Understandings● The characteristics of the pH curves produced by the different combinations of strong and weak
acid 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. Guidance ● For an indicator which is a weak acid: HIn(aq) s H+(aq) + In–(aq) ● For an indicator which is a weak base: BOH(aq) s B+(aq) + OH–(aq) ● Examples of indicators are listed in the data booklet in section 22. ● The relationship between the pH range of an acid–base indicator, which is a weak acid, and its pKa value. Guidance The colour change can be considered to take place over a range of pKa ± 1. ● 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. |
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.
Guidance ● 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 ● 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/base with 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. Guidance ● 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. |
Inquiry questions
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Introduction to Buffers
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Action of buffers
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Titration simulator |
Inquiry investigation - pH Curves
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