22.2 enthalpy changes in solution

• water molecules are able to break up the giant ionic lattice structure and overcome the strong electrostatic attractions between oppositely charged ions.
• the overall energy change associated with the dissolving process is called enthalpy change of solution
• the standard enthalpy change of solution is the enthalpy change that takes place when one mole of a 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 exothermic or endothermic.

  in figure one: compares the ions in the NaCl lattice and in a solution. the attraction exists in 3 dimensions, so there will be ions above and below the plane

  
• When a sold io if compound dissolves in water, two processes taken place

  • the ionic lattice breaks up
  • water molecules are attracted to and surround the ions

  two types of energy changes are involved

  1. the ionic lattice breaks up forming separate gaseous ions, this is the opposite from lattice energy, which form the ionic lattice from gaseous ions
  2. 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.
• When using the equation q=mc^T to calculate the energy change, m is the mass that is changing temperature (where the thermometer is). Uts the mas of the solution not the mass of the water.