Enthalpies and dissolving

Lattice enthalpy

• Measure of ionic bond strength.

• The standard lattice enthalpy, Δ_LEH^⦵, is the enthalpy change when 1 mole of an ionic lattice is formed from its gaseous ions under standard conditions.

• Lattice enthalpy is always negative because bonds are formed and energy is released.

• The more negative the lattice enthalpy, the stronger the bonding.

Enthalpy changes involved in dissolving

• 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.

• The enthalpy change of hydration of ions, Δ_hydH, is the enthalpy change when 1 mole of aqueous ions is formed from gaseous ions.

• The enthalpy change of solution is the overall effect on the enthalpy when a substance dissolves.

• The enthalpy change of solution, Δ_solH, is the enthalpy change when 1 mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution.

• An infinitely dilute solution is one which contains so much solvent that when more solvent is added, there is no change in concentration.

• Δ_solH = Δ_hydH - Δ_LEH (an enthalpy (Hess) cycle and enthalpy level diagram can be used to visualise this).

Measuring enthalpy change of solution experimentally

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

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

Effect of charge density on lattice enthalpy and enthalpy of hydration

• 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.

• Enthalpy of hydration: 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.