Essential ideas
  • The acid–base concept can be extended to reactions that do not involve proton transfer.
  • The equilibrium law can be applied to acid–base reactions. Numerical problems can be simplified by making assumptions about the relative concentrations of the species involved. The use of logarithms is also significant here.
  • pH curves can be investigated  experimentally  but are mathematically  determined by the dissociation constants of the acid and base. An indicator with an appropriate end point can be used to determine the equivalence point of the reaction.
18.1 Lewis theory of acids and bases 
Understandings:
A Lewis acid is a lone pair acceptor and a Lewis base is a lone pair donor.
When a Lewis base reacts with a Lewis acid a coordinate bond is formed.
A nucleophile is a Lewis base and an electrophile is a Lewis acid.
Please note there is a slight error in the video - when bonded to NH3, the boron atom is bonded to 3 hydrogen atoms, when it should be bonded to three fluorine atoms.
18.2 Temperature dependence of Kw
Guidance:
This video covers the effect of temperature on the value of the Kw. 
Temperature dependence of Kw worksheet 
18.2 Acid and base dissociation constants (Ka and Kb)
Understandings:
The expression for the dissociation constant of a weak acid (Ka) and a weak base (Kb).
Ka and Kb worksheet
18.2 Ka, pKa, Kb, pKb, Kw, pKw
Understandings:
For a conjugate acid base pair, Ka × Kb = Kw.
The relationship between Ka and pKa is (pKa = -log Ka), and between Kb and pKb is (pKb = -log Kb).
Applications and skills:
Discussion of the relative strengths of acids and bases using values of Ka, pKa, Kb and pKb.
18.2 Calculations with weak acids and bases 
Applications and skills:
Calculations involving pH, pOH, Ka and Kb. 
18.3 pH curves 
Understandings:
The characteristics of the pH curves produced by the different combinations of strong and weak acids and bases.
Applications and skills:
The general shapes of graphs of pH against volume for titrations involving strong and weak acids and bases with an explanation of their important features.
18.3 pH curves part 2 
Understandings:
The buffer region on the pH curve represents the region where small additions of acid or base result in little or no change in pH.
Applications and skills:
Only examples involving the transfer of one proton will be assessed. Important features are:
intercept with pH axis
equivalence point
buffer region
points where pKa = pH or pKb = pOH.
18.3 Salt hydrolysis 
Applications and skills:
Prediction of the relative pH of aqueous salt solutions formed by the different combinations of strong and weak acid and base.
18.3 Acid-base indicators 
Understandings:
An acid–base indicator is a weak acid or a weak base where the components of the conjugate acid–base pair have different colours.
Applications and skills:
Selection of an appropriate indicator for a titration, given the equivalence point of the titration and the end point of the indicator.
18.3 Buffer solutions 
Understandings:
The composition and action of a buffer solution.
Applications and skills:
While the nature of the acid–base buffer always remains the same, buffer solutions can be prepared by either mixing a weak acid/base with a solution of a salt containing its conjugate, or by partial neutralization of a weak acid/base with a strong acid/base.


Please note that calculating the pH of a buffer solution will only be assessed in option B (B.7) and option D (D.4).