Dissolution of ionic compounds

Question 1

Dissolving ionic solids (2 processes)

Answer 1

1. Breaking down of crystal lattice

2. Subsequent stabilization of ions by solvent molecule (Solvation)

Question 2

Breaking down crystal lattice

Answer 2

• requires a lot of energy
• energy required to break up the crystal lattice: lattice dissociation enthalpy
• e.g. NaCl (s) –> Na+ (g) + Cl- (g)
• solute-solvent forces of attraction must be sufficient to overcome attractive forces that hold solid tgt (LE)

Question 3

Solvation/stabilization of ions by solvent molecule

Answer 3

• solvent/water molecules w +ve ends point towards the anions, surround them [Cl- (g) –> Cl- (aq)]
• solvent/water molecules w -ve ends point towards cations, surround them [Na+(g) –> Na -(aq)]
• formation of ion-solvent interaction releases energy sufficient to overcome first endo process (lattice dissociation enthalpy)
• 2nd process is exo, counterbalances first process, allows for dissolving to take place

Question 4

Enthalpy change of hydration

Answer 4

• energy evolved
• 1 mol of (g) ions surrounded by water molecules
• form an infinitely dilute soln
• under std conditions

Question 5

Things to note for std enthalpy change of hyd

Answer 5

• always -ve (exo)

- magnitude proportional to ionic charge; inversely proportional to ionic radius

Question 6

Relationship between mag of enthalpy change of hyd and ionic charge

Answer 6

• proportional
• larger ionic charge, attraction btwn water molecules and ion stronger
• enthalpy change of hyd larger

Question 7

Relationship between mag of enthalpy change of hyd and ionic size/radius

Answer 7

• ionic radius smaller
• attraction between water molecule and ion stronger
• enthalpy change of hyd larger

Question 8

Enthalpy change of solution

Answer 8

• enthalpy change when 1 mol of solute
• dissolved in a solvent
• form infinitely dilute soln
• under std conditons
• e.g. NaCl (s) + aq –> Na+ (aq) + Cl- (aq)

Question 9

Solubility of salt/ionic compounds (Enthalpy change of soln)

Answer 9

• more exothermic std enthalpy change of soln, more soluble

- solubility depends on balance btwn hydration energy of ions and LE of compound

Question 10

Std enthalpy change of soln equation

Answer 10

std enthalpy change of hyd + lattice enthalpy