Nature of science: Increased power of instrumentation and advances in available techniques-development in pH meter technology has allowed for more reliable and ready measurement of pH. (3.7)
Essential idea: 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.
The characteristics of the pH curves produced by the different combinations of strong and weak acids and bases.
An acid-base indicator is a weak acid or a weak base where the components of the conjugate acid-base pair have different colours.
The relationship between the pH range of an acid-base indicator, which is a weak acid, and its pKa value.
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.
The composition and action of a buffer solution.
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.
Selection of an appropriate indicator for a titration, given the equivalence point of the titration and the end point of the indicator.
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/basewith a strong acid/base.
Prediction of the relative pH of aqueous salt solutions formed by the different combinations of strong and weak acid and base.
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.
For an indicator which is a weak acid:- HIn(aq) H+(aq) + In-(aq) Colour A Colour B - The colour change can be considered to take place over a range of pKa ± 1.
For an indicator which is a weak base:- BOH(aq) B+(aq) + OH-(aq) Colour A Colour B
Examples of indicators are listed in the data booklet in section 22.
Salts formed from the four possible combinations of strong and weak acids and bases should be considered. Calculations are not required.
The acidity of hydrated transition metal ions is covered in topic 13. The treatment of other hydrated metal ions is not required.
Is a pH curve an accurate description of reality or an artificial representation?
Does science offer a representation of reality?
Topic 5.1 - thermometric/conductiometric titrations
Topic 16.2 - What are the unusual mathematical features of a pH curve?
Students should also be familiar with the use of natural logs when using the Arrhenius expression in topic 16.2
Aim 6: Experiments could include investigation of pH curves, determination of the pKa of a weak acid, preparation and investigation of a buffer solution and the determination of the pKa of an indicator.
Aim 7:Data logging, databases, spreadsheets and simulations can all be used.
For example, the equivalence point could be determined by using a conductivity probe or a temperature probe.