Eduqas Chemistry for A Level Year 2: Student Bk

87 PI5.2 Acid-base equilibria Weak acid–strong base The titration curve for a weak acid with a strong alkali, e.g. ethanoic acid with NaOH of the same concentration, is shown in the graph below. 14 12 10 8 6 2 4 0 5 15 25 35 45 55 pH Volume NaOH added (cm 3 ) Key points The pH increases gradually to about 4, as the base is added. When the volume of sodium hydroxide is about half the volume of acid, and hence the number of moles of sodium hydroxide is half the number of moles of acid, the pH levels off. This is because a mixture is formed containing the unreacted acid and the salt formed by neutralisation of the acid, and this acts as a buffer. The pH therefore levels out slightly over a volume range of about 5 cm 3 . The pH then increases gradually towards pH 7, when volume of base added equals volume of acid before increasing vertically to about pH 12. The pH gradually increases up to about between 13 and 14. Strong acid–weak base The titration curve for a strong acid with a weak alkali, e.g. hydrochloric acid with ammonia of the same concentration, is shown in the graph below. 12 10 8 6 2 4 0 5 15 25 35 45 55 pH Volume NH 3 added (cm 3 ) Key points The pH up to 7 mirrors the original strong acid–strong base titration curve. After pH 7, the line becomes smoother and then levels off at a volume of about 35–45 cm 3 due to the buffer effect. The pH then increases gradually up to a pH of about 12. Examtip It is often useful to label the buffer region of the weak acid–strong base titration curve so that it is clear to any examiner. St u dy point The terms equivalence point and end point are used interchangeably in many texts. Although both very similar, they are not the same. The equivalence point is the point when the number of moles of alkali added is equal to the number of moles of acid. The end point is the point at which an indicator changes colour in a titration. Examtip When sketching any of the titration curves, it is important to start by noting any points that are known. These include: The pH of the initial acid (at a volume of alkali of 0 cm 3 ). The pH at half neutralisation (for weak acid–strong base calculations). The pH at the equivalence point (7 for a strong acid–strong alkali titration).