What a learner can do afterwards
- Puts the steps of a Born-Haber cycle in order and gives the sign of each
- Calculates a lattice enthalpy from the other terms in the cycle
- Explains why lattice enthalpy gets more exothermic for smaller ions and for larger charges
- Compares a cycle value with one calculated from a perfect ionic model and says what a gap suggests about the bonding
1 · Read
Lattice enthalpy is the energy released when one mole of solid forms from separate gaseous ions. The more exothermic the value, the stronger the lattice holds together. You cannot measure it directly, so you find it with a cycle.
A Born-Haber cycle splits formation into steps: atomisation of each element, ionisation of the metal, electron affinity of the non metal, then lattice formation. The steps must add to the overall formation enthalpy, which is Hess law applied to ionic compounds.
Signs are predictable. Atomisation and ionisation take energy in, so they are positive. Electron affinity and lattice formation usually release energy, so they are negative. Know every step but one and you can calculate the missing value.
Small highly charged ions pull hardest, so magnesium oxide far exceeds sodium chloride. If the cycle value disagrees with a perfect ionic model, the gap points to covalent character with shared electron density.
Build the cycle, balance the signs, solve for the missing step, and read size and charge for the trend.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.