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 )

air

beaker

NaOH(aq)

expanded polystyrene cup(發泡膠杯 )

Thermometer (溫度計 )

HCl(aq)

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)212 H2O) o

r acidic fertilizers (e.g. ammonium sulphate) to lower the soil pH.

Ex. 20.1 (p. 137)Q 20.1

NEUTRALIZATION OF EXCESS ACID IN STOMACH

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)

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).

Salts of common acids

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) + SO4

2(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 solutio

n.

Separating and purifying the salt

Separate out the precipitate by filtration.

Preparation of lead(II)sulphate

Pb2+(aq) SO42-(aq)

Q. 20.7 p. 148

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.

Suppose we have to prepare zinc sulphate crystals.

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

End point of titration (滴定終點 )

Preparation of sodium chloride

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

Conversion of a soluble salt into another soluble salt

insoluble salt

solution of a soluble salt

another insoluble salt

precipitation

soluble salt insoluble substance

another soluble salt

precipitation