Enthalpy changes in solution

• Dissolving ionic compounds - water molecules are able to break up the giant ionic lattice structure and overcome the strong electrostatic attractions between oppositely charged ions - this is what happens when salt dissolves in water
• Enthalpy change of solution - the overall energy change associated with the dissolving process is called the enthalpy change of solution
• the standard enthalpy change of solution is the enthalpy change that takes place when one mole of solute dissolves in a solvent. If the solvent is water, the ions from the ionic lattice finish up surrounded with water molecules as aqueous ions
• the enthalpy change of solution can be exo or endothermic
• attractions in an ionic lattice exist in 3 dimesnions
• NaCl (s): Na+ and Cl- ions are attracted together in a giant ionic lattice

  NaCl (aq): Na+ and Cl- are separate, but now surrounded by water molecules
• in the aqueous ions, the delta+ and delta- partial charges in the water molecules are attracted towards the positive and negative ions:

  • the delta- oxygen atom is attracted to the positive sodium ion
  • the delta+ hydrogen atoms are attracted to the negative chloride ion
• the dissolving process:

  • when a solid ionic compound dissolves in water, 2 processes take place
  • the ionic lattice breaks up
  • water molecules are attracted to, and surround the ions
• 2 types of energy are involved:

  • the ionic lattice is broken up forming separate gaseous ions - opposite energy change from lattice enthalpy, which forms the ionic lattice from gaseous ions
  • the separate gaseous ions interact with polar water molecules to form hydrated aqueous ions - the energy change involved is called the enthalpy change of hydration
• the enthalpy change of hydration is the enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions
• the equations show two enthalpy changes of hydrations that are involved in the dissolving of NaCl (s) - aq represents an excess of water:

  • Na+ (g) + aq = Na+ (aq) =-406 KJmol-1
  • Cl- (g) +aq = Cl- (aq) = -378KJmol-1
• the enthalpy change of solution can be exothermic or endothermic depending on the relative sizes of the lattice enthalpy and the enthalpy changes of hydration