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:
Soluble Salts
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Insoluble Salts
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- All common salts of sodium,
potassium, and ammonium
- All common chlorides, except silver
chloride, lead chloride and mercury (I) chloride
- All common tetraoxosulphate (VI).
Calcium tetraoxosulphate(VI) is sparingly soluble.
.
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- All common trioxocarbonate (IV),
except those of sodium, potassium and ammonium
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Preparation
of Salts by The Action of An Acid Upon a Metal (Crystalization method)
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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