Enthalpy change of solution

• The?standard enthalpy change of solution (ΔH_sol^?)?is the enthalpy change when?1 mole?of an ionic substance?dissolves?in sufficient water to form a?very dilute solution
• The symbol?(aq)?is used to show that the solid is dissolved in?sufficient water
  • For example, the enthalpy changes of solution for potassium chloride are described by the following equations:

KCl(s) + aq → KCl(aq)

OR

KCl(s) + aq → K⁺(aq) + Cl^-(aq)

• ΔH_sol^??can be?exothermic?(negative) or?endothermic?(positive)

Enthalpy change of hydration

• The lattice energy (ΔH_latt^?) of KCl is -711 kJ mol^-1
  • This means that 711 kJ mol-1?is?released?when the KCl ionic lattice is?formed
  • Therefore, to?break?the attractive forces between the K⁺?and Cl^-?ions, +711 kJ mol^-1?is needed
• However, the?ΔH_sol^??of KCl is +26 kJ mol^-1
• This means that another +685 kJ mol^-1?(711 - 26) is required to break the KCl lattice
• This is?compensated?for by the?standard enthalpy change of hydration?(ΔH_hyd^?)
• The?standard enthalpy change of hydration?(ΔH_hyd^?)?is the enthalpy change when?1 mole?of a specified?gaseous ion dissolves?in sufficient water to form a very dilute?solution
  • For example, the enthalpy change of hydration for magnesium ions is described by the following equation:

Mg²⁺(g) + aq → Mg²⁺(aq)

• • Hydration enthalpies are the measure of the energy that is released when there is an attraction formed between the ions and water molecules
  • Hydration enthalpies are?exothermic
• When an?ionic solid?dissolves in water, positive and negative ions are formed
• Water is a?polar?molecule with a δ- oxygen (O) atom and δ+ hydrogen (H) atoms which will form?ion-dipole attractions?with the ions present in the solution
• The oxygen atom in water will be attracted to the?positive ions?and the hydrogen atoms will be attracted to the?negative?ions
• Since the?ΔHhyd??of KCl is -685 kJ mol^-1, 685 kJ mol^-1?is released in forming these?ion-dipole attractions?when KCl dissolves in water
  • This compensates for the remaining +685 kJ mol^-1?which was needed to break down the KCl lattice

The polar water molecules will form ion-dipole bonds with the ions in solution (a) causing the ions to become hydrated (b)