Count first, calculate second
Before you touch the numbers, count the ions in one mole of solid. MgCl2 needs one Mg2+ ion and two Cl− ions, so the cycle uses IE1 + IE2 for magnesium, 2 × chlorine atomisation and 2 × EA1.
Year 13 · AQA & OCR A
Born–Haber cycles
Practise Born–Haber cycles for 1:1, 1:2 and 2:1 salts, including missing enthalpy terms and second electron affinity.
Find the missing term
Count the ions and required steps first, then work from the data provided. Open a clue or worked answer when you need one.
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Choose every step that one mole of the solid needs
Set the coefficient on each required term. Distractors are mixed in, so count the ions before you click.
Second electron affinity changes the sign pattern
EA1 for oxygen is exothermic. EA2 is endothermic because an electron is being added to O−(g). Enter each electron affinity with the sign given for that step.
AQA: read the lattice definition
AQA may give lattice enthalpy of formation or lattice enthalpy of dissociation. Read the definition before using the value; the two directions have opposite signs. This practice states the convention every time.
OCR A: use lattice formation
OCR A defines lattice enthalpy as the enthalpy change when one mole of ionic lattice forms from its gaseous ions. The OCR route therefore keeps the formation convention throughout.