The module of the A level course on acid-base equilibria extends and applies ideas learned earlier in the course regarding equilibrium constants, and topics in this section are exclusively second year A level material. Much of the section focuses on the concept of pH. Students must develop an intimate understanding of pH in terms of its rigorous definition as -log10[H3O+] (note that many syllabuses are not so rigorous and use the less accurate H+ in place of H3O+ and this practice is also common in text books). It is perhaps worth pointing out to students that the p of pH stands for -log10 of as this then makes sense of pKa which they will meet later.
The module is heavy on calculations and students must be able to calculate the pH of a range of solutions including strong acids, weak acids, strong bases and buffer solutions. Arguably,the degree of complexity of the calculations increases in this order and requires students to apply a number of additional concepts including that of the acid dissociation constant (Ka), the ion product of water (Kw) and to either derive or recal the equations for calculation of the pH of a weak acid and that for buffer calculations. Although the derivation of these equations is not a requirement, it is not that difficult and aids the students' understanding of the concepts. As such, it is worth going through with groups.
The module then moves on to titration curves and choice of indicators. Students often have difficulty understanding why a plot of pH against volume of reagent added in an acid-base titration is not a straight line, and need reminding that the graph is a log graph since pH is a logarithmic quantity. Students need to learn the form of the curves for various combinations of acid and base (strong-strong, strong-weak etc) and use the curves to choose suitable indicators for named titrations. Titration curves are then used to measure the pKa of a weak acid, and understanding how this works requires application of the ideas about buffer solutions developed earlier in the module.
Although ideas regarding the equilibrium constant for gas phase reactions (Kp) nest better with the concept of Kc developed in the first year of A level, this is a second year topic and is therefore included here for convenience thus collecting all second year topics in one list.
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Links and Resources
This comprehensive resource provides much detailed background material on the topics covered in this section of the A level. The resource would be useful for teachers when planning lessons, but also many of the exercises, with a few dated exceptions, are still useful and pertinent to current syllabuses and can be scanned or retyped and edited to provide class or homework material. This is an invaluable resource for planning and for setting example problems to support teaching and learning.
This resource helps students to understand the crucially important difference between acid strength and concentration. Although students will have met this distinction before at GCSE, they often still harbour misconceptions regarding this issue. The Student worksheet could be done individually, but is perhaps better used in small groups to stimulate discussion and exchange of ideas. Teaching notes with answers are included.
This is a nice activity to get students started on thinking about pH once the basic definition has been covered. It uses a simulation in order to investigate the pH of various mixtures. Student worksheets and teacher notes with answers to the problems are included. This would be a useful activity at the start of a module on acid-base equilibria.
This activity enables students to apply a number of the concepts learned in this module to a real life scenario, that of acid erosion of tooth enamel. Students need to be familiar with Ka, pKa, titration curves and indicators and so this activity is best included towards the end of the module where it would provide valuable consolidation. The activity requires students to design an experimental protocol involving acid-base titration.
Useful summary notes for the teacher, and answer sheets are included.
More able students will notice that there is no carboxyl group in ascorbic acid, and researching why the highlighted proton is acidic would be an interesting extension activity taking students' concepts of acidic groups beyond the confines of the A level syllabus.
The answer is that the negative charge produced on the oxygen is delocalised over the alkene and carbonyl functional groups and therefore stabilised. On ionisation the sp3 hybridised oxygen atom of the alcohol rehybridises to an sp2 state. The negative charge (a lone pair), formally on oxygen, is now in a p orbital which can overlap with the p orbitals of the conjugated alkene and carbonyl groups to form a delocalised pi system analagous to that in benzene for example.
This resource leads the student through the experimental determination of the pKa of 2-hydroxybenzoic acid by titration against soduim hydroxide and drawing a pH curve. Students who have studied arenes may ask why the phenol group doesn't ionise as well, and this could lead to an interesting discussion of relative acidities.This complication can be avoided if preferred by using the next resource in this list as an alternative.
Brief teacher notes are included.
This is a ver similar activity to the previous one but has two benefits. The first is that the complication of having two acidic groups in one molecule, whilst only one reacts, is avoided if so wished. The second, is that aspirin is a familar substance to students and therefore adds interest.
This resource from the Royal Society of Chemistry presents as a problem solving exercise on a weak acid. Data is given on the titration of the weak acid with sodium hydroxide from which a titration curve can be drawn. Students are given additional information about the acid and asked to find out as much as they can about the acid.
This is an open ended problem solving exercise best used towards the end of the module on acid-base equilibria, so that as many concepts can be applied to the problem as possible. It would be best used in small groups to stimulate discussion of ideas (the resource suggests groups of three).
From the information given, and by applying the principles learned in this module, students can derive the pKa of the acid (from the titration curve), the concentration of the acid solution (from the titre), the pH of the initial acid solution, and the Mr and identity of the organic acid.
There is a teacher guide that includes solutions. The teacher notes include a calculation of pKb for the conjugate base of the acid (ie its salt) and also the degree of dissociation. These are no longer specified requirements at A level and can be missed out without detracting from the activity at all.
The activity can be left completely open for able students, or could be facilitated by telling students which parameters they are expected to calculate from the information presented (as listed above).
This resource bundles together the students book and teachers' guide from this series. Topic 12 covers equilibria and in particular Kp. The quantitative relationship between Kp and Kc for a reaction is derived from an understanding of the equilibrium constant and the ideal gases equation (pV = nRT) and, although not a requirement on current syllabuses, is a frequently asked question by more able students. This is a valuable source of background information on Kp, and problems to use in class or as homework.
This resource contains a number of problems on Kp starting on page 46. The answers to the problems are collected at the back. Questions can be printed out or retyped to provide class or homework material.