AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: define each of the above terms and lattice…
Full specification wording
Born–Haber cycles (A-level only): Students should be able to: define each of the above terms and lattice enthalpy construct Born–Haber cycles to calculate lattice enthalpies using these enthalpy changes construct Born–Haber cycles to calculate one of the other enthalpy changes compare lattice enthalpies from Born–Haber cycles with those from calculations based on a perfect ionic model to provide evidence for covalent character in ionic compounds.
Born-Haber cycles use Hess's Law to calculate lattice enthalpy (enthalpy change when one mole of solid ionic lattice forms from gaseous ions).
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Born-Haber cycles use Hess's Law to calculate lattice enthalpy (enthalpy change when one mole of solid ionic lattice forms from gaseous ions). The cycle links standard enthalpy of formation (one mole of compound formed from elements in standard states) to atomisation (one mole of gaseous atoms formed from an element), ionisation (one mole of electrons removed from gaseous atoms), electron affinity (one mole of electrons added to gaseous atoms), and lattice enthalpy. Example: For NaCl, formation enthalpy equals the sum of the other steps. A theoretical lattice enthalpy assumes a perfect ionic model with undistorted spherical ions. If the experimental value is significantly more exothermic, the extra strength indicates covalent character: the cation polarises the anion's electron cloud.
What earns the marks
- Define lattice enthalpy as the enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions.
- Define standard enthalpy of formation, atomisation, ionisation energy, and electron affinity, specifying one mole and correct state symbols.
- Construct a Born-Haber cycle to calculate lattice enthalpy by applying Hess's Law to sum the constituent enthalpy changes.
- Compare experimental and theoretical lattice enthalpies, stating that a more exothermic experimental value indicates covalent character due to anion polarisation.
Common mistakes and how to put them right
Failing to construct the cycle correctly using Hess's Law.
Put it right by ensuring the sum of clockwise enthalpy changes equals the anticlockwise changes.
Concluding covalent character from a theoretical value being more exothermic.
Put it right by checking that the experimental value is more exothermic.
Describing polarisation backwards; correct by stating the cation polarises the anion.
Exam tips
- When constructing cycles, ensure arrows for endothermic processes point up and exothermic point down.
- State which value is more exothermic before interpreting covalent character.
Try the lesson
The start of the Born–Haber cycles (A-level only): Students should be able to: define each of the above terms and lattice enthalpy construct Born–Haber cycles to calculate lattice enthalpies using these enthalpy changes construct Born–Haber cycles to calculate one of the other enthalpy changes compare lattice enthalpies from Born–Haber cycles with those from calculations based on a perfect ionic model to provide evidence for covalent character in ionic compounds. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement defines the standard lattice enthalpy of formation of an ionic compound?
Listen
Born-Haber cycles use Hess's Law to calculate lattice enthalpy (enthalpy change when one mole of solid ionic lattice forms from gaseous ions). The cycle links standard enthalpy of formation (one mole of compound formed from elements in standard states) to atomisation (one mole of gaseous atoms formed from an element), ionisation (one mole of electrons removed from gaseous atoms), electron affinity (one mole of electrons added to gaseous atoms), and lattice enthalpy. Example: For NaCl, formation enthalpy equals the sum of the other steps. A theoretical lattice enthalpy assumes a perfect ionic model with undistorted spherical ions. If the experimental value is significantly more exothermic, the extra strength indicates covalent character: the cation polarises the anion's electron cloud. A strong answer earns marks for these points. Define lattice enthalpy as the enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions. Define standard enthalpy of formation, atomisation, ionisation energy, and electron affinity, specifying one mole and correct state symbols. Construct a Born-Haber cycle to calculate lattice enthalpy by applying Hess's Law to sum the constituent enthalpy changes. Compare experimental and theoretical lattice enthalpies, stating that a more exothermic experimental value indicates covalent character due to anion polarisation. A common slip: Failing to construct the cycle correctly using Hess's Law. Instead, ensuring the sum of clockwise enthalpy changes equals the anticlockwise changes. Exam tip: When constructing cycles, ensure arrows for endothermic processes point up and exothermic point down.
Key words
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Lattice enthalpy
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Perfect ionic model
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Covalent character
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Born–Haber cycle
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See every definition
- Lattice enthalpy
- The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions.
- Perfect ionic model
- A theoretical model that assumes undistorted spherical ions, used to calculate a theoretical lattice enthalpy.
- Covalent character
- Indicated when the experimental lattice enthalpy is significantly more exothermic than the theoretical value, as the extra strength shows the cation polarises the anion's electron cloud.
- Born–Haber cycle
- A cycle that applies Hess's Law to sum the constituent enthalpy changes to calculate lattice enthalpy or one of the other enthalpy changes.
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Test your understanding of Born–Haber cycles (A-level only): Students should be able to: define each of the above terms and lattice enthalpy construct Born–Haber cycles to calculate lattice enthalpies using these enthalpy changes construct Born–Haber cycles to calculate one of the other enthalpy changes compare lattice enthalpies from Born–Haber cycles with those from calculations based on a perfect ionic model to provide evidence for covalent character in ionic compounds.
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