Topic 18: Acids and Bases (HL) Flashcards
Is nucleophile a Lewis base or acid?
Lewis base
Is electrophile a Lewis base or acid?
Lewis acid
Electrophile
An electron-deficient species that can accept a lone pair from a nucleophile
Acid dissociation constant, Ka
For a reaction:
HA(aq) + H2O(l) ⇋ A^(-) (aq) + H3O+(aq)
Ka = ([A^(-)][H3O+])/[HA]
where HA is weak acid
Base dissociation constant, Kb
For a reaction:
B(aq) + H2O(l) ⇋ BH+(aq) + OH (aq)
Kb = ([BH+][OH-])/[B]
where B is a weak base
Formula to calculate Kw using Ka and Kb
Kw = Ka x Kb
Buffer
A solution that resists a change in pH upon the addition of small amounts of a strong base or strong acid, or upon the dilution of the buffer by adding water.
May be composed of a weak acid and its conjugate base or the other way around
Buffer capacity
the effectiveness of the buffer to resist changes in pH depends on the molar concentration of the acid and conjugate base. Te higher the concentration, the more effective the buffer at resisting the changes in pH.
Can be prepared from partial neutralisation of a weak acid with a strong base
Hydrolysis
The ionization of water that results from reaction with an ionic salt
e.g. Salts from strong acid and strong base do not react with H+ or OH- thus will not undergo hydrolysis.
Half-equivalence point of the weak acid
Stage of the titration at which half of the amount of weak acid has been neutralised.
pH = pKa
Titration of strong acid and a strong base
1) The starting point on the pH axis is an important feature of a pH curve as it is an indication of the relative strength of the acid. The strong acid gives an initial pH reading ≈ 1 .0.
2) There is a gradual rise in the pH as the titration approaches the equivalence point.
3) The sharp rise in pH at the equivalence point (pH = 7.0) is described as the point of inflection of the curve.
4) Once there is no remaining acid to be neutralized, the curve flattens and finishes at a high pH reflecting the strong base.
Titration of a weak acid and strong base.
ethanoic acid with sodium hydroxide
1) The weak acid gives an initial pH reading ≈ 3.0.
The initial rise is steep, as a strong base is being added to a weak acid and neutralization is rapid.
2) As the weak acid begins to be neutralized the strong conjugate base sodium ethanoate is formed, creating a buffer that resists change in pH.
3) Ethanoic acid is in equilibrium with the ethanoate ion: CH3COOH(aq) + H2O(l) ⇋ CH3COO (aq) + H3O+ (aq)
4) The continued addition of base to the solution uses up hydrogen ions, hence the forward reaction is favoured. This results in a very gradual change in pH in this region of the curve.
5) The half-equivalence point is the stage of the titration at which half of the amount of weak acid has been neutralized/
pKa = pH
6) There is a sharp rise in pH at the equivalence point (pH > 7) . The equivalence point is the result of salt hydrolysis.
7) With no remaining acid to be neutralized, the curve flattens and finishes at a high pH due to the presence of excess strong base.
Titration of a weak base with a strong acid.
hydrochloric acid and ammonia
1) The weak base gives an initial pH reading ≈ 11.0.
2) As the weak base begins to be neutralized, the ammonium ion NH4+, the conjugate acid, is created resulting in a buffer that resists
change in pH. Ammonia is in equilibrium with the ammonium ion:
NH3(aq) + H2O(l) ⇋ NH4 +(aq) + OH (aq)
3) At the half-equivalence point half of the amount of weak base has been neutralized. At this point, pOH = pKb
4) There is a gradual fall in the pH due to the buffering effect as the titration approaches the equivalence point.
5) The pH falls sharply at the equivalence point (pH < 7) .
6) The equivalence point is the result of salt hydrolysis.
7) With no remaining base to be neutralized, the curve flattens and ends at a low pH due to the presence of excess strong acid
Titration of weak base and weak acid
1) The weak base gives an initial pH reading ≈ 11 .0.
2) The change in pH throughout the titration is very gradual.
3) The point of inflection in the pH curve is not as steep as in the previous pH curves. The point of equivalence is difficult to determine, so this kind of titration has little or no practical use.
4) With no remaining base to be neutralized, the curve flattens and ends at a pH that indicates the presence of a weak acid
