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Acids, Bases and Salts - Preparation of salts

Grade 12IGCSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Solubility Rules: Understanding which salts are soluble is crucial. All nitrates (NO3−NO_3^-), all Group 1 salts, and all ammonium (NH4+NH_4^+) salts are soluble. Most sulfates (SO42−SO_4^{2-}) are soluble except for BaSO4BaSO_4, PbSO4PbSO_4, and CaSO4CaSO_4.

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Method 1: Excess Insoluble Base/Metal/Carbonate - Used to prepare a soluble salt from an insoluble reactant. Add the solid in excess to the acid to ensure all acid reacts, filter the mixture to remove unreacted solid, and crystallize the filtrate.

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Method 2: Titration - Used to prepare a soluble salt from a soluble base (alkali). An indicator is used to find the exact volume of acid needed to neutralize the alkali. The process is repeated without the indicator to obtain a pure salt solution, followed by evaporation and crystallization.

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Method 3: Precipitation - Used to prepare an insoluble salt. Two soluble salt solutions are mixed together. The precipitate is collected by filtration, washed with distilled H2OH_2O to remove impurities, and dried.

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Crystallization: The process of heating a salt solution until the 'crystallization point' (saturation) is reached, then allowing it to cool slowly so that crystals form from the lattice structure.

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The choice of acid determines the anion of the salt: HClHCl produces chlorides (Cl−Cl^-), H2SO4H_2SO_4 produces sulfates (SO42−SO_4^{2-}), and HNO3HNO_3 produces nitrates (NO3−NO_3^-).

📐Formulae

Acid+Metal→Salt+H2(g)Acid + Metal \rightarrow Salt + H_2(g) accounts for reactive metals.

Acid+Base→Salt+H2O(l)Acid + Base \rightarrow Salt + H_2O(l) is a standard neutralization.

Acid+Carbonate→Salt+H2O(l)+CO2(g)Acid + Carbonate \rightarrow Salt + H_2O(l) + CO_2(g) involves effervescence.

AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq) is a precipitation reaction.

H+(aq)+OH−(aq)→H2O(l)H^+(aq) + OH^-(aq) \rightarrow H_2O(l) is the ionic equation for neutralization.

💡Examples

Problem 1:

Describe how to prepare pure, dry crystals of Copper(II) Sulfate (CuSO4⋅5H2OCuSO_4 \cdot 5H_2O) from Copper(II) Oxide (CuOCuO) and Sulfuric Acid (H2SO4H_2SO_4).

Solution:

  1. Add excess CuOCuO to a fixed volume of warm H2SO4H_2SO_4. 2. Stir until no more CuOCuO dissolves. 3. Filter the mixture to remove the excess CuOCuO. 4. Heat the filtrate (CuSO4CuSO_4 solution) in an evaporating dish until the crystallization point. 5. Leave to cool; filter the crystals and pat dry with filter paper.

Explanation:

Since CuOCuO is an insoluble base, the 'excess solid' method is used. The reaction is: CuO(s)+H2SO4(aq)→CuSO4(aq)+H2O(l)CuO(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l). Excess solid ensures all H+H^+ ions are neutralized.

Problem 2:

Suggest a method to prepare the insoluble salt Lead(II) Sulfate (PbSO4PbSO_4).

Solution:

  1. Mix aqueous Lead(II) Nitrate (Pb(NO3)2Pb(NO_3)_2) with aqueous Sodium Sulfate (Na2SO4Na_2SO_4). 2. Filter the resulting mixture to collect the precipitate of PbSO4PbSO_4. 3. Wash the residue with distilled H2OH_2O. 4. Dry the precipitate in a warm oven.

Explanation:

The precipitation method is required for insoluble salts. Both starting reagents must be soluble. The ionic equation is: Pb2+(aq)+SO42−(aq)→PbSO4(s)Pb^{2+}(aq) + SO_4^{2-}(aq) \rightarrow PbSO_4(s). Washing removes traces of the soluble NaNO3NaNO_3 byproduct.

Problem 3:

How would you prepare Sodium Chloride (NaClNaCl) starting from Sodium Hydroxide (NaOHNaOH) and Hydrochloric Acid (HClHCl)?

Solution:

Use titration. 1. Pipette 25.0 cm325.0\ cm^3 of NaOHNaOH into a conical flask. 2. Add phenolphthalein indicator. 3. Add HClHCl from a burette until the color changes from pink to colorless. 4. Note the volume of HClHCl. 5. Repeat without indicator using the same volumes. 6. Evaporate the water to obtain NaClNaCl crystals.

Explanation:

Because both reactants are soluble, an indicator is necessary to find the stoichiometric point. The reaction is: NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l). Removing the indicator ensures the salt is not contaminated.

Preparation of salts - Revision Notes & Key Formulas | IGCSE Grade 12 Chemistry