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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
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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
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8All neutralization reactions give out heat.
NEUTRALIZATION is an exothermic reaction.
20.1 NEUTRALIZATION
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20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION
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.
20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION
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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)
20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION
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Figure 20.5
Milk of Magnesia or limewater can neutralize excess acid in the stomach.
20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION
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INDUSTRIAL WASTE DISPOSAL
Liquid wastes from industries are often acidic.
Figure 20.6
Industrial wastes are often acidic.
20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION
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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)
20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION
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Figure 20.7 A fertilizer production plant.
20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION
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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.
20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION
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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.
20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION
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
20.3 SALTS OF SOME COMMON ACIDS
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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
20.3 SALTS OF SOME COMMON ACIDS
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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.
20.3 SALTS OF SOME COMMON ACIDS
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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
20.3 SALTS OF SOME COMMON ACIDS
acid salt
normal salt
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Table 20.1 Examples of acid salts and normal salts, with their parent acids.
20.3 SALTS OF SOME COMMON 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.
20.3 SALTS OF SOME COMMON ACIDS
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Figure 20.10
A solid toilet cleaner which contains sodium hydrogensulphate.
20.3 SALTS OF SOME COMMON ACIDS
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25A20.3
(a) NaHSO4(aq) + NaOH(aq) Na2SO4(aq) + H2O(l)
(b) Sodium sulphate, normal salt.
20.3 SALTS OF SOME COMMON ACIDS
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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)
20.4 WATER OF CRYSTALLIZATION
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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.
20.4 WATER OF CRYSTALLIZATION
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20.4 WATER OF CRYSTALLIZATION
Driving away the water of crystallization from hydrated copper(II) sulphate by heat.
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20.7 PREPARATION OF SALTS
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.
20.7 PREPARATION OF SALTS
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A20.6
Water-soluble: (a), (b), (c), (g) and (h)
Water-insoluble: (d), (e) and (f)
20.7 PREPARATION OF SALTS
Soluble salts are separated from the reaction mixture by crystallization (and then filtration); insoluble salts are separated by filtration.
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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.
20.7 PREPARATION OF 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.
20.7 PREPARATION OF SALTS
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20.7 PREPARATION OF SALTS
Preparing an insoluble salt, lead(II) sulphate.
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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.
20.7 PREPARATION OF SALTS
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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.
20.7 PREPARATION OF SALTS
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Figure 20.18 Preparing hydrated zinc sulphate crystals from zinc oxide and dilute sulphuric acid.
20.7 PREPARATION OF SALTS
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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.
20.7 PREPARATION OF SALTS
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Suppose we have to prepare sodium chloride.
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
20.7 PREPARATION OF SALTS
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
20.7 PREPARATION OF SALTS
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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
20.7 PREPARATION OF SALTS
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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.