A guide for A level students KNOCKHARDY PUBLISHING INDICATORS & pH CURVES A guide for A level students KNOCKHARDY PUBLISHING

Indicators www.argonet.co.uk/users/hoptonj/sci.htm INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards. Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available. Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at... www.argonet.co.uk/users/hoptonj/sci.htm Navigation is achieved by... either clicking on the grey arrows at the foot of each page or using the left and right arrow keys on the keyboard

Indicators CONTENTS Acid-base indicators - theory Titration curves - introduction Titration curve; strong acid - strong base Titration curve; strong acid - weak base Titration curve; weak acid - strong base Titration curve; weak acid - weak base Titration curve; acid - carbonate Titration curve; phosphoric acid Check list

Acid-base indicators General Many indicators are weak acids and partially dissociate in aqueous solution HIn(aq) H+(aq) + In¯(aq) The un-ionised form (HIn) is a different colour to the anionic form (In¯).

Acid-base indicators General Many indicators are weak acids and partially dissociate in aqueous solution HIn(aq) H+(aq) + In¯(aq) The un-ionised form (HIn) is a different colour to the anionic form (In¯). Apply Le Chatelier’s Principle to predict any colour change In acid - increase of [H+] - equilibrium moves to the left to give red undissociated form In alkali - increase of [OH¯] - OH¯ ions remove H+ ions to form water; H+(aq) + OH¯(aq) H2O(l) - equilibrium will move to the right to produce a blue colour

Acid-base indicators General Many indicators are weak acids and partially dissociate in aqueous solution HIn(aq) H+(aq) + In¯(aq) The un-ionised form (HIn) is a different colour to the anionic form (In¯). Apply Le Chatelier’s Principle to predict any colour change In acid - increase of [H+] - equilibrium moves to the left to give red undissociated form In alkali - increase of [OH¯] - OH¯ ions remove H+ ions to form water; H+(aq) + OH¯(aq) H2O(l) - equilibrium will move to the right to produce a blue colour Summary In acidic solution In alkaline solution

COLOUR CHANGES OF SOME COMMON INDICATORS Acid-base indicators Must have an easily observed colour change. Must change immediately in the required pH range over the addition of ‘half’ a drop of reagent. COLOUR CHANGES OF SOME COMMON INDICATORS pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14 METHYL ORANGE CHANGE LITMUS CHANGE PHENOLPHTHALEIN CHANGE

Acid-base indicators Must change immediately in the required pH range Must have an easily observed colour change. Must change immediately in the required pH range over the addition of ‘half’ a drop of reagent. To be useful, an indicator must change over the “vertical” section of the curve where there is a large change in pH for the addition of a very small volume of alkali. The indicator used depends on the pH changes around the end point - the indicator must change during the ‘vertical’ portion of the curve. In the example, the only suitable indicator is PHENOLPHTHALEIN. PHENOLPHTHALEIN LITMUS METHYL ORANGE

strong acid (HCl) v. strong base (NaOH) pH curves Types There are four types of acid-base titration; each has a characteristic curve. strong acid (HCl) v. strong base (NaOH) weak acid (CH3COOH) v. strong alkali (NaOH) strong acid (HCl) v. weak base (NH3) weak acid (CH3COOH) v. weak base (NH3) In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M) End points need not be “neutral‘ due to the phenomenon of salt hydrolysis

strong acid (HCl) v. strong base (NaOH)

strong acid (HCl) v. strong base (NaOH) pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

strong acid (HCl) v. strong base (NaOH) Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

strong acid (HCl) v. strong base (NaOH) Very sharp change in pH over the addition of less than half a drop of NaOH Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

strong acid (HCl) v. strong base (NaOH) Curve levels off at pH 13 due to excess 0.1M NaOH (a strong alkali) Very sharp change in pH over the addition of less than half a drop of NaOH Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

strong acid (HCl) v. strong base (NaOH) PHENOLPHTHALEIN LITMUS METHYL ORANGE Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion

