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Salts, Pg 9

Salts, Pg 9. A salt is an ionic compound that is formed when the hydrogen ions of an acid are partly or completely replaced by metallic or ammonium ions. Salts, Pg 9. Hydrated copper (II) sulfate, CuSO 4 .5H 2 O, is blue. Hydrated Salts, Pg 9. Water of crystallisation.

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Salts, Pg 9

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  1. Salts, Pg 9 A salt is an ionic compound that is formed when the hydrogen ions of an acid are partly or completely replaced by metallic or ammonium ions.

  2. Salts, Pg 9 Hydrated copper (II) sulfate, CuSO4.5H2O, is blue.

  3. Hydrated Salts, Pg 9 Water of crystallisation Hydrated salts contain water of crystallisation CuSO4.5H2O Hydrated Copper (II) sulpahte anhydrous Copper (II) sulpahte

  4. Hydrated and Anhydrous Salts, Pg 9 Anhydrous salt Name of salt Hydrated salt Copper (II) sulfate CuSO4 CuSO4.5H2O Sodium carbonate Na2CO3 Nickel (II) sulfate NiSO4 Na2CO3.10H2O NiSO4.6H2O NiSO4.7H2O

  5. Hydrated Salts, Pg 9 How do you remove water of crystallisation from hydrated copper (II) sulfate? 1. Heat the hydrated copper (II) sulfate crystals until it turns from blue to white. CuSO4.5H2O CuSO4 + 5H2O 2. Add concentrated sulfuric acid, a dehydrating agent to hydrated copper (II) sulfate.

  6. Solubility of salts, Pg 9 1. All chlorides are soluble except silver chloride (AgCl)and lead (II) chloride, (PbCl2.) 2. All sulfates are soluble except bariumsulfate (BaSO4), calcium sulfate (CaSO4) andlead (II) sulfate (PbSO4). Note: Calcium sulfate is sparingly soluble. Silver sulfate is partially soluble.

  7. Solubility of salts, Pg 9 3. All nitrates are soluble. 4. All oxides, hydroxides and carbonates are insoluble except those of sodium, potassium and ammonium. Note: Calcium oxide and calcium hydroxide are partially soluble. Ba(OH)2 and Sr(OH)2 are soluble

  8. Solubility of salts, Pg 9 5. All salts ofsilverand lead are insoluble except those of their nitrates. 6. All salts of sodium, potassium(Group I metals) and ammonium are soluble. 7. All ethanoates are soluble.

  9. Methods of Preparing Salts, Pg 10 Acid + metal Acid + insoluble base Acid + soluble base (titration) Methods of preparing Soluble salts Acid + carbonate

  10. Preparing Soluble Salts, Pg 10 Preparing copper (II) sulfate by reacting copper (II) oxide with dilute sulfuric acid Starting materials: copper (II) oxide, dilute sulfuric acid 1. Add solid CuO tohot dilute sulfuric acid until some no longer dissolve. Why is an excess of copper (II) oxide used? To neutraliseall the dilute sulfuric acid, so that it will not contaminate the salt.

  11. Preparing Soluble Salts, Pg 10 Preparing copper (II) sulfate by reacting copper (II) oxide with dilute sulfuric acid 2. Filter the reaction products from step 1. Why? To remove unreacted/excess copper (II) oxide.

  12. Preparing Soluble Salts, Pg 10 Preparing copper (II) sulfate by reacting copper (II) oxide with dilute sulfuric acid 3. Heat the filtrate to evaporate some of the water to obtain a saturated solution.

  13. Preparing Soluble Salts, Pg 10 Preparing copper (II) sulfate by reacting copper (II) oxide with dilute sulfuric acid 4. Allow the solution to cool and crystals will form. 5. Filter the copper (II) sulfate crystals. Wash the crystals with a little cold deionised water and dry them by squeezing them between sheets of filter paper. Name two alternatives to copper (II) oxide. 1. Copper (II) hydroxide 2. Copper (II) carbonate

  14. Both reactants are soluble, hence exact quantities must be used. Titration method is used here. Preparing Soluble Salts, Pg 11 Preparing sodium sulfate (Critical thinking) 1. Can we add a metal to dilute acid? 2 Na + H2SO4 Na2SO4 + H2 Reaction is too violent and dangerous as sodium is a very reactive metal. Such an experiment should not be carried out in the laboratory. 2. How about metal hydroxide with dilute sulfuric acid? 2NaOH (aq) + H2SO4 (aq)  Na2SO4 (aq) + 2H2O (l)

  15. Preparing Soluble Salts Both reactants are soluble, hence exact quantities must be used. The method of titration has to be used here. Preparing sodium sulfate 3. How about metal carbonate with acid? Na2CO3 (aq) + H2SO4 (aq)  Na2SO4 (aq) + H2O (l) + CO2 (g) Salts of sodium, potassium (generally group I) and ammonium must be prepared using titration which uses exact quantities of reactants.

