Count first, calculate second
For MgCl2, one mole of solid needs one Mg2+ ion and two Cl− ions. That means IE1 + IE2 for magnesium, but 2 × chlorine atomisation and 2 × EA1.
Year 13 · AQA & OCR A
Born–Haber cycles
Build the route from elements to gaseous ions, then use Hess's law to find the missing enthalpy. The practice includes 1:1, 1:2 and 2:1 salts, second electron affinity and the lattice-sign convention your board expects.
Find the missing term
Nothing is drawn for you first. Build the Hess relationship yourself; ask for a clue only if you need one.
0first-time correct0completed
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). Do not copy the sign of EA1 onto EA2.
AQA: read the lattice definition
AQA may give lattice enthalpy of formation or lattice enthalpy of dissociation. They describe opposite directions, so their signs are opposite. 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.