strong acid (HCl) v. weak base (NH3) Curve levels off at pH 10 due to excess 0.1M NH3 (a weak alkali) Sharp change in pH over the addition of less than half a drop of NH3 Very little pH change during the initial 20cm3 pH 1 at the start due to 0.1M HCl

strong acid (HCl) v. weak base (NH3) PHENOLPHTHALEIN LITMUS METHYL ORANGE Only methyl orange is suitable - it is the only one to change in the ‘vertical’ portion

weak acid (CH3COOH) v. strong base (NaOH) Curve levels off at pH 13 due to excess 0.1M NaOH (a strong alkali) Sharp change in pH over the addition of less than half a drop of NaOH Steady pH change pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

weak acid (CH3COOH) v. strong base (NaOH) PHENOLPHTHALEIN LITMUS METHYL ORANGE Only phenolphthalein is suitable - it is the only one to change in the ‘vertical’ portion

weak acid (CH3COOH) v. weak base (NH3) Curve levels off at pH 10 due to excess 0.1M NH3 (a weak alkali) NO SHARP CHANGE IN pH Steady pH change pH 4 due to 0.1M CH3COOH (weak monoprotic acid) Types

weak acid (CH3COOH) v. weak base (NH3) PHENOLPHTHALEIN LITMUS METHYL ORANGE NOTHING SUITABLE There is no suitable indicator- none change in the ‘vertical’ portion. The end point can be detected by plotting a curve using a pH meter.

Other pH curves - acid v. carbonate Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2

Other pH curves - acid v. carbonate There are two sharp pH changes Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2 There are two sharp pH changes The second addition of HCl is exactly the same as the first because the number of moles of HCl which react with the NaHCO3 is the same as that reacting with the Na2CO3. 17.50cm3 35.00cm3

Other pH curves - acid v. carbonate There are two sharp pH changes Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2 There are two sharp pH changes First rapid pH change around pH = 8.5 due to the formation of NaHCO3 . Can be detected using phenolphthalein

Other pH curves - acid v. carbonate There are two sharp pH changes Sodium carbonate reacts with hydrochloric acid in two steps... Step 1 Na2CO3 + HCl ——> NaHCO3 + NaCl Step 2 NaHCO3 + HCl ——> NaCl + H2O + CO2 Overall Na2CO3 + 2HCl ——> 2NaCl + H2O + CO2 There are two sharp pH changes First rapid pH change around pH = 8.5 due to the formation of NaHCO3 . Can be detected using phenolphthalein Second rapid pH change around pH = 4 due to the formation of acidic CO2 . Can be detected using methyl orange.

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O There are three sharp pH changes Each successive addition of NaOH is the same as equal number of moles are involved.

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of H3PO4 = 1.5

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of NaH2PO4 = 4.4 pH of H3PO4 = 1.5

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of Na2HPO4 = 9.6 pH of NaH2PO4 = 4.4 pH of H3PO4 = 1.5

Other pH curves - polyprotic acids (H3PO4) Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps... Step 1 H3PO4 + NaOH ——> NaH2PO4 + H2O Step 2 NaH2PO4 + NaOH ——> Na2HPO4 + H2O Step 3 Na2HPO4 + NaOH ——> Na3PO4 + H2O pH of Na3PO4 = 12 pH of Na2HPO4 = 9.6 pH of NaH2PO4 = 4.4 pH of H3PO4 = 1.5

What should you be able to do? REVISION CHECK What should you be able to do? Recall the definition of a weak acid Understand why indicators can be made from weak acids Understand why indicators must change colour quickly over a small pH range Recall and explain the shape of titration curves involving acids and bases Explain why particular indicators are used for certain titrations Explain the shape of the titration curve for phosphoric acid Explain the shape of the titration curve for acid -sodium carbonate reactions CAN YOU DO ALL OF THESE? YES NO

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