Preparation of Salts PDF

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Exceptions:

Hydroxides

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Hydroxides of Group I Metals

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Ba(OH)2 (Barium Hydroxide)

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Ca(OH)2 (Calcium Hydroxide)

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Preparation of Salts The most efficient way of preparing salts is the way whereby the salt can be easily extracted without loss of yield. The table below shows the overview of the different preparation methods:

Salt Prepared

Soluble

Soluble Insoluble

Preparation Method

Extraction Method

Examples

Solution + Excess solid

Filter any excess solid Evaporate the filtrate

Acid + metal Acid + carbonate(s) Acid + base(s)

Evaporate

Acid + alkali(aq) Acid + carbonate(aq)

Solution + Solution (via titration) Solution + Solution

Filter out precipitate

Solution + solution

1. Group 1 and ammonium salts are formed by titration. [ For O level syllabus, remember the SPA (sodium, potassium and ammonium salts) and group I are prepared by titration] 2. All other soluble salts are formed by reacting an excess of their oxides, hydroxides or carbonates with acid. 3. All insoluble salts are formed by precipitation. Method 1: Crystallisation (Solution + Solid) – Preparation of Soluble Salts In the “solution + solid” method, a solid and solution are reacted together to form soluble salt.

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Follow the following procedures to prepare soluble salts:

Reacting Acid + Metal (Excess) Example: Zn(s) + H2SO4(aq) →ZnSO4(aq) + H2(g) Zinc + Sulfuric Acid → Zinc Sulfate + Hydrogen

Reacting Acid + Carbonate (Insoluble, Excess) Example: CaCO3(s) + 2HCl(aq) → CaCl2(aq)+ H2O(l) + CO2(g) Calcium Carbonate + Hydrochloride Acid → Calcium Chloride + Water + Carbon Dioxide

Reacting Acid + Base (Insoluble, Excess) Example: CuO(s) + H2SO4(aq) →CuSO4(aq)+H2O(l) Copper(II) Oxide + Sulfuric Acid → Copper (II) Sulfate + Water

Preparation: 1. Add solid (oxide, carbonate or metal) to acid until no more can dissolve. Stir the solution. 2. Filter the undissolved excess solid. 3. Evaporate the filtrate. 4. Either dry the salt with heat or crystallise the salt. Heat the solution gently until a thin film of crystals is formed on the surface. 5. Cool saturated solution slowly. 6. Filter out the crystals. 7. Dry crystals between sheets of filter papers.

Method 2: Titration (Solution + Solution) In the Titration method, a solution and another solution are reacted together to form soluble salt. The reactants must all be used up so that the reaction mixture only contains the required

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salt and other “harmless” products. Hence, exact quantities of the reactants must be used to ensure complete reaction. Follow the following titration procedures to prepare soluble salts:

Reacting Acid + Alkali

Reacting Acid + Carbonate (aq)

Example:

Example:

H2SO4(aq) + 2NaOH(aq)→ Na2SO4

K2CO3 (aq) + 2HNO3(aq)→

(aq) + 2H2O(l)

2KNO3(aq) +H2O(l) + CO2(g)

Sulfuric Acid + Sodium Hydroxide →

Potassium carbonate + Nitric acid →

Soluble Sodium Sulfate + Water

Potassium Nitrate + Water + Carbon Dioxide

Preparation: 1. Pipette a volume of acid into a conical flask. 2. Add a few drops of indicator (methyl orange) into the conical flask. 3. Using a burette, add the alkali, a little at a time, into the conical flask until the indicator changes colour. 4. Note the volume of alkali used and throw away the solution in the flask. 5. Repeat the titration without the indicator, using the same volume of alkali and a clean flask. 6. Evaporate the mixture obtained.

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Method 3: Precipitation (Solution + Solution) - Preparation of Insoluble Salts Insoluble salts are prepared by precipitation via a double decomposition method, as shown below:

Reacting with Insoluble Salts Example: AgNO3(aq) + NaCl(aq) → NaNO3(aq) + AgCl(s) Silver Nitrate + Sodium Chloride → Sodium Nitrate + Silver Chloride Preparation: 1. Mix the solutions. 2. Filter mixture to get the precipitate. 3. Wash the salt with deionised water. 4. Dry precipitate between sheets of filter paper. Qualitative Analysis Test for Cations Cations present in the solution

Observations on adding aqueous Sodium Hydroxide

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Observations on adding aqueous Ammonia...