chemrevise.org 20/08/2013 Titration curves N Goalby Chemrevise.org 25 cm 3 of base

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1 chemrevise.org 20/08/203 Titration curves N Goalby Chemrevise.org Titration curves 3 Titration curves are made by measuring the of the solution in the conical flask each time a small amount of acid or base is added to the mixture. Normally base is added into the acid

2 chemrevise.org 20/08/203 Constructing a PH curve Calibrate meter first by measuring known of a buffer solution. This is necessary because meters can lose accuracy on storage Method Measure initial of the acid Add alkali in small amounts noting the volume added Stir mixture to equalise the Measure and record the to dp When approaching endpoint add in smaller volumes of alkali Add until alkali in excess Can improve accuracy by maintaining solution constant temperature It is a standard question at A2 to be asked to sketch accurately a curve. There are 4 main types of curve Strong acid and strong base Weak acid and strong base Strong acid and weak base Weak acid and weak base The Key points to sketching a curve. Initial and final 2. Volume at neutralisation 3. General Shape ( at neutralisation) 2

3 chemrevise.org 20/08/203 3 Strong acid Strong base Long vertical part from around 3 to 9 e.g. HCl and NaOH ends near 2.5 at equivalence point = starts near Vol of neutralisation Equivalence Point 3 The equivalence point lies at the mid point of the extrapolated vertical portion of the curve. The equivalence point does not have to be at 3

4 chemrevise.org 20/08/203 3 Strong acid Strong base You may also have to work out the neutralisation volume from titration data given in the question Do not just guess a volume! In some questions you may have previously worked out the of the acid or alkali. So use that on your graph Weak acid Strong base e.g. CH 3 CO 2 H and NaOH ends near 3 3 Equivalence point > Vertical part of curve > (around to 9) starts near 3 At the start the rises quickly and then levels off. The flattened part is called the 25 buffer region cm and 3 of is base formed because a buffer solution is made 4

5 chemrevise.org 20/08/203 Strong acid Weak base 3 e.g. HCl and NH 3 ends near 9 Equivalence point < Vertical part of curve < (around 4 to ) starts near Weak acid Weak base e.g. CH 3 CO 2 H and NH 3 3 ends near 9 No vertical part of the curve starts near 3 5

6 chemrevise.org 20/08/203 3 SUMMARY strong base weak base weak acid strong acid Half neutralisation volume 3 Ka = [H + (aq)][a - (aq)] [HA (aq) ] pka At ½ the neutralisation volume the [HA] = [A - ] ½ V V So Ka= [H + ] and pka = If we know the ka we can then work out the at ½ V or vice versa 6

7 chemrevise.org 20/08/203 Choosing an Indicator Indicators can be considered as weak acids. The acid must have a different colour to its conjugate base HIn (aq) colour A In - (aq) + H + (aq) colour B We can apply Le Chatelier to give us the colour. In an acid solution the H + ions present will push this equilibrium towards the reactants. Therefore colour A is the acidic colour. In an alkaline solution the OH - ions will react and remove H + ions causing the equilibrium to shift to the products. Colour B is the alkaline colour. Indicator colour at low range of colour change colour at high methyl orange red yellow phenolphthalein colourless purple An indicator changes colour from HIn to In - over a narrow range. Different indicators change colours over a different ranges The end-point of a titration is reached when [HIn] = [In - ]. To choose a correct indicator for a titration one should pick an indicator whose end-point coincides with the equivalence point for the titration

8 chemrevise.org 20/08/203 Strong acid Strong base 3 range alizarin yellow range phenolphthalein range bromothymol blue range methyl red range thymol blue An indicator will work if the range of the indicator lies on the vertical part of the titration curve. In this case the indicator will change colour rapidly and the colour change will correspond to the neutralisation point Weak acid Strong base Equivalence point for weak acid /strong base range phenolphthalein = pka range methyl orange ½ V V Volume of alkali added 8

9 chemrevise.org 20/08/203 Strong acid Strong base 3 Equivalence point for strong acid /strong base range phenolphthalein range methyl orange Strong acid Weak base 3 range phenolphthalein range methyl orange 9

10 chemrevise.org 20/08/203 3 Weak acid Weak base range phenolphthalein range methyl orange SA/WB X SA/SB WA/WB X X WA/SB X 0