Born-Haber cycles Flashcards
(10 cards)
Definition of lattice energy/enthalpy
Energy change when one mole of an ionic solid is formed from its gaseous ions
e.g.
Na⁺(g) + Cl⁻(g) –> NaCl(s)
Definition of enthalpy change of atomisation (ΔatH°)
The enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions
e.g.
½ Cl₂(g) → Cl(g) ΔₐₜH
Definition of electron affinity
The enthalpy change when one mole of electrons is added to one mole of gaseous atoms to form one mole of gaseous negative ions
e.g.
Cl(g) + e⁻ → Cl⁻(g)
The steps in a born haber cycle (starting at the solid compound)
- Enthalpy of formation (pointing towards the solid)
- Atomise the elements
- Ionise the metal atom
- Electron affinity for non-metal atom
- Form the Ionic Solid (Lattice Formation) -Combine the gaseous ions into a solid lattice
What does lattice energy (gaseous ions into a solid lattice) prodvide a measure for?
lattice energy provides a measure of ionic bond strength
What do theoretical values of lattice enthalpy/energy assume?
theoretical values of lattice enthalpy/energy assume a purely ionic model
What does a large difference between theoretical and experimental values for lattice enthalpy indicate?
With small highly charged cations and large anions a degree of polarisation occurs
meaning the difference between the theoretical and experimental value for lattice enthalpy will be larger
Indicating it has a degree of covalency
What is polarisation?
Polarisation is when small highly charged cations distort (attracts the electron clouds of) large highly charged anions leading to a degree of covalency.
What is the polarising power of a cation dependent on?
- radius - stronger charge density, more polarising
- charge - larger charge = stronger attractive force
What is the polarisability of an anion dependent on?
- radius - larger radius = electron cloud more easily distorted
- charge - higher charge = more electrons, repel each other, easier to distort
