What is the sign of lattice energy, and what does a more negative value show?

It is exothermic (negative). The more exothermic, the stronger the ionic bonding.

Write the equation for the lattice energy of NaCl.

Na⁺(g) + Cl⁻(g) → NaCl(s)

How is the experimental lattice energy found?

From a Born-Haber cycle, using Hess's law and measured enthalpy changes.

How is the theoretical lattice energy found?

By calculation from electrostatic theory, assuming a purely ionic model.

What does the purely ionic model assume?

Ions are perfect spheres with charge evenly distributed, held only by electrostatic attraction.

What does close agreement of experimental and theoretical values show?

The bonding is (almost) purely ionic.

What does an experimental value more exothermic than the theoretical value show?

An additional covalent contribution to the bonding.

What does the size of the difference tell you?

The larger the difference, the greater the degree of covalent character.

Define polarisation of an ion.

Distortion of the electron cloud of an anion by a neighbouring cation.

What two factors determine the polarising power of a cation?

Its radius and charge: small and highly charged gives high polarising power.

What two factors determine the polarisability of an anion?

Its radius and charge: large and highly charged gives high polarisability.

Which is more polarisable, I⁻ or F⁻, and why?

I⁻: it is larger, so its outer electrons are further from the nucleus and more weakly held.

Which combination of ions gives the most covalent character?

A small, highly charged cation with a large, highly charged anion.

Ionic bonding: theoretical and experimental lattice energyEdexcel A-Level Chemistry: Flashcards

Card 1 of 140 of 14 known

Question

Define lattice energy (Edexcel).

Tap or press Space to reveal

Tap card or press Space to flip

See all 14 cards

Define lattice energy (Edexcel).: The enthalpy change when one mole of an ionic solid is formed from its gaseous ions.
What is the sign of lattice energy, and what does a more negative value show?: It is exothermic (negative). The more exothermic, the stronger the ionic bonding.
Write the equation for the lattice energy of NaCl.: Na⁺(g) + Cl⁻(g) → NaCl(s)
How is the experimental lattice energy found?: From a Born-Haber cycle, using Hess's law and measured enthalpy changes.
How is the theoretical lattice energy found?: By calculation from electrostatic theory, assuming a purely ionic model.
What does the purely ionic model assume?: Ions are perfect spheres with charge evenly distributed, held only by electrostatic attraction.
What does close agreement of experimental and theoretical values show?: The bonding is (almost) purely ionic.
What does an experimental value more exothermic than the theoretical value show?: An additional covalent contribution to the bonding.
What does the size of the difference tell you?: The larger the difference, the greater the degree of covalent character.
Define polarisation of an ion.: Distortion of the electron cloud of an anion by a neighbouring cation.
What two factors determine the polarising power of a cation?: Its radius and charge: small and highly charged gives high polarising power.
What two factors determine the polarisability of an anion?: Its radius and charge: large and highly charged gives high polarisability.
Which is more polarisable, I⁻ or F⁻, and why?: I⁻: it is larger, so its outer electrons are further from the nucleus and more weakly held.
Which combination of ions gives the most covalent character?: A small, highly charged cation with a large, highly charged anion.

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

See the full worksheet

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).