1 Chapter 20Neutralization and Salts 20.1Neutralization 20.2Practical applications of neutralization 20.3Salts of some common acids 20.4Water of crystallization 20.5Hydrolysis of salts 20.6Deliquescent and hygroscopic substances 20.7Preparation of salts CONTENTS OF CHAPTER 20

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) + H 2 O(l) Written in ionic form: H + (aq) + Cl  (aq) + Na + (aq) + OH  (aq)  Na + (aq) + Cl  (aq) + H 2 O(l) The ionic equation is: H + (aq) + OH  (aq)  H 2 O(l)

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

4 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) + H 2 SO 4 (aq)  CuSO 4 (aq) + H 2 O(l) black blue The ionic equation is: CuO(s) + 2H + (aq)  Cu 2+ (aq) + H 2 O(l) The essential process is: O 2  + 2H +  H 2 O 20.1 NEUTRALIZATION

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

6 DEFINING NEUTRALIZATION NEUTRALIZATION is the combination of hydrogen ions H + and hydroxide ions OH  (or oxide ions O 2  ) to form water molecules H 2 O. In the process, a salt is formed NEUTRALIZATION

7 air beaker NaOH(aq) expanded polystyrene cup thermometer HCl(aq) Figure 20.3 A simple set-up for measuring the temperature change during neutralization NEUTRALIZATION HEAT CHANGE DURING NEUTRALIZATION

8 All neutralization reactions give out heat. NEUTRALIZATION is an exothermic reaction NEUTRALIZATION

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.

10 Figure 20.4 Liming soil with powdered limestone or slaked lime to neutralize acids in soil PRACTICAL APPLICATIONS OF NEUTRALIZATION

11 A20.1 (a)CaCO 3 (s) + 2H + (aq)  Ca 2+ (aq) + CO 2 (g) + H 2 O(l) (b)Ca(OH) 2 (s) + 2H + (aq)  Ca 2+ (aq) + 2H 2 O(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)  MgCl 2 (aq) + 2H 2 O(l) 20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION

12 Figure 20.5 Milk of Magnesia or limewater can neutralize excess acid in the stomach PRACTICAL APPLICATIONS OF NEUTRALIZATION

13 INDUSTRIAL WASTE DISPOSAL Liquid wastes from industries are often acidic. Figure 20.6 Industrial wastes are often acidic PRACTICAL APPLICATIONS OF NEUTRALIZATION

14 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. HNO 3 (aq) + NH 3 (aq)  NH 4 NO 3 (aq) 20.2 PRACTICAL APPLICATIONS OF NEUTRALIZATION

15 Figure 20.7 A fertilizer production plant PRACTICAL APPLICATIONS OF NEUTRALIZATION

16 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 PRACTICAL APPLICATIONS OF NEUTRALIZATION

17 Figure 20.8 A bee. Bee stings are acidic. A suitable treatment is to wash the affected part with a weakly alkaline solution 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.

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. CH 3 COO H (aq) + NaOH(aq)  CH 3 COO Na (aq) + H 2 O(l) parent acidsalt ionizable hydrogen atom replaced by metal ion 20.3 SALTS OF SOME COMMON ACIDS

19 NAMING 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

20 ACID 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 SALTS OF SOME COMMON ACIDS

21 e.g. H H SO 4 (aq) + NaOH(aq)  Na H SO 4 (aq) + H 2 O(l) ionizable hydrogen atoms hydrogen atom replaced by metal ion e.g. Na H SO 4 (aq) + NaOH(aq)  Na Na SO 4 (aq) + H 2 O(l) ionizable hydrogen atom hydrogen atoms replaced by metal ions 20.3 SALTS OF SOME COMMON ACIDS acid salt normal salt

22 Table 20.1 Examples of acid salts and normal salts, with their parent acids SALTS OF SOME COMMON ACIDS

23 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. NaHCO 3 (aq) + NaOH(aq)  Na 2 CO 3 (aq) + H 2 O(l) Acid salts in water Most acid salts produce an acidic solution when dissolved in water. e.g. HSO 4  (aq)  H + (aq) + SO 4 2  (aq) Strange to say, a few acid salts, such as sodium hydrogencarbonate, react with water to give an alkaline solution SALTS OF SOME COMMON ACIDS

24 Figure A solid toilet cleaner which contains sodium hydrogensulphate SALTS OF SOME COMMON ACIDS

25 A20.3 (a)NaHSO 4 (aq) + NaOH(aq)  Na 2 SO 4 (aq) + H 2 O(l) (b)Sodium sulphate, normal salt SALTS OF SOME COMMON ACIDS

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. CuSO 4 5H 2 O(s)  CuSO 4 (s) + 5H 2 O(l) 20.4 WATER OF CRYSTALLIZATION

27 this dot means that the water of crystallization is loosely combined chemically with the rest of the compound water of crystallization Figure The formula of a hydrated salt, copper(II) sulphate-5-water WATER OF CRYSTALLIZATION

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

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.

30 Table 20.3 Rules on the solubilities of common salts (and metal hydroxides) in water PREPARATION OF SALTS

31 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.

32 GENERAL METHODS OF PREPARING SALTS Table 20.4 General methods of preparing salts PREPARATION OF SALTS

33 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(NO 3 ) 2 (aq) + Na 2 SO 4 (aq)  PbSO 4 (s) + 2NaNO 3 (aq) (orPb 2+ (aq) + SO 4 2  (aq)  PbSO 4 (s)) The spectator ions Na + (aq) and NO 3  (aq) remain in solution. Separating and purifying the salt Separate out the precipitate by filtration PREPARATION OF SALTS

PREPARATION OF SALTS Preparing an insoluble salt, lead(II) sulphate.

35 A20.7 (a) (i)AgNO 3 (aq) + NaBr(aq)  AgBr(s) + NaNO 3 (aq) (ii)Mg(NO 3 ) 2 (aq) + Na 2 CO 3 (aq)  MgCO 3 (s) + 2NaNO 3 (aq) (or other suitable combinations) (b) (i)Ag + (aq) + Br  (aq)  AgBr(s) (ii)Mg 2+ (aq) + CO 3 2  (aq)  MgCO 3 (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 PREPARATION OF SALTS

36 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 PREPARATION OF SALTS

37 Figure Preparing hydrated zinc sulphate crystals from zinc oxide and dilute sulphuric acid PREPARATION OF SALTS

38  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 Preparation of potassium, sodium or ammonium salts PREPARATION OF SALTS

39 Suppose we have to prepare sodium chloride. HCl(aq) + NaOH(aq)  NaCl(aq) + H 2 O(l) 20.7 PREPARATION OF SALTS Preparing sodium chloride by titration.

40 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

41 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

42 SUMMARY 1.Neutralization is the combination of hydrogen ions H + and hydroxide ions OH  (or oxide ions O 2  ) to form water molecules H 2 O. 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 production SUMMARY

43  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: NaHSO 4, NaHCO 3. SUMMARY

44 7.A normal salt is a salt in which all the ionizable hydrogen atoms of the parent acid have been replaced. Examples: NaCl, KNO 3, Na 2 SO 4, CH 3 COONa, Na 3 PO 4. 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.