Preparation of Salts

Preparation, Separation & Purification of Salts

Salts

• A salt is a compound that is formed when the hydrogen atom in an acid is replaced by a metal
• For example if we replace the H in HCl with a potassium atom, then the salt potassium chloride is formed, KCl
• Salts are an important branch of chemistry due to the varied and important uses of this class of compounds
• These uses include fertilisers, batteries, cleaning products, healthcare products and fungicides

Naming salts

• The name of a salt has two parts
• The first part comes from the metal, metal oxide or metal carbonate used in the reaction
• The second part comes from the acid
• The name of the salt can be determined by looking at the reactants
• For example hydrochloric acid always produces salts that end in chloride and contain the chloride ion, Cl^–
• Other examples:
  • Sodium hydroxide reacts with hydrochloric acid to produce sodium chloride
  • Zinc oxide reacts with sulfuric acid to produce zinc sulfate

Preparing salts

• Some salts can be extracted by mining but others need to be prepared in the laboratory.
• There are two key ideas to consider when preparing salts:
  • Is the salt being formed soluble or insoluble in water?
  • Is there water of crystallisation present in the salt crystals?

Solubility of the common salts

Preparing soluble salts

Method A: adding acid to a solid metal, base or carbonate

Diagram showing the preparation of soluble salts

Method:

• Add dilute acid into a beaker and heat using a bunsen burner flame
• Add the insoluble metal, base or carbonate, a little at a time, to the warm dilute acid and stir until the base is in excess (i.e. until the base stops disappearing and a suspension of the base forms in the acid)
• Filter the mixture into an evaporating basin to remove the excess base
• Heat the solution to evaporate water and to make the solution saturated. Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end
• Leave the filtrate in a warm place to dry and crystallize
• Decant excess solution and allow crystals to dry

Preparation of pure, Hydrated Copper (II) Sulfate Crystals using Method A

Acid = Dilute Sulfuric Acid

Insoluble base = Copper (II) Oxide

Method:

• Add dilute sulfuric acid into a beaker and heat using a bunsen burner flame
• Add copper (II) oxide (insoluble base), a little at a time to the warm dilute sulfuric acid and stir until the copper (II) oxide is in excess (stops disappearing)
• Filter the mixture into an evaporating basin to remove the excess copper (II) oxide
• Leave the filtrate in a warm place to dry and crystallize
• Decant excess solution
• Blot crystals dry

Equation of reaction:

Copper (II) Oxide + Dilute Sulfuric Acid → Copper (II) Sulphate + Water

CuO (s) + H2SO4 (aq) → CuSO4 (s) + H2O (l)

Method B: reacting a dilute acid and alkali

Diagram showing the apparatus needed to prepare a salt by titration

Method:

• Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (phenolphthalein or methyl orange)
• Add the acid into the burette and note the starting volume
• Add the acid very slowly from the burette to the conical flask until the indicator changes to appropriate colour
• Note and record the final volume of acid in burette and calculate the volume of acid added (starting volume of acid – final volume of acid)
• Add this same volume of acid into the same volume of alkali without the indicator
• Heat to partially evaporate, leaving a saturated solution
• Leave to crystallise decant excess solution and allow crystals to dry

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Preparing Insoluble Salts

• Insoluble salts can be prepared using a precipitation reaction
• The solid salt obtained is the precipitate, thus in order to successfully use this method the solid salt being formed must be insoluble in water

Using two soluble reactants

Diagram showing the filtration of the mixture to remove the precipitate

Method:

• Dissolve soluble salts in water and mix together using a stirring rod in a beaker
• Filter to remove precipitate from mixture
• Wash filtrate with water to remove traces of other solutions
• Leave in an oven to dry

Preparation of Pure, Dry Lead (II) Sulfate Crystals using a precipitation reaction

Soluble Salt 1 = Lead (II) Nitrate

Soluble Salt 2 = Potassium Sulfate

Method:

• Dissolve Lead (II) Nitrate and Potassium Sulfate in water and mix together using a stirring rod in a beaker
• Filter to remove precipitate from mixture
• Wash precipitate with water to remove traces of potassium nitrate solution
• Leave in an oven to dry

Equation of reaction:

Lead (II) Nitrate + Potassium Sulfate → Lead (II) Sulfate + Potassium Nitrate

Pb(NO3)2  (aq) + K2SO4 (aq) → PbSO4 (s) + 2KNO3 (aq)

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Selecting a Method of Preparation

Selecting a method of preparation

• When deciding the method of preparation, it is important to first know whether the salt being produced is soluble or insoluble
• If it is soluble then it can be prepared using either method (A or B) for preparing a soluble salt
• If it is insoluble then it must be prepared by using precipitation