1 chapter 20neutralization and salts 20.1neutralization 20.2practical applications of neutralization...

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Chapter 20 Neutralization and Salts

20.1 Neutralization

20.2 Practical applications of neutralization

20.3 Salts of some common acids

20.4 Water of crystallization

20.5 Hydrolysis of salts

20.6 Deliquescent and hygroscopic substances

20.7 Preparation of salts

CONTENTS OF CHAPTER 20

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20.1 NEUTRALIZATION

20.1 NEUTRALIZATION neutralization

acid + base salt + water

NEUTRALIZATION OF ACID AND ALKALI

Suppose we slowly add dilute hydrochloric acid to dilute aqueous sodium hydroxide solution in a beaker.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

Written in ionic form:

H+(aq) + Cl(aq) + Na+(aq) + OH(aq) Na+(aq) + Cl(aq) + H2O(l)

The ionic equation is:

H+(aq) + OH(aq) H2O(l)

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Changes of particles in the neutralization reaction between hydrochloric acid and sodium hydroxide solution.

20.1 NEUTRALIZATION

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NEUTRALIZATION OF ACID AND INSOLUBLE BASE

Copper(II) oxide is an insoluble base. If we add it to warm dilute sulphuric acid, the solution will gradually turn blue.

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

black blue

The ionic equation is:

CuO(s) + 2H+(aq) Cu2+(aq) + H2O(l)

The essential process is:

O2 + 2H+ H2O

20.1 NEUTRALIZATION

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Copper(II) oxide reacts with dilute sulphuric acid to form copper(II) sulphate and water.

20.1 NEUTRALIZATION

6DEFINING NEUTRALIZATION

NEUTRALIZATION is the combination of hydrogen ions H+ and hydroxide ions OH (or oxide ions O2) to form water molecules H2O. In the process, a salt is formed.

20.1 NEUTRALIZATION

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air

beaker

NaOH(aq)

expanded polystyrene cup

thermometerHCl(aq)

Figure 20.3

A simple set-up for measuring the temperature change during neutralization.

20.1 NEUTRALIZATION

HEAT CHANGE DURING NEUTRALIZATION

8All neutralization reactions give out heat.

NEUTRALIZATION is an exothermic reaction.

20.1 NEUTRALIZATION

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20.2 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 (a natural form of calcium carbonate) or slaked lime (calcium hydroxide) to neutralize acids in soil.

Farmers can add alums or acidic fertilizers (e.g. ammonium sulphate) to lower the soil pH.

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Figure 20.4

Liming soil with powdered limestone or slaked lime to neutralize acids in soil.


11A20.1

(a) CaCO3(s) + 2H+(aq) Ca2+(aq) + CO2(g) + H2O(l)

(b) Ca(OH)2(s) + 2H+(aq) Ca2+(aq) + 2H2O(l)

NEUTRALIZATION OF EXCESS ACID IN STOMACH

Milk of Magnesia is a suspension 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)


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Figure 20.5

Milk of Magnesia or limewater can neutralize excess acid in the stomach.


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INDUSTRIAL WASTE DISPOSAL

Liquid wastes from industries are often acidic.

Figure 20.6

Industrial wastes are often acidic.


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To reduce pollution, the liquid waste should be treated before disposal — by addition of slaked lime or sodium carbonate to the acidic solution.

FERTILIZER PRODUCTION

Many common fertilizers are made by neutralization.

e.g. HNO3(aq) + NH3(aq) NH4NO3(aq)


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Figure 20.7 A fertilizer production plant.


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ACID BURNS TREATMENT

If concentrated acid is spilt on the skin, wash the affected area immediately with plenty of water. Then wash with very dilute sodium hydrogencarbonate solution, which is weakly alkaline.


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Figure 20.8

A bee. Bee stings are acidic. A suitable treatment is to wash the affected part with a weakly alkaline solution.


INSECT STING TREATMENT

Figure 20.9

A wasp. Wasp stings are alkaline. A suitable treatment is to wash the affected part with a weakly acidic solution.

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20.3 SALTS OF SOME COMMON ACIDS

A SALT AND ITS PARENT ACID

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).

e.g. CH3COO H (aq) + NaOH(aq) CH3COO Na (aq) + H2O(l)

parent acid salt

ionizable hydrogen atom

hydrogen atomreplaced by metal ion


19NAMING OF SALTS

A20.2

(a) Copper(II) chloride, hydrochloric acid

(b) Iron(III) sulphate, sulphuric acid

(c) Ammonium nitrate, nitric acid

(d) Calcium ethanoate, ethanoic acid


20ACID SALTS AND NORMAL SALTS

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.


