20.4    Enthalpy Changes in Solutions

Recall enthalpy change of  solution and hydration

• Standard enthalpy change of solution, ΔH°sol is the enthalpy change when one mole of ionic compounds is dissolved in water to form an infinitely dilute solution under standard conditions.
  MgCl2(s) + aq → MgCl2(aq)        ; ΔH = ΔH°sol

 

• Standard enthalpy change of solution can be exothermic or endothermic, that   is, some substance dissolve exothermically while others dissolve So, why is there a difference?

 

• Standard enthalpy change of hydration, ΔH°hyd is the enthalpy change when one mole of gases ions dissolves in water to form hydrated ions of infinite dilution under standard conditions
  Ca²⁺(g) + aq → Ca²⁺(aq)           ; ΔH = ΔH°hyd

• Standard enthalpy change of hydration is always exothermic, this is because when an  ion dissolves in water, bonds are formed between the ions and water molecules. This bond is called the ion-dipole bond.

• Factors affecting the magnitude of hydration enthalpy are:
  1. the size of the ion
  2. the charge on the ion

• • The larger the size of the ion, the lower the hydration For example, hydration enthalpy of the Group II ions decreases down the  Group.
  • The higher the charge of the ion, the higher the hydration enthalpy. For example, Group II ions have higher hydration enthalpy compared to their corresponding Group I ions.

 

The process of dissolving

 

• • The process of dissolving is imagined as if the solid ionic lattice is ftrst broken down to form gaseous ions. Lattice energy is absorbed.
  • These gaseous ions are then dissolved in water, this is how they exist in the final solution. The energy involved here is hydration enthalpy
  • The energy involved in the entire process(from solid ionic lattice to hydrated ions) is the solution enthalpy

• Therefore, take sodium chloride, NaCl as an example:

• An example:

 

Solubility of Group II sulfates

 

• The solubility of Group II sulfates decreases down the Group

 

• The solubility is governed by the sign of the enthalpy change of  solution, ΔH°sol. If ΔH°sol > 0, the sulfate is relatively insoluble. If ΔH°sol < 0, the sulfate is relatively soluble

 

• The sign of ΔH°sol is governed by the relative magnitude of  ΔH°hyd and ΔH°latt.
• • Going down the Group, if ΔH°hyd decreases faster than ΔH°latt, the sulfates become more insoluble down the Group.
  • Going down the Group, if ΔH°latt decreases faster than ΔH°hyd, the sulfates become more soluble down the Group.

• In the case of Group II sulfates, the former is true
• • Hydration enthalpy decreases faster down the Group because the size of the cations increases dramatically down the Group, the anion is a constant
  • Lattice energy decreases slower down the Group because the sulfate ion is much larger than the Group II ions. As a result, the sulfate ion contributes to a greater part in the magnitude of lattice energy. That is, the increase in size of the cations is relatively insignificant.