Enthalpy and Entropy

Definition of lattice enthalpy (Δ_LEH)

Enthalpy change due to the formation of 1 mole of ionic lattice from gaseous ions

Measure of ionic strength

Always exothermic

What 2 routes are there to form ionic lattices from elements in their standard states?

Route 1: atomise each element (standard enthalpy change of atomisation), then make ions (ionisation energy / electron affinity), then form the lattice (lattice enthalpy)

Route 2: enthalpy of formation of lattice

Definition of standard enthalpy change of solution (Δ_solH^°)

Enthalpy change due to 1 mole of a solute dissolving in a solvent

Can be exothermic or endothermic

aq: excess of water

Definition of enthalpy change of hydration (Δ_hydH^o)

Enthalpy change due to dissolving gaseous ions in water to form 1 mole of aqueous ions

Always exothermic

What are the 2 routes of forming aqueous ions from gaseous ions?

Route 1: form a lattice (lattice enthalpy) then dissolve the lattice (enthalpy change of solution)

Route 2: hydrate the gaseous ions (enthalpy change of hydration for all atoms involved)

What factors affect lattice enthalpy and enthalpy of hydration?

Ionic size

Ionic charge

How does ionic size affect lattice enthalpy and enthalpy of hydration?

• Ionic radius increases

• Attraction between ions / ions and water molecules decreases

• Lattice enthalpy / enthalpy of hydration less negative

How does ionic charge affect lattice enthalpy and enthalpy of hydration?

• Ionic charge increases

• Attraction between ions / ions and water molecules increases

• Lattice enthalpy / enthalpy of hydration more negative

What is entropy (S)?

A measure of the dispersal of energy in a system, which is greater the more disordered a system is.

Units: J K^-1 mol^-1

What state has the highest entropy?

Gas > liquid > solid

What is standard entropy (S°)?

The entropy of 1 mole of a substance, under standard conditions (100 kPa and 298 K)

Always positive value

How do you calculate the entropy change of a reaction?