Eduqas Chemistry for A Level Year 2: Student Bk

86 Eduqas A Level Chemistry Year 2: Component 1 Acid-base titration curves When a base is added to an acid, a neutralisation reaction occurs, according to the equation: H + (aq) + OH – (aq) H 2 O (l) This is the reaction that occurs during acid-base titrations. The effect of this is to reduce the concentration of H + ions, and therefore increase the pH. The increase in pH is not a straight line, however, and it depends on whether the acid and alkali are strong or weak. Strong acid–strong base The titration curve for a strong acid with a strong alkali, e.g. HCl with NaOH of the same concentration, is shown in the graph below. 14 12 10 8 6 2 4 0 0 10 20 30 40 50 pH Volume of NaOH (cm 3 ) In this case, a 0.1 mol dm –3 solution of NaOH is added to 25 cm 3 of a solution of 0.1 mol dm –3 hydrochloric acid. The graph starts at pH 1, the pH of 0.1 mol dm –3 solution of a strong acid. There is a slow gradual increase in pH as the first 20 cm 3 is added. There is a sudden increase from about pH 2 to pH 12 as a very small volume of base is added around 25 cm 3 , which is approximately vertical – use a ruler for this. There is then a slow gradual increase in pH as the last 20 cm 3 of base is added. The graph ends at just below pH 13. The vertical region of the curve occurs when the number of moles of alkali added equals the number of moles of acid in the original solution. This is called the equivalence point – when the concentrations of the two solutions are the same it will occur when the volume of alkali added is equal to the volume of acid. The pattern is always similar to this for a strong acid with a strong base, but when a weak acid is used the acid quadrant (from pH 0–7) is changed and when a weak alkali is used the alkali quadrant (pH 7–14) is changed. The pattern is changed by the fact that the molecules are not totally dissociated. These give different patterns, with the sudden increase in pH being much smaller. YOU SHOULD KNOW › › › ››› how to undertake an acid-base titration. Examtip All the curves shown here are for bases added to acids. This means that the pH starts low (acidic) and increases to a high value (bases). Titrations can also be undertaken by adding acid to alkali. In these cases the curves will appear similar but in reverse, with the initial pH high and decreasing in a similar pattern to that seen in the curves here