[CHEM] Chapter 1.3 - Qualitative Analysis

1 DARRELL ER (COPYRIGHTED) © TOPIC 1.3: QUALITATIVE ANALYSIS DARRELL ER (COPYRIGHTED) ©

2 CHAPTER ANALYSIS THE ABOUT TIME EXAM WEIGHTAGE • Heavy memorising • 3 key concepts • Always tested • Typical format is a flowchart question which involves backtracking to identify ions • Medium overall weightage • Constitute to 3.5% of marks for past 5 year papers DARRELL ER (COPYRIGHTED) ©

3 QUALITATIVE ANALYSIS TEST FOR CATIONS TEST FOR ANIONS TEST FOR GASES KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

4 TEST FOR CATIONS KEY CONCEPT DARRELL ER (COPYRIGHTED) © Cation Reaction with NaOH (aq) (strong alkaline) Reaction with NH3 (aq) (weak alkaline) Copper(II) Cu2+ Forming blue precipitate of Cu(OH)2 Precipitate is insoluble in excess NaOH Forming blue precipitate of Cu(OH)2 Precipitate dissolves in excess NH3 to give dark blue complex ion Iron(II) Fe2+ Forming green precipitate of Fe(OH)2 Precipitate is insoluble in excess NaOH Forming green precipitate of Fe(OH)2 Precipitate is insoluble in excess NH3 Iron(III) Fe3+ Forming reddish-brown precipitate of Fe(OH)3 Precipitate is insoluble in excess NaOH Forming reddish-brown precipitate of Fe(OH)3 Precipitate is insoluble in excess NH3 Calcium Ca2+ Forming white precipitate of Ca(OH)2 Precipitate is insoluble in excess NaOH No observable reaction Aluminium Al3+ Forming white precipitate of Al(OH)3 Precipitate dissolves in excess NaOH to give colourless solution Forming white precipitate of Al(OH)3 Precipitate is insoluble in excess NH3 Lead(II) Pb2+ Forming white precipitate of Pb(OH)2 Precipitate dissolves in excess NaOH to give colourless solution Forming white precipitate of Pb(OH)2 Precipitate is insoluble in excess NH3 Zinc Zn2+ Forming white precipitate of Zn(OH)2 Precipitate dissolves in excess NaOH to give colourless solution Forming white precipitate of Zn(OH)2 Precipitate dissolves in excess NH3 to give colourless solution Ammonium NH4+ No precipitate formed Warming the solution produces pungent ammonia gas which turns moist red litmus blue No observable reaction PPT formed is amphoteric oxide and is able to react with alkaline to produce soluble salt. Gives exactly same result for both reaction. Add potassium iodide, lead (II) ions will give a yellow ppt (insoluble lead iodide). BLUE GREEN REDDISH BROWN WHITE

5 TEST FOR ANIONS KEY CONCEPT DARRELL ER (COPYRIGHTED) © Anions Test Observation Carbonate CO32- Add dilute H2SO4 and bubble gas produced through limewater. (Any other suitable dilute acid can be used.) 2H+ (aq) + CO32- (aq)  CO2 (g) + H2O (l) Bubbles of CO2 gas produced which gives a white precipitate of CaCO3 in limewater. Sulfate SO42- Add dilute HNO3, followed by adding Ba(NO3)2 solution. Ba2+ (aq) + SO42- (aq)  BaSO4 (s) A white precipitate of BaSO4 is formed. Chloride Cl- Add dilute HNO3, followed by adding AgNO3 solution. Ag+ (aq) + Cl-(aq)  AgCl (s) Alternative test: Add dilute HNO3, followed by adding Pb(NO3)2 solution. Pb2+ (aq) + 2Cl- (aq)  PbCl2 (s) A white precipitate of AgCl is formed. For alternative test: A white precipitate of PbCl2 is formed. Iodide I- Add dilute HNO3, followed by adding Pb(NO3)2 solution. Pb2+ (aq) + 2I- (aq)  PbI2 (s) Alternative test: Add dilute HNO3, then add AgNO3 solution. Ag2+ (aq) + I- (aq)  AgI (s) A yellow precipitate of PbI2 is formed. For alternative test: A yellow precipitate of AgI is formed. Nitrate NO3- Add dilute NaOH, followed by adding a little aluminium powder. Warm the mixture. Aluminium reduces NO3- to NH4+ ions: NH4+ (aq) + OH- (aq)  NH3 (aq) + H2O (l) Pungent NH3 gas is produced which turns moist red litmus blue. We are adding reagents with cations that can form an insoluble salt with the anion. The insoluble salt is the coloured ppt we observe. Dilute HNO3 is first added to remove other possible anions present such as carbonates and hydroxides. As presence of these anions might also produce precipitates with silver or lead. Nitric acid is not needed if there is only one anion.

6 TEST FOR GASES KEY CONCEPT DARRELL ER (COPYRIGHTED) © Gas Smell Test Observation Ammonia, NH3 colourless, pungent Test with a moist piece of red litmus paper Moist red litmus paper turns blue Carbon dioxide, CO2 colourless, odourless Bubble the gas through limewater, Ca(OH)2 A white precipitate (CaCO3) is formed Chlorine, Cl2 greenish- yellow, pungent Test with a moist piece of blue litmus paper Blue litmus paper first turns red and then bleached Hydrogen, H2 colourless, odourless Place a lighted splint near the gas Gas extinguishes lighted splint with a “pop” sound Oxygen, O2 colourless, odourless Place a glowing splint near the gas Gas reignites glowing splint Sulfur dioxide, SO2 colourless, pungent Bubble the gas through a solution of acidified potassium dichromate(VI), K2Cr2O7. Alternative: Bubble the gas through a solution of acidified potassium manganate(VII), KMnO4. Recall: SO2 is a reducing agent! Redox reaction occurs. Acidified potassium dichromate (VI) turns from orange to green Acidified potassium manganate (VII) turns from purple to colourless

7 Try it yourself! (TYS Question) Answer:

8 Try it yourself! (TYS Question) Answer:

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