Acids, bases and salts - Preparation of salts (3)
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1.
Describe the preparation of an insoluble salt by precipitation. Your answer should include the reactants, the process involved, and how you would identify the formation of a precipitate.
To prepare an insoluble salt by precipitation, you need to react a soluble salt with a soluble salt to form an insoluble salt. For example, barium chloride (BaCl2) reacts with sodium sulfate (Na2SO4) to produce barium sulfate (BaSO4), which is insoluble.
Reactants: You would need a solution of a soluble salt (e.g., BaCl2) and a solution of another soluble salt (e.g., Na2SO4). The concentrations of the solutions will affect the amount of precipitate formed.
Process: The two solutions are mixed together in a beaker. As the ions from the two soluble salts combine, they form an insoluble salt that comes out of solution as a solid – this is the precipitation reaction. The reaction can be represented as:
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
Identifying the formation of a precipitate: The formation of a precipitate can be identified by observing the solution. The solution will become cloudy or opaque as the solid forms. You can also use a filter paper to collect the solid precipitate. The precipitate can then be dried and weighed to determine its mass.
2.
Describe the preparation, separation and purification of a soluble salt by reaction of an alkali with an excess of a metal. Include in your description the reagents used, the observations made, and the steps involved in isolating the pure salt.
Preparation: The salt is prepared by reacting a solution of an alkali (e.g., sodium hydroxide, NaOH) with a solution of a metal salt (e.g., copper(II) sulfate, CuSO4). The reaction produces a soluble salt and a precipitate. The balanced equation for the reaction between sodium hydroxide and copper(II) sulfate is:
| CuSO4(aq) + 2NaOH(aq) -> Cu(OH)2(s) + Na2SO4(aq) |
Separation: The precipitate (e.g., copper(II) hydroxide, Cu(OH)2) is separated from the solution by filtration. This is done using a Buchner funnel and filter paper. The filtrate contains the desired soluble salt (e.g., sodium sulfate, Na2SO4).
Purification: The filtrate is then evaporated to dryness. This leaves the solid salt, which may be impure. The salt can be further purified by recrystallization. This involves dissolving the impure salt in a minimum amount of hot water, filtering the hot solution to remove any insoluble impurities, and then allowing the solution to cool slowly. As the solution cools, the pure salt crystallizes out, leaving the impurities behind in the solution. The crystals are then filtered, washed with cold water, and dried.
3.
Define a hydrated substance and an anhydrous substance in terms of their chemical composition. Explain the difference between them.
Hydrated substance: A hydrated substance is a chemical compound that has water molecules chemically bound within its crystal structure. These water molecules are typically held in a specific ratio to the compound. The water molecules are not simply present as free water; they are an integral part of the compound's structure.
Anhydrous substance: An anhydrous substance is a chemical compound that contains no water molecules chemically bound within its structure. It is a pure substance devoid of water.
Difference: The key difference lies in the presence or absence of chemically bound water. Hydrated substances *contain* water as part of their chemical formula and structure, while anhydrous substances *do not*.