4.2.6Additional lessons
Born-Haber Cycles (HL) — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 78 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 4Enthalpy Terms in the Cycle
IB defines lattice enthalpy as the breaking of the lattice, so values are positive, unlike some other courses.
Atomisation
- Atomisation ΔHatom⦵ is the enthalpy change to form one mole of gaseous atoms from the element in its standard state;
- always endothermic.
First ionisation energy
- First ionisation energy The energy to remove one mole of electrons from one mole of gaseous atoms, giving gaseous 1+ ions;
- endothermic.
Electron affinity
- Electron affinity The first electron affinity is the enthalpy change when one mole of gaseous atoms each gain an electron;
- usually exothermic.
Second electron affinity
- Second electron affinity Adding a second electron to a negative ion is endothermic because the incoming electron is repelled by the ion.
Lattice enthalpy
- Lattice enthalpy In IB, the enthalpy change when one mole of solid ionic compound separates into gaseous ions;
- endothermic and positive.
Section 2 of 4Building the Cycle for NaCl
1
Start with the elements in their standard states, Na(s) and ½Cl₂(g), at the lower level of the diagram.
2
Atomise sodium to give Na(g), then atomise chlorine to give Cl(g), with both steps shown as upward arrows.
3
Ionise Na(g) to Na⁺(g) using the first ionisation energy, which is another upward step.
4
Add an electron to Cl(g) to give Cl⁻(g) using the electron affinity, which is a small downward step.
5
Combine the gaseous ions into solid NaCl, shown as the formation of the lattice, and compare with direct formation from elements.
Section 3 of 4Why the Cycle Matters
Why the Cycle Matters
Hess's law
- Hess's law The direct formation enthalpy equals the sum of all steps in the indirect route through gaseous atoms and ions.
Cannot measure directly
- Cannot measure directly Lattice enthalpy cannot be found by experiment alone, so it is calculated from the cycle.
Charge and size
- Charge and size Higher ion charges and smaller ionic radii give stronger attraction and larger lattice enthalpy values.
Theory versus experiment
- Theory versus experiment Calculated values from an ideal ionic model differ from cycle values when bonding has covalent character.
| Ion pair | Lattice enthalpy trend |
|---|---|
| NaCl | Lower: singly charged ions |
| MgO | Much higher: doubly charged and smaller ions |
Section 4 of 4Exam Checklist & Tips
4.2.6 – Key Points for the Exam
- Lattice enthalpy Enthalpy change when one mole of a solid ionic compound separates into its gaseous ions.
- Atomisation enthalpy Enthalpy change to produce one mole of gaseous atoms from an element in its standard state.
- Electron affinity Enthalpy change when gaseous atoms gain electrons to form gaseous negative ions.
- Born-Haber cycle A Hess's law energy cycle linking formation of an ionic solid with its gaseous ions.
Common Mistakes
- Writing lattice enthalpy as the energy released on forming the lattice when the IB definition describes its breaking.
- Treating the first electron affinity as endothermic, when it is usually exothermic for non-metals.
- Forgetting to atomise ½Cl₂(g) and using the bond enthalpy of the whole molecule.
- Mixing up the order of the steps so that ionisation happens before atomisation.
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