Acid-base equilibria

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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