22.2 - enthalpy changes in solution

• What happens when salt dissolves in water?


  Water molecules are able to break up the giant ionic lattice structure and overcome the strong electrostatic attractions between oppositely charged ions.
• Enthalpy change of solution


  The overall energy change associated with the dissolving process.
• Standard enthalpy change of solution


  The enthalpy change that takes place when one mole of a solute dissolves in a solvent.
• What is the structure of particles when a salt has been dissolved in water?


  The ions from the ionic lattice (of the salt) end up surrounded with water molecules, as aqueous ions.
• Is enthalpy change of solution endothermic or exothermic?


  It can be exothermic or endothermic.
• How are water molecules attracted to towards aqueous ions (of a salt)?


  The δ+ and δ- partial charges in the water molecules are attracted towards the positive and negative ions.

  
  

  E.g. NaCl

  - The δ- oxygen atom is attracted to the positive sodium ion.

  - The δ+ hydrogen atoms are attracted ti the negative chloride ion.
• What mass is used in enthalpy calculations


  Mass of whats changing in temperature (where the thermometer is). Often water and solid.
• What two processes take place when a solid ionic compound dissolves in water?


  1. The ionic lattice breaks up.

  2. Water molecules are attracted to, and surround, the ions.
• How many types of energy change are involved when a solid ionic compound is dissolved in water?


  Two.
• First type of energy change that occurs when a solid ionic compound is dissolved in water


  The ionic lattice is broken up forming separate gaseous ions. (opposite energy change from lattice energy, which forms the ionic lattice from gaseous ions - known as lattice enthalpy change of dissociation).
• Second type of energy change that occurs when a solid ionic compound is dissolved in water?


  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.
• Enthalpy change of hydration


  The enthalpy change that accompanies the dissolving of gaseous ions in water to form one mole of aqueous ions.