  16. Soluble Salt, Pg 11 Preparing sodium sulfate by titration H2SO4 (aq) + 2NaOH (aq)  Na2SO4 (aq) + 2H2O (l) Apparatus for titration Pipette filler burette pipette Conical flask

  17. Soluble Salt, Pg 11 Preparing sodium sulfate by titration The titration technique Volume of H2SO4 needed = (V2 - V1) cm3 Pipette NaOH into a conical flask Add 2 drops of methyl orange. Titrate NaOH with dilute H2SO4 until methyl orange changes from yellow to orange. This shows that neutralisation is complete.

  18. Soluble Salt, Pg 12 Titration using pH meter (Enrichment) pH meter can be used in place of an indicator.

  19. Preparing Insoluble Salt (Pg 12) The most important method for preparing insoluble salts is by precipitation (or direct union of ions) A precipitate is an insoluble solid obtained when two solutions are mixed. Fundamentally the method consists of mixing two soluble salts or solutions which between them contain the ions making up the insoluble compound.

  20. E.g To prepare silver chloride (Pg 12) Always choose nitrate as all nitrates are soluble For the salt containing the chloride, always choose Gp I salts as all gp I salts are soluble Reagents are aqueous silver nitrate and aqueous sodium chloride, both of which are soluble. Equation: AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq) A white precipitate (AgCl) will be formed.

  21. Experimental procedure, Pg 12 • Pour about 20 cm3 of aqueous silver nitrate in a beaker. Add aqueous sodium chloride until no more white precipitate (silver chloride) is formed. • Stir the mixture thoroughly. • Filter the mixture. The residue is the insoluble silver chloride. • Wash the residue with plenty of deionised water to remove soluble impurities such as excess reactants and aqueous sodium nitrate. • Dry the silver chloride precipitate by pressing them between sheets of filter paper.

  22. Preparation of Salts Soluble salts Insoluble salts E.G AgCl, BaSO4, ZnCO3 Salts of Gp I & ammonium Soluble Salts other than Gp I & ammonium Titration(Acid with alkali (soluble base)/soluble carbonate) Precipitation Add excess metal carbonate to warmdilute acid Add excess metal to warmdilute acid Add excess metal oxide to warmdilute acid Add excess metal hydroxide to warmdilute acid Pg 14 Based on reactions of acid Acid with insoluble metal/carbonate/base

  23. Practice, Pg 13 Refer to the concept map on page 14 when you attempt part (a). • Salt = MgSO4 • Things to consider: • MgSO4 is soluble (refer to solubility rules on page 9 of notes); • MgSO4 is not Gp I or ammonium salt • Hence should be prepared by metal/metal oxide/metal hydroxide/metal carbonate + acid method Answers: Reagents: magnesium/magnesium oxide/magnesium hydroxide with dilute sulfuric acid Eqn: MgO (s) + H2SO4 (aq) → MgSO4 (aq) + H2O (l)

  24. Practice, Pg 13 Part (a) • Salt = PbI2 • Things to consider: • PbI2 is insoluble (refer to solubility rules on page 9 of notes); • Should use precipitation • Should choose two reagents which are soluble, one of them should contain Pb2+ (choose a nitrate here as all nitrates are soluble) and the other should contain I- (choose Gp I iodide as Gp I salts are always soluble) Answers: Reagents: aqueous lead (II) nitrate, Aqueous sodium iodide Eqn: Pb(NO3)2 + 2NaI (aq) → PbI2 (s) + 2NaNO3 (aq)