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e.g. H H SO4(aq) + NaOH(aq) Na H SO4(aq) + H2O(l)

ionizable hydrogen atoms

hydrogen atomreplaced by metal ion

e.g. Na H SO4(aq) + NaOH(aq) Na Na SO4(aq) + H2O(l)

ionizable hydrogen atom

hydrogen atomsreplaced by metal ions


acid salt

normal salt

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Table 20.1 Examples of acid salts and normal salts, with their parent acids.


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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.


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Figure 20.10

A solid toilet cleaner which contains sodium hydrogensulphate.


25A20.3

(a) NaHSO4(aq) + NaOH(aq) Na2SO4(aq) + H2O(l)

(b) Sodium sulphate, normal salt.


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20.4 WATER OF CRYSTALLIZATION

Crystals of many salts contain water molecules loosely combined chemically. This water is called water of crystallization, and salts containing it are called hydrated salts or hydrates. A hydrated salt loses the water of crystallization quite easily on heating, forming an anhydrous salt.

Water of crystallization gives the crystals their shapes, and in many cases, their colours.

CuSO4 • 5H2O(s) CuSO4(s) + 5H2O(l)


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this dot means that the water of crystallization is loosely combined chemically with the rest of the compound

water of crystallization

Figure 20.11 The formula of a hydrated salt, copper(II) sulphate-5-water.


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Driving away the water of crystallization from hydrated copper(II) sulphate by heat.

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20.7 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.

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Table 20.3 Rules on the solubilities of common salts (and metal hydroxides) in water.


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A20.6

Water-soluble: (a), (b), (c), (g) and (h)

Water-insoluble: (d), (e) and (f)


Soluble salts are separated from the reaction mixture by crystallization (and then filtration); insoluble salts are separated by filtration.

32GENERAL METHODS OF 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

Table 20.4 General methods of preparing salts.


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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.


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Preparing an insoluble salt, lead(II) sulphate.

35A20.7

(a) (i) AgNO3(aq) + NaBr(aq) AgBr(s) + NaNO3(aq)

(ii) Mg(NO3)2(aq) + Na2CO3(aq) MgCO3(s) +

2NaNO3(aq)

(or other suitable combinations)

(b) (i) Ag+(aq) + Br(aq) AgBr(s)

(ii) Mg2+(aq) + CO32(aq) MgCO3(s)

(c) A full equation shows clearly what substances are used for the reaction. On the other hand, an ionic equation shows what particles actually take part in the reaction. Thus each type of equation has its own merits.


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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 out from the solution after some time.

Suppose we have to prepare zinc sulphate crystals.


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Figure 20.18 Preparing hydrated zinc sulphate crystals from zinc oxide and dilute sulphuric acid.


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Action of acid on alkali/ soluble carbonate

Potassium, sodium and ammonium salts are prepared by neutralization reactions between solutions of an acid and an alkali (or soluble carbonate).

acid solution

burette

conical flask

aqueous alkali + indicator

Figure 20.19

Preparation of potassium, sodium or ammonium salts.


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Suppose we have to prepare sodium chloride.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)


Preparing sodium chloride by titration.

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Separating and purifying the salt

After the soluble salt is formed, separate it out from the solution by crystallization.

A20.8

(a) No (b) Yes (c) No


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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


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SUMMARY

1. Neutralization is the combination of hydrogen ions H+ and hydroxide ions OH (or oxide ions O2) to form water molecules H2O. In the process, a salt is formed.

2. Neutralization is an exothermic reaction.

3. Practical applications of neutralization:

Soil pH control

Neutralization of excess acid in stomach

Industrial waste disposal

Fertilizer productionSUMMARY

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Acid burns treatment

Insect sting treatment

4. 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).

5. Rules for naming ionic compounds also apply to salts.

6. An acid salt is a salt in which only part of the ionizable hydrogen atoms of the parent polybasic acid has been replaced.

Examples: NaHSO4, NaHCO3.

SUMMARY

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7. A normal salt is a salt in which all the ionizable hydrogen atoms of the parent acid have been replaced.

Examples: NaCl, KNO3, Na2SO4, CH3COONa, Na3PO4.

8. Some salts are soluble in water, some are not. Refer to Table 20.3 on p. 146 for rules on solubilities of common salts (and metal hydroxides) in water.

SUMMARY

9.

1 chapter 20neutralization and salts 20.1neutralization 20.2practical applications of neutralization...

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