Eduqas Chemistry for A Level Year 2: Student Bk

88 Eduqas A Level Chemistry Year 2: Component 1 Weak acid–weak base The titration curve for a weak acid with a weak alkali, e.g. ethanoic acid with ammonia of the same concentration, is shown in the graph below. 10 8 6 2 4 0 0 10 20 30 40 50 pH Volume of base Titration of a weak base into a weak acid The graph for weak acids with weak bases lacks the clear vertical region that is present in the other graphs. This makes it much harder to study using an indicator in titration. In place of an indicator a pH probe is used, measuring the pH throughout the titration. A plot of the data is then used to identify the equivalence point, as this is the point of inflection. Obtaining data from titration curves Equivalence points Titration curves can be used to find the volume of alkali needed to neutralise the acid. For titration curves where there are vertical regions these mark the volume required for neutralisation. The value obtained can be used in the titration calculations to find the concentration of one solution. pH of salt formed The pH at the end point of each of these titrations can be found from the midpoint of the vertical region. In the case of strong acid–strong base this will be 7; however, this will shift away from neutral when we use a weak acid or a weak base. At the end point the solution only includes a salt, and the salts formed from weak acids are basic whilst those formed from weak alkalis are acidic. The pH at neutralisation is equal to the pH of the salt solution. p K a of a weak acid In the discussion of buffers the pH of a buffer was expressed as: pH buffer = pK a + log [SALT] [ACID] When half the volume of base needed for neutralisation is added to the acid, half the acid will have been converted into salt. This means that the concentration of the salt will equal the concentration of the acid, and so the log term would be log (1) which equals zero. This simplifies the equation above to: pH buffer = p K a So the p K a equals the pH at the point that half the alkali has been added for neutralisation. St u dy point All these diagrams are plotted for concentrations of acid and base that are the same. If a titration is undertaken with different concentrations of acid and base, the shape of the plot will be unchanged but the graph will be compressed along the volume axis (when the base has a higher concentration) or extended along the volume axis (when the acid has a higher concentration). Use the volume when the vertical region occurs to find the equivalence point and this will tell you whether the concentrations of the two are the same, as it will occur at the same volume of base as the original volume of acid. & S C Stretch & Challenge The curves already seen are drawn for monobasic acids only, such as HCl or HNO 3 . Dibasic acids such as H 2 SO 4 lose each hydrogen in turn and so there are two equivalence points, with tribasic acids showing three equivalence points. The graph below shows what this pattern would look like for a tribasic acid. 12 10 8 6 2 4 0 pH Volume of Titrant Added First Equivalence Point Second Equivalence Point Third Equivalence Point This graph allows us to check our calculations as the volume needed for the first equivalence point should be half that for the second, with the third value being three times the original.