In order to understand why many ionic compounds are soluble we first need to consider our model of an ionic lattice in a little more detail.
Lattice energy or lattice enthalpy is both the energy needed to from gaseous ions from one mole of a solid ionic compound / ionic lattice AND the energy released when one mole of a solid ionic compound / ionic lattice is formed from its ions in the gaseous state.
And herein lies much confusion … text books can use the term to mean both!
Clearly forming a lattice is highly exothermic as millions of strong electrostatic attractions pull the gaseous ions into a regular crystalline arrangement. Breaking up a lattice to from gaseous ions must be an highly endothermic process.
Na+(g) + Cl–(g) ⇾ NaCl(s) ΔH⦵ = -787 kJ mol-1
NaCl(s) ⇾ Na+(g) + Cl– (g) ΔH⦵ = +787 kJ mol-1
How do we tell which version of lattice energy a question or text book is talking about?
- Check the sign – if it is negative then we are forming a lattice and if it is positive we are breaking it apart
- Text books and exam questions often talk in terms of ‘enthalpy of formation of a lattice’ which equates to the exothermic process
To be clear, I make the assumption that lattice energy or enthalpy is an endothermic process to convert one mole of an ionic solid into its constituent ions which is the most common definition outside of A level.
It doesn’t actually make any difference to our explanations or our calculations as long as we all agree on a definition before beginning. Just be aware!
A note on the nature of ionic lattices
We are all familiar with the classic picture of an ionic lattice – for example, for a compound such as sodium chloride where sodium and chlorine exist as discrete ions in a regular 3-D arrangement.
Let’s take a look at the evidence for the model and its limitations …
Cations are positively charged and anions are negatively charged.
Born-Haber cycles
Born-Haber cycles are simply a fancy Hess cycle transposed onto an energy level diagram and we can use them to calculate the lattice energy for an ionic compound.
They might seem a little complicated when you first meet them but if you employ your logic skills, you will have them mastered in no time at all.
Why are ionic compounds soluble? (part 1)
Quick answer?
It’s all about competing strengths of attraction between ions, between ions and water molecules, between water molecules themselves …
A quick note on solvents
Almost all of the chemical reactions we study take place in a solvent, often water, and the choice of solvent can have a significant effect on the rate and the products of a reaction.
Why are ionic compounds soluble? (part 2)
For a full understanding of why ionic compounds are soluble we need to consider both the enthalpy and entropy changes involved …
