Born-Haber lattice enthalpy
sum of enthalpy changes around a Hess cycle = 0
Give every cycle arrow its sign; electron affinity is often already negative.
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
For NaCl(s): enthalpy of formation = -411, Na atomisation = +108, half Cl-Cl bond dissociation = +121, first ionisation energy of Na = +496 and first electron affinity of Cl = -349 kJ/mol. Calculate the lattice enthalpy of formation.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Changing the supplied sign of electron affinity.
- Using lattice dissociation enthalpy when the question asks for lattice formation enthalpy.
Exam tip: Write one algebraic Hess equation from gaseous atoms and ions to the solid before substituting values.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.