Preparation of Salts GCSE Chemistry

Preparation of Salts

There are 3 major methods of preparing salts; however the method chosen depends on the solubility of the salt, and of the compounds which may be used to prepare it.

There are also some general rules when it comes to solubilities

• All common sodium, potassium ad ammonium compounds are soluble.
•   All common chlorides are soluble, except for silver and lead chlorides
•  All common sulphates are soluble except for lead barium and calcium sulphates
•   All common hydroxides are insoluble , except for sodium, potassium and ammonium hydroxides
•   All common nitrates are soluble

Preparing INSOLUBLE Salts

Precipitation is used to prepare insoluble salts. Two solutions are mixed each containing one of the two ions required to form the salt. When these two solutions are mix the ions combine and form the insoluble salt- simple! The precipitate is then filtered off, washed with a little distilled water and then left to air dry.

Example: The preparation of Barium Sulphate.

First we need two solutions, one with the barium ions and the other with the sulphate ions. AS we can see from the solubility table all nitrates and sodium compounds are soluble so we will use barium nitrate and sodium sulphate. When the two solutions are mixed the barium sulphate precipitates out.

Equation

Ba(NO₃)_{2 (aq)} + Na₂SO_{4 (aq)} à BaSO₄ (s) +2NaNO_3(aq)

Ionic Equation

Ba²⁺ _(aq) + SO₄^2- _(aq) à BaSO₄ (s)

As the other ions present remain unchanged in solution and do not react they are called “spectator ions”. Spectator ions are not included in ionic equations.

Preparing SOLUBLE Salts from an ACID and a SOLUBLE base

Titration is used to prepare soluble salts from an acid and a soluble base (i.e. an alkali). Often to prepare sodium, potassium and ammonium salts titration is used.

A known volume of the acid (commonly 25 cm³) is measured into a conical flask using a pipette. An indicator, such as phenolphthalein, is then added. The alkali is placed in a burette, and then slowly added to the conical flask containing acid drop by drop. When the indicator changes colour this shows that the acid has been neutralised, the volume of alkali that has neutralised the alkali is then recorded. The process is the repeated to obtain an accurate, average value. Once the exact volume of alkali required is known the process is repeated without the indicator to get a neutral pure salt solution.

N.B. It doesn’t matter whether the acid placed in the conical flask and the alkali is added from the burette, or vice-versa.

To obtain the salt from the solution you warm the solution gently until half the water is boiled off to concentrate the solution, then the remainder is allowed to cool, and to evaporate slowly forming large crystals.

Example: Making Sodium Chloride

Sodium Chloride is a chloride soluble salt as all sodium compounds are soluble and must be made by titration. To make a chloride salt hydrochloric acid must be used. Let’s use Sodium Hydroxide as the alkali containing sodium.

Preparing SOLUBLE Salts from an INSOLUBLE Base

The Reaction of an acid with an insoluble base (or a suitably reactive metal) is used to prepare soluble salts from an insoluble base (a metal oxide or carbonate). The appropriate acid is warmed with a Bunsen burner and a small portion of the appropriate metal oxide or carbonate (which must be insoluble) is added to the warm acid and stirred. The oxide or carbonated reacts with the acid, forming a soluble salt and appears to dissolve in the acid. Note- If a carbonate is used then effervescence will also occur as CO₂ is produced. More of the base is then added, a little at a time, with stirring. When the acid has all been neutralised, the base has nothing to react with and will sink to the bottom of the beaker unreacted, this can then be filtered off. You are then left with the pure salt solution.

The same process can also be used with a moderately reactive metal, e.g. using zinc and nitric acid to make zinc nitrate. Again, fizzing will be observed as hydrogen gas is produced.

To obtain the salt from this solution it is warmed gently until half the water is boiled off to concentrate the solution, then the remainder is allowed to cool, and to evaporate slowly forming large crystals.

Example: Making copper (II) sulphate

As it is a sulphate salt sulphuric acid must be used. Copper (II) Sulphate is soluble, so precipitation won’t work. Copper (II) Hydroxide is insoluble, so titration won’t work.

Excess copper (II) oxide is added to warm sulphuric acid, forming blue copper (II) sulphate solution. The excess copper (II) oxide is filtered off. The copper (II) Sulphate is then evaporated to form blue crystals. Copper (II) Carbonate could also have been used, as it is also insoluble.