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Acids and Bases - Preparation and Uses of Salts

Grade 9IB

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

🔑Concepts

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A salt is an ionic compound formed when the hydrogen ions (H+H^+) of an acid are replaced by metal ions or the ammonium ion (NH4+NH_4^+).

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The naming of a salt depends on the acid used: Hydrochloric acid forms chlorides (Cl−Cl^-), Sulfuric acid forms sulfates (SO42−SO_4^{2-}), and Nitric acid forms nitrates (NO3−NO_3^-).

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Solubility rules are critical for choosing a preparation method: All Nitrates and Group 1 salts are soluble. Most Chlorides are soluble except AgClAgCl and PbCl2PbCl_2. Most Sulfates are soluble except BaSO4BaSO_4, PbSO4PbSO_4, and CaSO4CaSO_4.

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Soluble salts can be prepared by reacting an acid with a metal, an insoluble base (metal oxide/hydroxide), or a carbonate. This is often called the 'Excess Base Method'.

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Soluble salts involving Group 1 metals or Ammonium are typically prepared via 'Titration' because both reactants (acid and alkali) are aqueous.

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Insoluble salts are prepared using a 'Precipitation Reaction' by mixing two solutions of soluble salts.

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Common uses of salts include: NaClNaCl for food seasoning and preservation, NH4NO3NH_4NO_3 as a fertilizer, CaCO3CaCO_3 in construction (limestone), and CuSO4CuSO_4 as a fungicide.

📐Formulae

Acid+Metal→Salt+H2(g)\text{Acid} + \text{Metal} \rightarrow \text{Salt} + H_2(g) (Note: Metal must be above hydrogen in the reactivity series)

Acid+Base→Salt+H2O(l)\text{Acid} + \text{Base} \rightarrow \text{Salt} + H_2O(l) (Neutralization)

Acid+Metal Carbonate→Salt+H2O(l)+CO2(g)\text{Acid} + \text{Metal Carbonate} \rightarrow \text{Salt} + H_2O(l) + CO_2(g)

Soluble Salt (aq)+Soluble Salt (aq)→Insoluble Salt (s)+Soluble Salt (aq)\text{Soluble Salt (aq)} + \text{Soluble Salt (aq)} \rightarrow \text{Insoluble Salt (s)} + \text{Soluble Salt (aq)}

💡Examples

Problem 1:

Describe the preparation of Magnesium Sulfate (MgSO4MgSO_4) using Magnesium Oxide (MgOMgO) and Sulfuric Acid (H2SO4H_2SO_4).

Solution:

MgO(s)+H2SO4(aq)→MgSO4(aq)+H2O(l)MgO(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + H_2O(l)

  1. Add excess MgOMgO to H2SO4H_2SO_4 to ensure all acid is neutralized.
  2. Filter the mixture to remove the unreacted MgOMgO.
  3. Heat the filtrate (MgSO4MgSO_4 solution) to the point of crystallization.
  4. Allow to cool, filter the crystals, and dry them.

Explanation:

This is the 'Excess Base Method' used for preparing a soluble salt from an insoluble starting material.

Problem 2:

Write the ionic equation for the preparation of the insoluble salt Silver Chloride (AgClAgCl) from Silver Nitrate (AgNO3AgNO_3) and Sodium Chloride (NaClNaCl).

Solution:

Ag+(aq)+Cl−(aq)→AgCl(s)Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s) Full equation: AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)

Explanation:

This is a precipitation reaction. The Ag+Ag^+ ions from the silver nitrate and Cl−Cl^- ions from the sodium chloride combine to form a solid precipitate of silver chloride because it is insoluble in water.

Problem 3:

A student needs to prepare Sodium Nitrate (NaNO3NaNO_3). Which method should be used and why?

Solution:

The titration method should be used. Reaction: NaOH(aq)+HNO3(aq)→NaNO3(aq)+H2O(l)NaOH(aq) + HNO_3(aq) \rightarrow NaNO_3(aq) + H_2O(l)

Explanation:

Since both the starting material (Sodium Hydroxide) and the resulting salt (Sodium Nitrate) are soluble, an indicator is needed to determine the exact volume of acid required to neutralize the alkali without adding an excess that cannot be filtered out.