  25. Practice, Pg 13 Part (a) • Salt = NH4Cl • Things to consider: • NH4Cl is soluble (refer to solubility rules on page 9 of notes); • It is an ammonium salt, so use titration which involves neutralising an alkali with an acid. • Choose an alkali which contains ammonium ion (aqueous ammonia) and an acid which contains chloride ion (hydrochloric acid) Answers: Reagents: aqueous ammonia, dilute hydrochloric acid Eqn: NH3.H2O (aq) + HCl (aq) → NH4Cl (aq) + H2O (l)

  26. Practice, Pg 13 Part (a) • Salt = NaNO3 • Things to consider: • NaNO3 is soluble (refer to solubility rules on page 9 of notes); • It is Gp I salt, so use titration which involves neutralising an alkali/soluble carbonate with an acid. • Choose an alkali/carbonate which contains sodium ion (NaOH or Na2CO3 ) and an acid which contains nitrate ion (HNO3 acid) Answers: Reagents: Aqueous sodium hydroxide, dilute nitric acid Eqn: NaOH(aq) + HNO3 (aq) → NaNO3 (aq) + H2O (l)

  27. Practice, Pg 13 Part (a) • Salt = CaCO3 • Things to consider: • CaCO3 is insoluble (refer to solubility rules on page 9 of notes); • So use precipitation – mixing 2 solutions to produce an insoluble product • Should choose two reagents which are soluble, one of them should contain Ca2+ (choose a nitrate here as all nitrates are soluble) and the other should contain CO32- (choose Gp I carbonate as Gp I salts are always soluble) Answers: Reagents: Aqueous calcium nitrate, aqueous sodium carbonate Eqn: Ca(NO3)2 (aq) + Na2CO3 (aq) → CaCO3 (s) + 2NaNO3 (aq)

  28. Practice, Pg 13 • Part (b) From the table above, choose salt(s) which are • Prepared by titration method • NH4Cl and NaNO3 • (Choose Gp I and ammonium salts) • Prepared by precipitation method • PbI2 and CaCO3 • (Choose insoluble salts)

  29. Critical thinking, Pg 13 *1. Can you prepare an insoluble salt calcium sulfate by reacting calcium carbonate with dilute sulfuric acid. Why? Coat around H2SO4 CaSO4 forms a protective layer around CaCO3 , preventing acid from coming into contact with CaCO3 CaCO3 CaCO3 CaCO3 CaCO3 H2SO4 Eqn: CaCO3 (s) + H2SO4 (aq) → CaSO4 (s) + H2O (l) + CO2 (g) No. Calcium carbonate reacts with dilute sulfuric acid to form calcium sulfate, an insoluble salt which forms a coating over calcium carbonate, preventing it from coming into contact with dilute sulfuric acid, thus stopping the reaction. * Important question

  30. Critical thinking, Pg 13 2. Can you prepare barium sulfate by adding dilute sulfuric acid to aqueous barium nitrate? Yes. Dilute sulfuric acid provides sulfate ions that react with barium ions to form the insoluble salt, barium sulfate. Ba(NO3)2 (aq) + H2SO4 (aq) → BaSO4 (s) + 2HNO3 (aq)

  31. Critical thinking, Pg 13 3. Starting from lead (II) oxide, describe briefly how you would prepare a sample of pure dry lead (II) chloride. • Thought process • PbCl2 is insoluble, hence must be prepared by precipitation • For precipitation, the 2 reagents used must be aqueous. • PbO is insoluble and cannot be used directly. Hence must convert it to a soluble form. This is achieved by adding dilute nitric acid, giving Pb(NO3)2 (Recall all nitrates are soluble, so it would be a good choice to add nitric acid here). • The rest of the procedure is precipitation, involving filtering, washing and drying…

  32. HCl/NaCl (aq) Excess HNO3 Aqueous Pb(NO3)2 PbO PbCl2 (s) Critical thinking, Pg 13 3. Starting from lead (II) oxide, describe briefly how you would prepare a sample of pure dry lead (II) chloride. With stirring, add solid lead (II) oxide to excess dilute nitric acid to obtain aqueous lead (II) nitrate. To the aqueous lead (II) nitrate, add dilute hydrochloric acid/aqueous sodium chloride. Lead (II) chloride will be formed as a white precipitate. Filter the mixture to obtain lead (II) chloride as the residue. Wash the residue with copious amounts of deionised water. Dry lead (II) chloride between sheets of filter paper.

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