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Methods of Preparation of Salts

Salts can be prepared from several methods. Any particular method chosen depends on whether the salts are soluble or not. Soluble salts are prepared by methods which involve crystallization, while insoluble salts are prepared by methods which involve precipitation.

The table below summarizes soluble and insoluble salts:

1. Preparation of Salts by The Action of An Acid Upon a Metal

This method is used to prepare soluble salts. E.g. the addition of granulated zinc to a dilute solution of tetraoxosulphate (VI) acid - effervescence of hydrogen occurs.

Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

The solution is filtered, leaving insoluble impurities, such as excess zinc and particles of carbon on the filter paper. The ZnSO4 is then crystallized from the solution.

Note: excess of H₂SO₄ in the solution is removed by adding more zinc, otherwise, the acid will become concentrated during the process of crystallization. Other salts that may be produced this way are FeSO₄ and MgSO₄ - from their respective metals.

Dilute acids are usually used. However, if the metal is less reactive, e.g. copper, concentrated acid is then used.

To prepare CuSO4. A mixture of copper turnings and dilute acid will not react. However, if the acid is concentrated and warmed, effervescence of SO₂ is produced. After a time, the salt, CuSO₄ is produced as solid, together with certain impurities - such as: solid dark brown copper(II) sulphide (formed in small amount); excess copper; and excess conc. H₂SO₄ - take note of these impurities.

To remove the impurities: - the excess H₂SO₄ is poured off. - the solid dark Cu₂S and excess copper are both insoluble in water. To remove them, a considerable amount of water is added and heated gently to boiling. The solution is then filtered, leaving the impurities on the filter paper. The crystals of the salt are obtained by crystallization.

Cu(s) + 2H₂SO₄(aq) → CuSO₄(aq) + 2H₂O(l) + SO₂(g)

CuSO₄(s) + 5H₂O(l) → CuSO₄.5H₂O

Note: * The process heating copper in conc. H₂SO₄ is the best laboratory method of preparing SO₂(g).

* Trioxonitrate (V) of certain metals are also prepared by action of the metals on dilute or conc. trioxonitrate (V) acid:

e.g. 3Pb(s) + 8HNO₃(aq) → 3Pb(NO₃)₂(aq) + 4H₂O(l)+ 2NO(g) (dilute)

3Cu(s) + 8HNO₃(aq) → 3Cu(NO3)₂(aq) + 4H₂O(l) + 2NO(g) (dilute)

Zn(s) + 4HNO₃(aq) → Zn(NO₂)₂(aq) + 2NO₂(g) + 2H₂O(l) (conc)

Chlorides of heavy metals are generally prepared in the anhydrous state (dry state) by heating the metal in a current of dry HCl. This is because the salts crystallize from their solutions with water of crystallization if produced using HCl solution.

Attempt to remove the water of crystallization will result in hydrolysis to form basic salts.

l.e. ZnCl₂.H₂O heat → Zn(OH)Cl(s) + HCl(g)

2. Preparation of Salts by Double Decomposition

This method is used to prepare insoluble salts. lt involves the reaction between two soluble compounds to produce one soluble and one insoluble products. Amongst the products, the one of importance is the insoluble salt which is gotten as it precipit- ates out of the solution.

E.g. To prepare lead (II) tetraoxosulphate (VI) - suitable reactants for this are lead (II) trioxonitrate (V) and tetraoxosulphate (VI) acid (or any other soluble tetraoxosulphate (VI), e.g. Na₂SO₄). Pb(NO₃)₂(aq) + H₂SO₄(aq) → PbSO₄(s) + 2HNO₃(aq)

Or Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq)

Other salts which can be prepared by double decomposition are:

- Barium tetraoxosulphate (VI)

BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(aq) white precipitate + 2HCl(aq) 

- Lead (II) chloride

Pb(NO₃)₂(aq) + 2NaCl(aq) → PbCl₂(s) + 2NaNO₃(aq)

- Calcium trioxocarbonate (IV)

CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2NaCl(aq)

The principle behind double decomposition reactions is that the reactants decompose to their respective ions, and then reconstitution of the ions take place to form the insoluble salts, which are precipitated.

