Weak acids and bases Can perform calculations using weak acids and bases.

Terminology check up – mix and match Hydronium ionFormed from an acid that has lost a proton Bronsted – Lowry acidA measure of the concentration of aqueous hydronium ions in solution Bronsted – Lowry baseA substance which can donate protons Conjugate acidReleases one mole of hydrogen ions per one mole of acid Conjugate baseSolids which react with both acids and bases eg: aluminium hydroxide MonoproticFormed from a base that has accepted a proton AmphiproticA substance that can act as both an acid and a base AmphotericA substance which can accept protons pHFormed when a proton attaches to a water molecule to become H 3 O +

Identify the acid base reactions from the list below. Label the acids with A and Bases with B CuO + NH 4 +  Cu NH 3 + H 2 O ZnCl 2 + 2NaOH  Zn(OH) 2 + 2NaCl Na 2 CO 3 + 2HCl  2NaCl + H 2 O + CO 2

K a – acid dissociation constant K a is very large > 1 for a strong acid K a is very small and < 1 for a weak acid

pK a and K a As pH = - log [H 3 O + ] so is pK a calculated the same way… pK a = -log [Ka] And vice versa K a = 10 -pKa The larger the pK a, the smaller the K a value and degree of dissociation. pKa tells us directly the equilibrium position. Eg: HF acid has a pKa of 3.17 and HClO has a pKa of Which dissociates more in water?

Calculating pH of a weak acid

Sample calculation

Your turn Pg 240 Q 5 a-d

You may also be asked to work backwards from pH Eg: What is the concentration of a solution of ethanoic acid that has a pH of 2.85? First find [H 3 O + ] from pH (1.41 x molL -1 )then solve using the same equation but solving for acid concentration rather than H 3 O +. c(ethanoic acid) = 0.11molL -1

Conjugate base strengths A small K a means the acid is very _______ and has not ______________ much. The pK a of this acid will be __________. This means the equilibrium concentration of the conjugate base of the acid will be _______________. By implication, the strength of the conjugate base (ability to accept protons) is _____________ compared to the strength of the acid. The weaker the acid the _____________the conjugate base.

Calculating pH of weak base eg: Calculate pH of molL -1 sodium ethanoate CH 3 COONa. Ka = 1.74 x 10 -5

Your turn – Calculate pH of 0.02 molL - solution of aminomethane CH 3 NH 2. Ka(CH 3 NH 4 + ) = 2.78 x