pH and buffers
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Content (from A-level)
5.1.3 Acids, bases and buffers
(a) (i) a Brønsted–Lowry acid as a species that donates a proton and a Brønsted–Lowry base as a species that accepts a proton (see also 2.1.4 Acids)
(ii) use of the term conjugate acid–base pairs
(iii) monobasic, dibasic and tribasic acids
(b) the role of H+ in the reactions of acids with metals and bases (including carbonates, metal oxides and alkalis), using ionic equations (see also 2.1.4 c, 2.1.5 e)
(c) (i) the acid dissociation constant, Ka, for the extent of acid dissociation (see also 2.1.4 b)
(ii) the relationship between Ka and pKa
(d) use of the expression for pH as:
pH = –log[H+]
[H+] = 10–pH
(e) use of the expression for the ionic product of water, Kw
(f) calculations of pH, or related quantities, for:
(i) strong monobasic acids
(ii) strong bases, using Kw
(g) calculations of pH, Ka or related quantities, for a weak monobasic acid using approximations
(h) limitations of using approximations to Ka related calculations for ‘stronger’ weak acids
(i) a buffer solution as a system that minimises pH changes on addition of small amounts of an acid or a base
(j) formation of a buffer solution from:
(i) a weak acid and a salt of the weak acid, e.g. CH3COOH/CH3COONa
(ii) excess of a weak acid and a strong alkali, e.g. excess CH3COOH/NaOH
(k) explanation of the role of the conjugate acid–base pair in an acid buffer solution, e.g. CH3COOH/CH3COO–, in the control of pH
(l) calculation of the pH of a buffer solution, from the Ka value of a weak acid and the equilibrium concentrations of the conjugate acid–base pair; calculations of related quantities
(m) explanation of the control of blood pH by the carbonic acid–hydrogencarbonate buffer system
(n) pH titration curves for combinations of strong and weak acids with strong and weak bases, including:
(i) sketch and interpretation of their shapes
(ii) explanation of the choice of suitable indicators, given the pH range of the indicator
(iii) explanation of indicator colour changes in terms of equilibrium shift between the HA and A– forms of the indicator
(o) the techniques and procedures used when measuring pH with a pH meter.
Approaches to teaching the content
pH and buffers can be one of the harder topics in the A Level course, as there are so many types of calculations, assumptions and exceptions for learners to remember.
Section 5.1.3 has four main parts:
- introduction/review of acids, bases and pH
- calculating the pH of strong acids and bases, weak acids and water
- buffers – how they work and calculations
- titration curves and indicators.
The section also provides an excellent opportunity to review titration calculations. Do not be tempted to skip over these just because they have been covered previously.
Learners will have a basic understanding of acids as H+ donors, the pH scale and neutralisation. They should have covered the distinction between a base and an alkali, but may be vague on this. They will probably not know the meaning of pH, and the terms monobasic, dibasic etc. Lay firm foundations of these things in order to ensure understanding when moving on to the more tricky parts. Reviewing the formulae for all common acids and bases and whether they are strong or weak is also recommended.
It is easy to cover the content without showing the relevance to everyday life, but there are many areas of interest in the context of the human body, the environment and development of products that will reinforce the relevance of chemistry.
Common misconceptions or difficulties learners may have
1) Assumptions: In some respects this topic is good for learners with a good grasp of maths, but unfortunately they will often struggle to accept the assumptions. Keep reinforcing the fact that in chemistry assumptions must be made in order for us to understand concepts as best we can. One of the main problems that learners have when doing calculations is remembering which assumptions apply to which type of calculation. Reinforce the need to learn the assumptions by getting learners to learn them for homework and regularly test them on them; there are clear benefits of doing this in terms of tackling calculation questions.
2) Percentage dissociation: Even when learners are able to do the pH calculations, questions that involve percentage dissociation can sometimes confuse them. Ensure you teach learners what it means and give them plenty of questions which include it.
3) Indicators: Learners sometimes struggle to distinguish between the equivalence point of an acid/base titration and the pH range of an indicator when choosing a suitable indicator. Use annotated pH curves that have the pH range of the indicator labelled as well as the appropriate parts coloured in, in the appropriate colour according to the indicator (see Learner and Teacher Resource 1 in this guide). Give each group one of the four possible titrations to do and then they can either rotate or compare their answers at the end. pH probes are not always available in schools so it is worth noting that fine range indicator paper can work well in taking readings to plot titration curves.
4) Buffers: One of the difficulties with buffer calculations is that there are two types of acidic buffer made in different ways and that many calculations involve the addition of an acid or base to the buffer, which confuses learners further. Make it explicit to learners that there are two types of acidic buffer and name them:
- Type 1: Weak acid and the salt of that weak acid
- Type 2: Weak acid and a small amount of alkali.
This will remind them that there are two types and help them more easily identify the method to use when doing calculations. Another difficulty with buffer calculations is the incorrect rearrangement of the Ka equation. You can avoid this by ensuring learners learn it with [H+] as the subject so they can reduce the risk of making errors through the pressure of the exam.
A quiz testing knowledge of which substances are acid/bases, H+ and OH– ions. Although this is a GCSE exercise it is good to use this at the start of the topic to gauge learners’ knowledge.
Use this once you have taught learners how to convert H+ to pH and vice versa. These are based on the Sudoku principle where learners must use their knowledge to complete boxes with words, numbers or pictures.
Website that provides a guide to titrations and questions covering most aspects of the topic. Many of these questions would be useful for stretch and challenge.
Worksheet that takes learners step by step through the difference between acid concentration and strength.
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