Reactivity 3.1 Proton transfer reactions (HL)
Reactivity 3.1.9
Understandings: 
  • ​The pOH scale describes the [OH–] of a solution. pOH = –log10[OH–]; [OH–] = 10–pOH
Learning outcomes:
  • Interconvert [H+], [OH–], pH and pOH values.
Additional notes:
  • The equations for pOH are given in the data booklet.
Video coming soon. 
Reactivity 3.1.10
Understandings: 
  • The strengths of weak acids and bases are described by their Ka, Kb, pKa or pKb values.
Learning outcomes:
  • Interpret the relative strengths of acids and bases from these data.
This video covers Ka, Kb, pKa and pKb.
Reactivity 3.1.11
Understandings: 
  • For a conjugate acid–base pair, the relationship Ka × Kb = Kw can be derived from the expressions for Ka and Kb.
Learning outcomes:
  • Solve problems involving these values.
Additional notes:
  • The use of quadratic equations is not expected in calculations.
Linking questions:
  • Reactivity 2.3 How can we simplify calculations when equilibrium constants Ka and Kb are very small?
This video covers how to solve problems involving Ka, Kb and Kw. 
Reactivity 3.1.12
Understandings: 
  • The pH of a salt solution depends on the relative strengths of the parent acid and base.
Learning outcomes:
  • Construct equations for the hydrolysis of ions in a salt, and predict the effect of each ion on the pH of the salt solution.
Additional notes:
  • Examples should include the ammonium ion, the carboxylate ion, the carbonate ion and the hydrogencarbonate ion.
  • The acidity of hydrated transition element ions and (aq) is not required.​
This video covers salt hydrolysis. 
Reactivity 3.1.13
Understandings: 
  • pH curves of different combinations of strong and weak monoprotic acids and bases have characteristic shapes and features.
Learning outcomes:
  • Interpret the general shapes of pH curves for all four combinations of strong and weak acids and bases.
Additional notes:
  • Interpretation should include: intercept with the pH axis, equivalence point, buffer region, points where pH = pKa or pOH = pKb.
This video covers pH curves. 
Reactivity 3.1.14 and 3.1.15
Understandings: 
  • Acid–base indicators are weak acids, where the components of the conjugate acid–base pair have different colours (3.1.14).
  • The pH of the end point of an indicator, where it changes colour, approximately corresponds to its pKa value (3.1.14).
  • ​An appropriate indicator for a titration has an end point range that coincides with the pH at the equivalence point (3.1.15).
Learning outcomes:
  • Construct equilibria expressions to show why the colour of an indicator changes with pH (3.1.14).
  • Identify an appropriate indicator for a titration from the identity of the salt and the pH range of the indicator (3.1.15).
Additional notes:
  • The generalised formula HInd(aq) can be used to represent the undissociated form of an indicator.
  • Examples of indicators with their pH range are given in the data booklet.
  • Include universal indicator as a mixture of many indicators with a wide pH range of colour change.
  • ​Distinguish between the terms “end point” and “equivalence point”.
This video covers acid-base indicators. 
Reactivity 3.1.16
Understandings: 
  • A buffer solution is one that resists change in pH on the addition of small amounts of acid or alkali.
Learning outcomes:
  • Describe the composition of acidic and basic buffers and explain their actions.
Linking question(s):
  • Reactivity 2.3 Why must buffer solutions be composed of weak acid or base conjugate systems, not of strong acids or bases?
This video covers buffer solutions. 
Reactivity 3.1.17
Understandings: 
  • The pH of a buffer solution depends on both:
         the pKa or pKb of its acid or base
        the ratio of the concentration of acid or base to the concentration of the conjugate base or acid.
Learning outcomes:
  • Solve problems involving the composition and pH of a buffer solution, using the equilibrium constant.
Additional information:
  • Include explanation of the effect of dilution of a buffer.
Linking question(s):
  • Reactivity 2.3 How does Le Châtelier’s principle enable us to interpret the behaviour of indicators and buffer solutions?
Video coming soon.