Enthalpies and dissolving

Standard lattice enthalpy (Δ_LEH^⦵)

The enthalpy change when 1 mole of an ionic lattice is formed from its gaseous ions under standard conditions.

What is lattice enthalpy a measure of

Ionic bond strength

What sign does lattice enthalpy have and why

Negative because bonds are formed and energy is released.

What does a more negative lattice enthalpy mean

Stronger bonding

What 2 things occur when an ionic lattice dissolves

1. Bonds between the ions break - this is endothermic and the enthalpy change is the opposite of the lattice enthalpy.

2. Bonds between the ions and water are made - this is the enthalpy change of hydration and is always exothermic.

Enthalpy change of hydration (Δ_hydH)

The enthalpy change when 1 mole of aqueous ions is formed from gaseous ions.

Enthalpy change of solution (Δ_solH)

The enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution. (Overall effect on the enthalpy when a substance dissolves)

Infinitely dilute solution

A solution which contains so much solvent that when more solvent is added, there is no change in concentration (to reduce interaction between solute particles).

What is the equation linking Δ_solH, Δ_hydH, and Δ_LEH

Δ_solH = Δ_hydH - Δ_LEH

What diagrams can be used to visualise the relationship between Δ_solH, Δ_hydH, and Δ_LEH

Enthalpy (Hess) cycles and enthalpy level diagram.

How can enthalpy change of solution be measured experimentally

Using a polystyrene cup, lid and thermometer, the solute can be added to the solvent and the largest change in temperature measured.

What calculations must be performed to find the enthalpy change of solution experimentally

Using q = mcΔT, the energy (in joules) that is transferred can be calculated. Then converting into KJ and dividing by the moles of solute.

Effect of charge density on lattice enthalpy

Smaller ions with greater charges can get closer to each other and attract each other more strongly, so the ionic bonding is stronger, therefore as charge density increases, the lattice enthalpy gets more exothermic.

Effect of charge density on hydration enthalpy

Water molecules can get closer to smaller ions and are more strongly attracted to ions with greater charges, so the ion-dipole bonds are stronger, therefore as charge density increases, the enthalpy of hydration gets more exothermic.