Preparation of salts.(鹽的製法)
A. Neutralization (中和)
CuO(s) + 2 H+(aq)  Cu2+(aq) + H2O(l)
Neutralization is an exothermic reaction(放熱) 
HCl(aq)
Thermometer
(溫度計)
expanded
polystyrene
cup(發泡膠杯)
NaOH(aq)
beaker
air
Insulator of heat(熱的
絕緣體) : preventing
heat lost to
surroundings.
B.
PRACTICAL APPLICATIONS OF
NEUTRALIZATION
SOIL pH CONTROL
Most plants grow well only in soils which are neither
too acidic nor too alkaline.
Farmers often add powdered limestone(石灰石) (a
natural form of calcium carbonate) or slaked lime (熟石
灰)(calcium hydroxide) to neutralize acids in soil.
Farmers can add alums(明礬,AlK(SO4)212 H2O) or
acidic fertilizers (e.g. ammonium sulphate) to lower the
soil pH.
Ex. 20.1 (p. 137)
Q 20.1
NEUTRALIZATION
STOMACH
OF
EXCESS
ACID
IN
Milk of Magnesia(鎂氧乳) is a suspension(懸浮體) of
magnesium hydroxide in water. It is often used to
neutralize excess acid in the stomach, so as to relieve
stomach pain.
Mg(OH)2(s) + 2HCl(aq)  MgCl2(aq) + 2H2O(l)
INDUSTRIAL WASTE DISPOSAL
Liquid wastes from industries are often acidic.
1. Electrolyte used in
electroplating industry
2. Dyeing industry(染
料工業)
CaCO3 , Ca(OH)2,
NaHCO3, Na2CO3 are
used usually.
Fertilizer production (肥料製造)
HNO3 + NH3  NH4NO3
H2SO4 + 2 NH3  (NH4)2SO4
Acid burn treatment : wash with plenty of water , and then
wash with dilute sodium hydrogencarbonate(NaHCO3)
Insect sting treatment :sting by ant and bee are acidic(weakly
acidic solution)
Salts of common acids
A SALT is a compound formed when the ionizable
hydrogen atoms of an acid are partly or completely
replaced by metallic ions (or ammonium ions).
An ACID SALT is a salt in which only part of the
ionizable hydrogen atoms of the parent polybasic acid
has been replaced.
A NORMAL SALT is a salt in which all the ionizable
hydrogen atoms of the parent acid have been replaced.
Since an acid salt still contains ionizable hydrogen
atoms, it can react with a base to form a normal salt:
acid salt
+
base
 normal salt + water
e.g. NaHCO3(aq) + NaOH(aq) Na2CO3(aq) + H2O(l)
Acid salts in water
Most acid salts produce an acidic solution when
dissolved in water.
e.g.
HSO4-(aq)  H+(aq) + SO42-(aq)
Strange to say, a few acid salts, such as sodium
hydrogencarbonate, react with water to give an alkaline
solution.
Water of crystallization (結晶水)[hydrated salt水合鹽]
CuSO4 H2O  CuSO4 + 5 H2O
Blue
white (anhydrous salt)
PREPARATION OF SALTS
INTRODUCTION
Any method to prepare a salt involves two important
steps:
(A) Making the salt by a suitable reaction.
(B) Separating the salt from the reaction mixture
and purifying it.
We have to consider whether the salt is
(1) * soluble or insoluble in water
(2) hydrated or anhydrous.
Methods for preparing salts
Salt
Methods of preparation
Insoluble salt
precipitation
Soluble salt
neutralization
 action of acid on a metal, an insoluble
base or an insoluble carbonate
 action of acid on an alkali or a soluble
carbonate
PREPARATION OF INSOLUBLE SALTS
Mixing two solutions to get a precipitate
We can prepare insoluble salts by precipitation.
Take the example of preparing the insoluble salt
lead(II) sulphate.
Pb(NO3)2(aq)+Na2SO4(aq) PbSO4(s) + 2NaNO3(aq)
or Pb2+(aq) + SO42-(aq)  PbSO4(s)
The spectator ions Na+(aq) and NO3-(aq) remain in
solution.
Separating and purifying the salt
Separate out the precipitate by filtration.
Preparation of lead(II)sulphate
Pb2+(aq)
Q. 20.7 p. 148
SO42-(aq)
PREPARATION OF SOLUBLE SALTS
Formation of salt
 Action of acid on metal/ insoluble base/ insoluble
carbonate
To prepare a soluble salt, add an excess of the metal,
insoluble base or insoluble carbonate to the required acid.
This is to ensure that all the acid is used up.
Remove the excess solid by filtration. Boil the filtrate for
some time to concentrate the solution. Leave the hot
concentrated solution to cool slowly at room temperature.
Crystals will separate(by filtration) out from the solution after
some time.
Preparation of zinc sulphate
Preparation of potassium, sodium or ammonium salts.
acid solution
Burette(滴定管)
conical flask(錐形燒瓶)
aqueous alkali (or soluble
carbonate)+ indicator
Preparation of sodium chloride
End point of titration (滴定終點)
To obtain the salt without any indicator, use one of the
following methods :
1. Add a little activated charcoal(活性碳) to the above orange
solution. Warm and then filter the mixture. The filtrate would
be a colourless solution of sodium chloride.
2. Repeat the experiment with exactly the same volume of acid
and alkali needed for neutralization (as found by the above
experiment). However, do not add methyl orange indicator at
the start of this time.
Separate the salt form the solution by the method of
crystallization. Filter the solid formed. Then wash them with a
little cold distilled water. Finally, dry the crystal by using filter
paper or an oven.
A NOTE ON PREPARATION OF SALTS
Conversion of an insoluble salt into another insoluble salt
insoluble
salt
solution of
a soluble
salt
precipitation
another
insoluble
salt
Conversion of a soluble salt into another soluble salt
precipitation
soluble salt
insoluble
substance
another
soluble
salt