9701 · 23.1
Common exam mistakes on 9701 Lattice energy and Born-Haber cycles. Learn what loses marks, then practise the topic with Examiner’s Ink.
Pay close attention to the direction of arrows in a Born-Haber cycle diagram. Upward arrows represent endothermic processes (energy input), while downward arrows represent exothermic processes (energy release). The sum of clockwise energy changes must equal the sum of anticlockwise energy changes.
When asked to compare lattice energies, always discuss both ionic charge and ionic radius. Charge is almost always the more significant factor. A perfect answer will state which factor is dominant.
They are the reverse of each other. Lattice energy (as defined by Cambridge International) is the enthalpy change when 1 mole of solid is formed from its gaseous ions (exothermic, negative value). Lattice dissociation enthalpy is the enthalpy change to break 1 mole of solid into its gaseous ions (endothermic, positive value). They have the same magnitude but opposite signs.
The first electron affinity (O(g) + e⁻ → O⁻(g)) is exothermic. However, to form the O²⁻ ion, you must add a second electron to the already negative O⁻ ion (O⁻(g) + e⁻ → O²⁻(g)). There is strong electrostatic repulsion between the negative electron and the negative ion, so energy must be supplied to force the electron on. This makes the process endothermic.
No, you cannot. The concept of a Born-Haber cycle is fundamentally based on the formation of an ionic lattice from gaseous ions. Covalent compounds like methane consist of discrete molecules held together by weak intermolecular forces in the solid state, not a lattice of ions. The energy terms, such as lattice energy and electron affinity, are not applicable in the same way.
It is practically impossible to perform an experiment that directly measures the process of gaseous ions coming together to form a solid lattice. We cannot easily create a gas of just cations and anions and measure the heat released as they combine. The Born-Haber cycle is a clever application of Hess's Law that allows us to calculate this value using other enthalpy changes that can be measured experimentally.