Sources of ions in aqueous solutions:
Ionization – Chemical reaction
Ion formation from solute molecules by solvent action
Water tears polar solutes apart into ions
Acids ionize in aqueous solutions
Dissolve acid in water, form a proton
H – Cl + water → H + (aq) + Cl − (aq)
H + (proton transfer to electronegative Oxygen atom of H2O to form ions)
H + + H2O → H3O + + Cl –
HCl (g) + H2O(l) → H3O+ (aq) + Cl – (aq)
H3O + = Hydronium ion, (hydrated proton)
Dissociation – Physical change
A soluble ionic solid separates into ions when dissolved in water
KI (s) → K+ (aq) + I − (aq)
CaCl2 (s) → Ca +2 (aq) + 2 Cl – (aq)
Ba(NO3)2 (s) → Ba +2 (aq) + 2 NO3 – (aq)
Coefficients represent moles of ions
Write Dissociation Equation for Aluminum sulfate in water.
# moles of ions from 1 mole of solid ?
Al2(SO4)3 (s) → 2Al+3 (aq) + 3SO4 − 2 (aq)
2 moles of Al ions, 3 moles sulfate ions
Dissolution
Ionic solid dissolves in a polar liquid; (ions enter solution)
AgCl (s) → Ag + (aq) + Cl – (aq)
Precipitation
Ions leave solution and regenerate an ionic solid
Ag + (aq) + Cl – (aq) → AgCl (s)
Solubility Equilibrium
AgCl (s) → Ag+ (aq) + Cl – (aq)
Dissolution rate = precipitation rate
Precipitation Reaction
Two ionic compounds dissolved in water mix and a solid precipitate forms
Double Replacement reaction:
AB + CD → AC + BD
(Reactant ions switch partners)
AgNO3 (aq) + KBr(aq) → AgBr(s) + KNO3(aq)
AgBr = Solid Precipitate Product
Precipitate identity confirmed experimentally
Use Solubility Rules to predict solids
SS = sparingly soluble = Insoluble
= 0.01 mole ≤ dissolves in 1.0 L H2O
Net Ionic Equations (Steps)
Write:
Formula Equation - shows reactants and products
Complete Ionic Equation –
Shows each original solution as a sum of its constituent ions,
(Total charge of both sides = 0)
Identify which ions unchanged (no reaction)
Spectator Ions:
Do not react, they “Watch” more reactive ions undergo a chemical change.
Net Ionic Equation – remove spectator ions
Show only compounds and ions that react in an aqueous solution.