Ionic bonding: theoretical and experimental lattice energyEdexcel A-Level Chemistry: Revision notes
Section 1
Lattice energy and ionic bond strength
Lattice energy (Edexcel) is the enthalpy change when one mole of an ionic solid is formed from its gaseous ions, for example Na⁺(g) + Cl⁻(g) → NaCl(s). It is always exothermic (negative).
The more exothermic the lattice energy, the stronger the ionic bonding, because more energy is released as oppositely charged ions come together. The value depends on ionic charge (higher charge, more exothermic) and radius (smaller ions, more exothermic).
Do not confuse the lattice energy (gaseous ions to solid, exothermic) with the enthalpy of formation (elements in their standard states to compound).
Section 2
Two ways to find a lattice energy
The experimental lattice energy cannot be measured directly. It is found using a Born-Haber cycle: Hess's law applied to measured enthalpy changes (formation, atomisation, ionisation energies, electron affinities).
The theoretical lattice energy is calculated from electrostatic theory. This purely ionic model assumes that the ions are perfect spheres with charge spread evenly, behaving as point charges.
Section 3
Comparing experimental and theoretical values
If the experimental and theoretical values agree closely, the compound is behaving as purely ionic (e.g. NaCl: −787 and −770 kJ mol⁻¹).
If the experimental value is more exothermic than the theoretical value, there is an additional covalent contribution to the bonding (e.g. AgI: −889 and −778 kJ mol⁻¹). The bigger the difference, the greater the degree of covalent character.
Use the words: 'experimental more exothermic than theoretical, so extra covalent bonding'. Say which value is larger and why.
Section 4
Polarisation of ions
Polarisation is the distortion of the electron cloud of an anion by a neighbouring cation. The cation attracts the anion's electrons towards itself, so some electron density is found between the two nuclei: this is a partial covalent bond.
The more the anion is polarised, the greater the covalent character.
Section 5
Polarising power and polarisability
The polarising power of a cation depends on its radius and charge: a small, highly charged cation has a high charge density and a high polarising power (e.g. Al³⁺ > Mg²⁺ > Na⁺).
The polarisability of an anion depends on its radius and charge: a large, highly charged anion has outer electrons far from the nucleus, weakly held, so it is easily distorted (e.g. I⁻ > Cl⁻ > F⁻; S²⁻ > O²⁻).
Most covalent character = small, highly charged cation + large, highly charged anion.
Cations polarise and anions are polarised. Do not say that an anion has 'polarising power' or that a cation is 'polarisable' in this context.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Ionic bonding: theoretical and experimental lattice energy
- A chemist is comparing the strength of the ionic bonding in sodium chloride and magnesium oxide. She uses lattice energy values, defined as the enthalpy change when one mole of an ionic solid is formed from its gaseous ions.The lattice energy of magnesium oxide is much more exothermic than that of sodium chloride. State what this shows about the ionic bonding in magnesium oxide and explain why.2 marks
- A teacher asks a class to predict which ionic compounds will show the most covalent character. The students must decide which cations are best at distorting the electron cloud of a neighbouring anion, and which anions are most easily distorted.Explain what is meant by the polarisation of an anion.2 marks
- A data table gives, for three silver and sodium compounds, the lattice energy from a Born-Haber cycle (experimental) and the lattice energy calculated using a purely ionic model (theoretical). Experimental and theoretical values in kJ mol⁻¹ are: sodium chloride, −787 and −770; silver chloride, −905 and −833; silver iodide, −889 and −778.Calculate the difference between the experimental and theoretical lattice energy for each compound, and deduce which compound has the greatest covalent character. Justify your answer.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).