3. Preparation of Salts by Neutralization

Recall that neutralization is the reaction between acids and bases to form salts and water only. To actually apply this method would depend on whether the base is soluble (i.e. alkali) or insoluble in water. (a).

Using A Soluble Base (alkali)

Example - to prepare sodium tetraoxosulphate (VI) from dilute sodium hydroxide solution and dilute tetraoxosulphate (VI) acid. An indicator, e.g. litmus solution is added to the solution of NaOH to obtain a pale blue solution. The dilute H₂SO₄ is then added until the solution becomes purple (indicating the equivalence point). The litmus is removed by boiling the solution with animal charcoal, and filtered. Crystals of the salt are then obtained from the solution by crystallization.

2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)

Na₂SO₄(s) + 10H₂O(l) → Na₂SO₄.10H₂O(s)

Other salts that can be prepared by this method are:

- sodium chloride NaOH (aq) + HCl (aq) → NaCl(aq) + H₂O(l)

- sodium trioxonitrate (V) NaOH(aq) + HNO₃(aq) → NaNO₃(aq) + H₂O(l)

- Ammonium chloride NH₃(aq) + HCl(aq) → NH₄Cl(aq)

- Ammonium tetraoxosulphate (VI) 2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq)

(b). Using An Insoluble Base

Example - to prepare copper (II) tetraoxosulphate (VI) crystals from the insoluble base, copper (II) oxide and tetraoxosulphate(VI) acid

- some dilute H₂SO₄ are heated, and black CuO is added gradually to obtain a blue solution of CuSO₄. More of CuO is added to reduce or to remove excess H₂SO₄ from the solution. A permanent black precipitate is obtained (indicating that all the H₂SO₄ have reacted). The solution is filtered, and the salt is obtained by crystallization.

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

CuSO₄(s)+ 5H₂O(l) → CuSO₄.5H₂O(s)

Other salts that can be prepared by this method are:

- Zinc tetraoxosulphate (VI) ZnO(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l) Or Zn(OH)₂(s) + H₂SO₄(aq) → ZnSO₄(aq) + 2H₂O(l)

- Lead trioxonitrate (V) PbO(s) + 2HNO₃(aq) → Pb(NO₃)₂(aq) + H₂O(l) Or Pb(OH)₂(s) + 2HNO₃(aq) → Pb(NO₃)₂(aq) + 2H₂O(l)

4. Preparation of Salts by the Action of An Acid on The Trioxocarbonate (IV) of A Metal

Note: trioxocarbonate(IV) of any metal will react will the mineral acids to produce the corresponding salt of the metal, water, and carbondioxide - this is one of the properties of acids.

E.g. ZnCO₃(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l) + CO₂(g)

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

PbCO₃(s) + 2HNO₃(aq) → Pb(NO₃)₂(aq) + H₂O(l) + CO₂(g)

Limitations of this Method - the limitations or disadvantages of this method arise if the carbonate and the salt produced are both insoluble in water. The salt formed is precipitated on the carbonate before all of it completely react, thereby stopping the reaction prematurely.

E.g. CaCO₃(s) (marble) + H₂SO₄(aq) → CaSO₄(s) (slightly soluble) + H₂O(l)+ CO₂(g)

The slightly soluble CaSO₄ precipitates on the unchanged marble, stopping the reaction after few seconds. Note: considering the above limitation, it is not advisable to try to prepare an insoluble salt from a compound which is also insoluble in water by a single process.

E.g. to produce insoluble lead (II) tetraoxosulphate (VI), PbSO₄ from insoluble lead (II) oxide,

PbO: do not go through this single process PbO(s) + H₂SO₄(aq) → PbSO₄(s) + H₂O(l) but, first convert the insoluble PbO to soluble compound. l.e. PbO(s) + 2HNO₃(aq) → Pb(NO₃)₂(aq) (soluble) + H₂O(l) 

Then, the soluble lead (II) trioxonitrate (V) is reacted with tetraoxosulphate (VI) acid to give the required salt - lead (II) tetraoxosulphate (VI). l.e. Pb(NO₃)₂(aq) +H₂SO₄(aq) → PbSO₄(s) + 2HNO₃(aq)

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