Preparation of 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 Zn(OH) 2 (s) + H 2 SO 4 (aq)  ZnSO 4 (aq) + H 2 0 (l) ZnSO 4 Zn 2+ comes from the base, Zn(OH) 2 SO 4 2- comes from sulfuric acid, H 2 SO 4

Two factors to consider…  Is the salt soluble in water?  Are the starting materials soluble in water?

Soluble SaltsInsoluble Salts All Sodium All Potassium All Ammonium All Nitrates All ChloridesLead(II) Silver All SulfatesBarium Calcium Lead Potassium Sodium Ammonium All Carbonates Potassium Sodium Ammonium Hydroxides and Oxides except

Methods of preparing salts Yes No YesNo Is the starting material for making the salt soluble in water? Titration method Acid + alkali solution Acid + carbonate solution Reaction with acids Acid + excess metal * Acid + excess insoluble base Acid + insoluble carbonate Salt solution Salt crystals (dry with filter paper) 1.Concentrating 2.Crystallizing 3.Filtering

 All insoluble salts can be prepared by 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. PbSO 4 Pb 2+ comes from soluble lead(II) salt SO 4 2- comes from sulfuric acid, H 2 SO 4 or any soluble sulfate

Methods of preparing salts Yes No YesNo Is the starting material for making the salt soluble in water? Titration method Acid + alkali solution Acid + carbonate solution Reaction with acids with insoluble substances Acid + excess metal * Acid + excess insoluble base Acid + insoluble carbonate Salt solution Salt crystals (dry with filter paper) 1.Concentrating 2.Crystallizing 3.Filtering

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

E.g. Prepare zinc sulfate 1. Think of the relevant starting materials. Zinc + Sulfuric acid  Zinc Sulfate + Hydrogen Zn(s) + H 2 SO 4 (aq)  ZnSO 4 (aq) + H 2 (g) ZnSO 4 Zn 2+ comes from the metal, Zn SO 4 2- comes from sulfuric acid, H 2 SO 4

To ensure that the sulfuric acid is completely used up (Why?) To ensure that the unreacted zinc does not contaminate the salt formed By heating to dryness, ZnSO 4 will lose its water of crystallisation Salts combine with water molecules to form crystals

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

Methods of preparing salts Yes No YesNo Is the starting material for making the salt soluble in water? Titration method Acid + alkali solution Acid + carbonate solution Reaction with acids with insoluble substances Acid + excess metal * Acid + excess insoluble base Acid + insoluble carbonate Salt solution Salt crystals (dry with filter paper) 1.Concentrating 2.Crystallizing 3.Filtering

E.g. Prepare copper sulfate 1. Think of the relevant starting materials. Copper(II) oxide + Sulfuric acid  Copper (II) Sulfate + Water CuO(s) + H 2 SO 4 (aq)  CuSO 4 (aq) + H 2O (l) CuSO 4 Cu 2+ comes from the insoluble base SO 4 2- comes from sulfuric acid, H 2 SO 4

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, CuSO 4 will lose its water of crystallisation.

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.

Methods of preparing salts Yes No YesNo Is the starting material for making the salt soluble in water? Titration method Acid + alkali solution Acid + carbonate solution Reaction with acids with insoluble substances Acid + excess metal * Acid + excess insoluble base Acid + insoluble carbonate Salt solution Salt crystals (dry with filter paper) 1.Concentrating 2.Crystallizing 3.Filtering

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.

 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.