Acids & Bases

Acids & Bases

1. Bronsted-Lowry

   • Acid = proton, H+, donator
   • Base = proton, H+, acceptor
   • Strong acids/bases fully ionised in aq solutions
     • HCl, H₂SO₄, HNO₃ NaOH, KOH
   • Weak acids/bases partially ionised in aq solutions
     • Ethanoic acid Aqueous ammonia
     • Equilibrium to the left
   • Monoprotic = donates 1 H⁺ per molecule Diprotic = donates 2 H⁺ per molecule

2. pH = -log₁₀[H⁺]

   • [H⁺] = 10⁻ᵖᴴ
   • pH of strong acids & alkalis mixtures can be calculated
     • You need to:  1) Work out moles for each reactants 2) Find the moles of the excess reactant 3) Use c = 1000n ÷ v where N is the moles of excess and V is the total volume  4) If the acid is in excess, use c to find pH. If the alkali is in excess, use  [H⁺] = Kw ÷ [OH⁻] and then find pH. 

3. 298K (25℃) Kw = 1x10⁻¹⁴ mol²dm⁻⁶

   • H₂O can be a acid or base
   • Temp increases, Kw increases
     • Shifts equilibrium to right
   • H₂O ⇌ H⁺ + OH⁻ ΔH = +ve
   • Kw = [H⁺] [OH⁻]
     • [H⁺] = √Kw
       • Use this if you need: 1) To find [H⁺] in water at 298K 2) To find the pH of water at 298K
     • [H⁺] = Kw [OH⁻]
       • Use this if you need: 1) To find pH of strong bases at 298K 2) To find conc of a strong base, rearrange to [OH⁻] = Kw ÷ [H⁺] 

4. Weak acids

   • HA ⇌ H⁺ +A⁻ ΔH = +ve
     • Temp changes rate of dissociation
     • Equilibrium constant is Ka
       • Ka = [H⁺] [A⁻] [HA]
         • mol dm ⁻³
         • Larger Ka = Stronger acid
         • Temp increases, Ka increases
         • If acid is 50% ionised, Ka = [H⁺]
         • [H⁺] = √Ka x [HA]
           • Use when you have a concentration of a weak acid and a Ka constant
           • [H⁺] = Ka x [HA] ÷ [A⁻]
             • Use when there is a mixture of excess weak acid and strong alkali
         • p Ka = -log₁₀Ka
           • Use this when you are not given a concentration for the weak acid and so can't use [H⁺] = √Ka x [HA]
     • pH of weak acids & strong alkalis mixtures can be calculated
       • You need to (if acid is excess):  1) Work out moles for each reactants 2) Find the moles of the excess reactant 3) Find the amount of [A⁻] formed. This is just the same as the moles of alkali added 4)  Use  [H⁺] = Ka x [HA] ÷ [A⁻] and then find pH. 
       • You need to (if alkali is excess):  1) Work out moles for each reactants 2) Find the moles of the excess reactant 3) Use c = 1000n ÷ v where N is the moles of excess and V is the total volume  4) If the alkali is in excess, use [H⁺] = Kw ÷ [OH⁻] and then find pH.