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CHM 217H1 Mid-Term Round-Up During the first part of this course, various narrative elements and examples have been used throughout the lectures. These have been used to illustrate the main points of the lectures. This provides a summary of those narratives and examples within the appropriate course context. Qualitative Analysis Many simple reactions can be used to identify different cations and anions in a sample at the macro and semimicro level. A number have been mentioned during the first two sections of the course. Litmus test: Litmus is a blue colouring matter obtained from various lichens, especially archil (Roccella tinctori). The active ingredient is erythrolitmin, which turns red in acidic solutions (pH < 4.5) and back to blue in alkaline ones (pH > 8.3): HO Me Me OH Me Me N OH OH O O O Repeat unit of the erythrolitmin polymer, consisting of a substituted phenoxazone and two resorcinol units. The phenolic groups are acidic; deprotonation affects the colour of the highly conjugated aromatic ring systems. Precipitation – silver nitrate for halides: Acidify the sample with dilute nitric acid then add silver nitrate solution. Halides yield characteristic precipitates: white indicates the presence of chloride, cream the presence of bromide, and yellow the presence of iodide. Precipitation – test for lead: Potassium iodide is added to the solution; if lead is present, a bright yellow precipitate of lead iodide (PbI )2is formed. Precipitation – test for sulphate: Acidify the sample with dilute hydrochloric acid then add barium chloride solution; if sulphate is present, a white precipitate (BaSO ) 4s formed. Complexation – the “Hollywood” reaction: A solution of ferric chloride is reacted with a solution of potassium thiocyanate; a blood red colour is formed due to the progressive formation of a series of complex species: 3+ – 2+ Fe + SCN → [Fe(SCN)] 2+ – 2+ Fe(SCN) + SCN → [Fe(SCN) ] 2 ... etc. Eventually, the overall reaction can be written as: 3+ – 3– Fe + 6SCN → [Fe(SCN) ] 6 2+ Note that the f–rrous ion (Fe ) does not react with thiocyanate. Cyanide (CN ) does, however, form blue complexes with both oxidation states of iron. Complexation – reaction with ammonia: Ammonia is a well-known ligand that forms many complexes with metal cations. When the cation is a transition metal, complex formation is often accompanied by a dramatic change in colour. It is essential to control pH, however, in order to avoid precipitation of weakly coloured salts. 2+ 2+ Cu (aq) 4NH 3(aq) [Cu(NH ) ]3 4 (pale blue → intense royal blue) Ni2+(aq) 6NH 3(aq) [Ni(NH ) 3 6 2+ (pale green → intense purple) Marie Curie – Purification and Analysis The procedure followed by Marie Curie to isolate new radioactive elements from ore samples was used to illustrate both methods of purification and methods of analysis. When the ores pitchblende, chalcolite, and autunite were examined, they were found to exhibit considerably more radioactivity than could be accounted for by their radium and thorium content. Marie Curie postulated that there must be other radioactive substances present, and set out to find, isolate, and identify them. These turned out to be radium, the recently–discovered element actinium, and the previously–unknown element polonium. Here is the outline of the procedure used to isolate “a few decigrams” of the elements from “several [metric] tons” of ore: • Crush ore • Roast ore with sodium carbonate • Wash with warm water • Wash with sulphuric acid – removes uranium in solution – leaves residue of insoluble sulphates • Boil sulphate residue with sodium carbonate – removes many remaining metals • Digest sulphate residue with hydrochloric acid – forms soluble chlorides • Precipitate polonium with hydrogen sulphide • Precipitate actinium with ammonium hydroxide • Dry residue to obtain radium chloride Note the extensive use of general ‘solubility rules’ (below) to selectively precipitate different species. Such principles form the basis of a number of qualitative tests (see preceding section), as well as quantitative volumetric analysis via gravimmetry. A list of specific gravimmetric reactions and reagents can be found in the course text. Solubility Rules for Inorganic Compounds: Rule Applies to: Solubility Exceptions + + + + 1 Li , Na , K , NH 4 Soluble – 2 Acetates and nitrates Soluble – 3 Cl , Br , I Soluble Ag, Hg, Pb 4 Sulphates Soluble Ca, Sr, Ba, Ag, Hg, Pb 5 Carbonates Insoluble Group 1 and NH 4+ 3– + 6 Phosphates (PO 4 ) Insoluble Group 1 and NH 4 7 Sulphides Insoluble Group 1 and NH 4+ + 8 Hydroxides* Insoluble Group 1, NH ,4Ca, Sr, Ba * Some hydroxide precipitates will redissolve at high pH due to the formation of complex anions. E.g. Zn(OH)2can form the [Zn(OH)4] anion. Solutions to Suggested Problems: You will find worked solutions for the questions from the course text in the study guide. Solutions to other questions are provided below. Checklist Questions: See the course text and solutions manual
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