1.8.2Physical Chemistry
Constructing Born–Haber Cycles — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 40 of 63
Part 1 of 5Rules for the Diagram
Rules for the Diagram
Energy axis
- Energy increases up the page, so endothermic steps are drawn as upward arrows and exothermic steps as downward arrows.
Energy levels
- Each species sits on its own horizontal line with full state symbols, so Na(g) and Na⁺(g) are shown on different levels.
Two routes
- The lower route goes from the elements directly to the solid using the formation enthalpy;
- the upper route goes through atoms and ions.
Labels
- Every arrow carries the name of the enthalpy change and its value, and doubled quantities are labelled with the factor of two.
Part 2 of 5Drawing the Cycle for Sodium Chloride
1
Write Na(s) + ½Cl₂(g) on the left about a third of the way up, then draw a line well below it for NaCl(s).
2
Draw an upward arrow to Na(g) + ½Cl₂(g) for the atomisation of sodium, then another upward arrow to Na(g) + Cl(g) for chlorine.
3
Draw an upward arrow from Na(g) to Na⁺(g) for the first ionisation energy, keeping Cl(g) alongside it.
4
Draw a downward arrow from Cl(g) to Cl⁻(g) for the electron affinity, which gives Na⁺(g) + Cl⁻(g) at a slightly lower level.
5
Draw the long downward arrow to NaCl(s) for lattice formation, and the arrow for formation from the elements to complete the loop.
Part 3 of 5Arrow Directions
Atomisation
- Always upwards, since energy is needed to convert a solid or diatomic gas into separate gaseous atoms.
Ionisation
- Always upwards, because every ionisation energy is positive and each one lifts the system to a higher level.
First electron affinity
- Usually downwards, as the atom releases energy on gaining an electron, although the step is small compared with others.
Second electron affinity
- Upwards, because adding a second electron to an anion needs energy to overcome repulsion.
Lattice formation
- Downwards and normally the longest arrow, since it releases a large amount of energy.
| Compound | Extra features to show |
|---|---|
| NaCl | One ionisation and one electron affinity only |
| MgCl₂ | Two ionisation energies; atomisation of two chlorine atoms; two electron affinities |
| MgO | Two ionisation energies; first and second electron affinity of oxygen |
Part 4 of 5Adapting for Other Formulae
Adapting for Other Formulae
Group 2 metals
- Show first and second ionisation energies as two separate upward arrows, giving the 2+ ion step by step.
Two chlorines
- MgCl₂ needs Cl₂(g) split into two Cl(g) atoms, so the arrow is labelled 2 × atomisation of chlorine.
Oxide ions
- Oxygen needs both electron affinities, so draw a downward arrow to O⁻(g) followed by an upward arrow to O²⁻(g).
Check
- The sum of all upward and downward arrows on the long route must equal the single formation arrow.
Part 5 of 5Exam Checklist & Tips
1.8.2 – Key Points for the Exam
- Energy level A horizontal line on the diagram representing the enthalpy of one particular set of species.
- Direct route The single step from elements in standard states to the ionic solid, the enthalpy of formation.
- Indirect route The multi-step path through gaseous atoms and ions that ends at the lattice.
- Ionisation energy Energy needed to remove one mole of electrons from one mole of gaseous atoms or ions.
Common Mistakes
- Omitting state symbols, so that Cl(g) and ½Cl₂(g) cannot be told apart.
- Drawing the electron affinity of oxygen as a single downward arrow instead of a down and an up arrow.
- Forgetting to double the atomisation enthalpy of chlorine in MgCl₂.
- Drawing endothermic steps downwards, which reverses the sign of the final result.
Last updated:
\n