Enthalpy

Lattice enthalpy is the enthalpy change that takes place when one mole of an ionic compound is formed from its gaseous ions under standard conditions.

• Lattice enthalpy is exothermic because energy is given out when ionic bonds are being formed
• Lattice enthalpy indicates the strength of an ionic lattice and is a measure of the ionic bond strength
• A covalent substance does not have a lattice enthalpy because there are no ions in it's structure
• Lattice enthalpies cannot be measured directly. It is impossible to form one mole of an ionic lattice from gaseous ions

Enthalpy changes can be calculated using Hess' Law. Born-Haber cycles can be used to represent these energy changes. 

Can you define the following enthalpy changes? What are their symbols?

Standard enthalpy change of formation

Standard enthalpy change of atomisation

First ionisation energy

Second ionisation energy

First electron affinity

Second electron affinity

A Born-Haber cycle is similar to a Hess' law energy cycle. It allows the calculation of an enthalpy change which cannot be measured directly.

• The top line (the datum line) has the elements in their standard states above it. This is the highest energy level.
• The bottom line (the base line) has the ionic solid above it. This is the lowest energy level.
• All ΔH values pointing upwards are endothermic reactions
• All ΔH values pointing downwards are exothermic reactions

In an exam, it is important to show all the equations with the correct state symbol, o/w marks will be lost. 

Enthalpy change of solution is the enthalpy change that takes place when one mole of a compound dissolves completely in water under standard conditions. When a solid dissolves, two processes take place:

• breakdown of the ionic lattice into gaseous ions

This involves breaking down the crystal lattice and separating the ions. This requires energy, and is therefore ΔH-ve. The energy involved has the same magnitude as lattice enthalpy but the sign is opposite. 

• hydration of the ions

The positively charged ions will be slightly attracted to the negative oxygen atoms in water. The negatively charged ions will be slightly attracted to the slightly positive hydrogen ions. Standard enthalpy change of hydration releases energy. It is always exothermic. It is the enthalpy change that takes place when one mole of isolated gaseous ions dissolves completely in water, forming one mole of aqueous ions under standard conditions.

Large exothermic value = large electrostatic force of attraction between oppositely charged ions = strong ionic bonds.

Ionic size

• as the ionic radius increases, the attraction between ions decreases and the lattice enthalpy becomes less negative. 

Ionic charge

• The compounds with the most negative lattice enthalpies are those which have small, highly charged ions.

Enthalpy change of hydration

The enthalpy change that takes place when one mole of isolated gaseous ions dissolves in water forming one mole of aqueous ions under standard conditions. Ionic size

• As the ionic radius becomes smaller, the enthalpy change of hydration becomes more negative. Small ions exert more attraction on water molecules and more energy is released

Ionic charge

• As the charge on an ion increases it has greater attraction for water molecules, and hence hydration enthalpy is more negative.