trend
- Lattice energy becomes less exothermic down a group because the ions get larger.
Comparison
- Compare ionic charge first and then size when ranking lattice energies.
Part 5 of 5Exam Checklist & Tips
4.3.1 – Key Points for the Exam
- Lattice energy of formation The enthalpy change when one mole of an ionic solid forms from its gaseous ions.
- Atomisation enthalpy The enthalpy change to form one mole of gaseous atoms from an element in its standard state.
- First ionisation energy The enthalpy change to remove one electron from each atom in one mole of gaseous atoms.
- First electron affinity The enthalpy change when one mole of gaseous atoms each gain one electron.
- Standard state The most stable physical form of a substance at 100 kPa and a stated temperature, usually 298 K.
Common Mistakes
- Writing the electron affinity for the second electron as exothermic.
- Using the bond enthalpy of Cl₂ directly as the atomisation enthalpy of chlorine without halving it.
- Omitting 'gaseous' from the definitions of ionisation energy and electron affinity.
- Mixing up the lattice energy of formation with the lattice energy of dissociation.
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