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Preparation of Salts

Preparation of Salts. Is this assault?. What are salts?. In a acid-base reaction, a salt is formed when a metallic ion or an ammonium ion replaces one or more hydrogen ions in an acid . E.g. Zinc hydroxide + sulfuric acid  zinc sulfate + water

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Preparation of Salts

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  1. Preparation of Salts

  2. Is this assault?

  3. What are salts? • In a acid-base reaction, a salt is formed when a metallic ion or an ammonium ionreplaces one or more hydrogen ions in an acid. • E.g. Zinc hydroxide + sulfuric acid  zinc sulfate + water Zn(OH)2 (s) + H2SO4 (aq)  ZnSO4 (aq) + H20 (l) ZnSO4 Zn2+ comes from the base, Zn(OH)2 SO42- comes from sulfuric acid, H2SO4

  4. Preparing Salts in the Laboratory Two factors to consider… • Is the salt soluble in water? • Are the starting materials soluble in water?

  5. Solubility Table except except except except

  6. Methods of preparing salts Is the salt to be prepared soluble in water? Yes No • Reaction with acids • Acid + metal • Acid + base • Acid + carbonate 3) Precipitation Is the starting material for making the salt soluble in water? Yes No • Titration method • Acid + alkali solution • Acid + carbonate solution • Reaction with acids • Acid + excess metal * • Acid + excess insoluble base • Acid + insoluble carbonate • Concentrating • Crystallizing • Filtering Salt solution Salt crystals (dry with filter paper)

  7. Preparation of insoluble salts • All insoluble salts can be prepared by precipitation.

  8. Precipitation • Aqueous solutions of two suitable soluble salts are mixed. Eg. Prepare lead(II) sulfate • The insoluble salt appears as a precipitate and can be obtained by filtration. SO42- comes from sulfuric acid, H2SO4 or any soluble sulfate PbSO4 Pb2+ comes from soluble lead(II) salt

  9. Making PbSO4 Preparation, Separation and Purification

  10. Methods of preparing salts Is the salt to be prepared soluble in water? Yes No • Reaction with acids • Acid + metal • Acid + base • Acid + carbonate 3) Precipitation Is the starting material for making the salt soluble in water? Yes No • Titration method • Acid + alkali solution • Acid + carbonate solution • Reaction with acids with insoluble substances • Acid + excess metal * • Acid + excess insoluble base • Acid + insoluble carbonate • Concentrating • Crystallizing • Filtering Salt solution Salt crystals (dry with filter paper)

  11. Preparation of soluble salts Reason:This ensures that all the acid is used up. The reaction is complete. 1. We react the acid with an excessof the substance (MCB). Reason:This allows the excess (unreacted) substance to be filtered from the salt solution produced. 2. The substance (MCB) must also be insolublein water.

  12. Preparing a soluble salt:Reacting acid with insoluble metal E.g. Prepare zinc sulfate • Think of the relevant starting materials. Zinc + Sulfuric acid  Zinc Sulfate + Hydrogen Zn(s) + H2SO4(aq)  ZnSO4(aq)+ H2(g) ZnSO4 Zn2+ comes from the metal, Zn SO42- comes from sulfuric acid, H2SO4

  13. Making ZnSO4 To ensure that the sulfuric acid is completely used up (Why?) To ensure that the unreacted zinc does not contaminate the salt formed Salts combine with water molecules to form crystals By heating to dryness, ZnSO4 will lose its water of crystallisation

  14. Making ZnSO4Preparation, Separation and Purification

  15. Making ZnSO4 Preparation, Separation and Purification

  16. Reacting acid with insoluble metal It is not suitable for reactive metals such as potassium, sodium and calcium. They react violently with acids. Potassium Sodium Calcium Magnesium* Aluminium* Zinc* Iron Lead Copper Silver This method is suitable for moderately reactive metals such as magnesium, aluminium, zinc, iron and lead. It is not suitable for unreactive metals such as copper and silver. These metals do not react with dilute acids.

  17. Methods of preparing salts Is the salt to be prepared soluble in water? Yes No • Reaction with acids • Acid + metal • Acid + base • Acid + carbonate 3) Precipitation Is the starting material for making the salt soluble in water? Yes No • Titration method • Acid + alkali solution • Acid + carbonate solution • Reaction with acids with insoluble substances • Acid + excess metal * • Acid + excess insoluble base • Acid + insoluble carbonate • Concentrating • Crystallizing • Filtering Salt solution Salt crystals (dry with filter paper)

  18. Preparing a soluble salt:Reacting acid with insoluble base E.g. Prepare copper sulfate • Think of the relevant starting materials. Copper(II) oxide + Sulfuric acid  Copper (II) Sulfate + Water CuO(s) + H2SO4(aq)  CuSO4(aq)+ H2O(l) CuSO4 Cu2+ comes from the insoluble base SO42- comes from sulfuric acid, H2SO4

  19. Making CuSO4 To ensure that the sulfuric acid is completely used up (Why?) To ensure that the unreacted CuO does not contaminate the salt formed By heating to dryness, CuSO4 will lose its water of crystallisation.

  20. Think! In the making of copper(II) sulfate, why is copper not used to react with sulfuric acid? This method is used to prepare metal salts whose metal is not too reactive.

  21. Methods of preparing salts Is the salt to be prepared soluble in water? Yes No • Reaction with acids • Acid + metal • Acid + base • Acid + carbonate 3) Precipitation Is the starting material for making the salt soluble in water? Yes No • Titration method • Acid + alkali solution • Acid + carbonate solution • Reaction with acids with insoluble substances • Acid + excess metal * • Acid + excess insoluble base • Acid + insoluble carbonate • Concentrating • Crystallizing • Filtering Salt solution Salt crystals (dry with filter paper)

  22. Think! In the preparation of soluble salt, what happens if the solid reactant is soluble in water? How can we tell if the acid has completely neutralised? We use the method TITRATION! Use of indicator help identify that the acid has been completely used up.

  23. Titration • Suitable for preparing sodium, potassium, ammonium salts. (Why?) • Find your end point with the help of a suitable indicator then repeat experiment without indicator to get a pure salt sample.

  24. So, what have you learnt?

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