BASANT S SCIENCE ACADEMY CLASS-X SUB- CHEMISTRY CHAPTER- ACID, BASE & SALT-NOTE

Transcription

1 ACID RAIN When the ph of rain water is less than 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the ph of the river water. Since our body works within a narrow ph range close to 7, the survival of aquatic life in river waters mixed with rain water becomes difficult. ROLE OF PH IN OUR DAILY LIFE Plants and Animals. Living organisms can be survive only in a narrow ph range, i.e., 6 8. Our digestive system. Our body works well within the ph range of During digestion the stomach produces acid. Too much of acid causes pain and irritation. In such cases antacids are used to neutralize excess of acid. Milk of Magnesia (magnesium hydroxide) is a mild base and used as an antacid. Our Tooth Decay. Bacteria present in the mouth produce acid by degradation of sugar and food particles remaining in the mouth after food. If the ph falls below 5.5, the concentration of H + ions is enough to dissolve calcium phosphate, one of the ingredients of tooth enamel. This can be checked by using toothpastes, (which are generally basic) which neutralize the excess acid and prevent tooth decay. SALTS- A salt is a compound formed by replacement of either all or a part of the ionisable hydrogen atoms of an acid by some other cation. A salt is formed by neutralization of an acid by a base. Acid + Base Salt + Water For example, sodium chloride (NaCl) is formed by neutralization of hydrochloric acid with sodium hydroxide. HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) Hydrochloric acid Sodium hydroxide Sodium chloride Water Similarly, potassium nitrate (KNO 3 ) is formed by neutralization of nitric acid with potassium hydroxide. HNO 3 (aq) + KOH (aq) KNO 3 (aq) + H 2 O (l) Nitric acid Potassium hydroxide Potassium nitrate Water TYPES OF SALTS Normal Salts Normal salts are the salts formed by complete replacement of all the ionisable hydrogen atoms of an acid by metallic or ammonium ions. Some examples of normal salts are: NaCl, KNO 3, Na 2 CO 3, Na 2 SO 4, (NH 4 ) 3 PO 4 HCl + NaOH NaCl + H 2 O H 2 SO 4 + 2NaOH Na 2 SO 4 + 2H 2 O Acid Salts Acid salts are the salts formed by partial replacement of ionizable hydrogen atoms by metal or ammonium ions. For example, NaHSO 4, NaHCO 3, KH 2 PO 4, K 2 HPO 4 Because there is still some replaceable hydrogen present in this type of salts, these salts behaves as an acid as well as a salt and hence are known as acid salts. Acid salts ionize in water to yield hydrogen ions. NaHSO 4, Na + + H + + SO 4 2 Basic Salts Basic salts are the salt formed by partial replacement of hydroxyl groups of a base by some other onion. For example, Basic lead chloride, Pb(OH)Cl, basic magnesium bromide, Mg(OH)Br. P H OF SALTS The aqueous solutions of salts may be neutral, acidic or basic nature depending upon the type of salt. The aqueous solutions of salts of strong acid and strong base are neutral and have ph value of 7. For example, sodium chloride, potassium nitrate, sodium sulphate, potassium chloride. The aqueous solutions of salts of strong acid and weak base are acidic and have ph value less than 7. For example, zinc sulphate, calcium chloride, copper sulphate, ammonium chloride. The aqueous solutions of salts of weak acid and strong base are basic and have ph value more than 7. For example, sodium acetate, sodium carbonate, sodium phosphate, potassium carbonate. The aqueous solutions of salts of weak acid and weak base are almost neutral and have ph value close to 7. For example, ammonium phosphate, ammonium acetate, zinc carbonate, zinc acetate, etc. FAMILY OF SALTS How many families can you identify among the salts given above?

2 Potassium sulphate, sodium sulphate, sodium chloride, potassium nitrate, calcium irate, calcium sulphate, magnesium sulphate, copper sulphate, sodium nitrate, sodium arbonate, calcium carbonate, ammonium chloride, copper chloride, ammonium nitrate, zinc sulphate. Ans. Family of sodium salts. Sodium sulphate, sodium chloride, sodium nitrate, sodium carbonate. Family of potassium salts. Potassium sulphate, potassium nitrate. Family of calcium salts. Calcium nitrate, calcium sulphate, calcium carbonate. Family of copper salts. Copper sulphate, copper chloride. Family of ammonium salts. Ammonium nitrate, ammonium chloride. Family of chloride salts. Sodium chloride, calcium chloride, ammonium chloride, copper chloride. Family of sulphate salts. Potassium sulphate, sodium sulphate, calcium sulphate, magnesium sulphate, copper sulphate, zincs sulphate. Family of nitrate salts. Potassium nitrate, calcium nitrate, sodium nitrate, ammonium nitrate. Family of carbonate salts. Sodium carbonate, calcium carbonate. CHEMICALS FROM SALT The common salt is an important raw material for many materials of daily use. Some of these compounds are: Caustic soda (NaOH), Washing soda (Na 2 CO 3. 0H 2 O), baking soda (NaHCO 3 ) and bleaching powder (CaOCI 2 ). SODIUM HYDROXIDE, CAUSTIC SODA (NaOH) PREPARATION Sodium hydroxide is prepared by passing electricity through an aqueous solution of sodium chloride (also known as brine) in a specially designed cell. As a result of electrolysis chlorine gas is given off at anode and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode. 2NaCl (aq) +2H 2 O 2NaOH (aq) +Cl 2 (g) + H 2 This process is known as chlor-alkali process due to the formation of chlorine and sodium hydroxide (an alkali) as the products. PROPERTIES OF SODIUM HYDROXIDE Sodium hydroxide is a white, translucent solid. It melts at 59 K. It is readily soluble in water to give a strongly alkaline solution. Crystals of sodium hydroxide are deliquescent. It is highly soluble in water and the solution has a very corrosive action on animal tissue. All the three products formed in chlor-alkali process are used for variety of purposes. For example, Sodium hydroxide is used in the manufacture of soaps, detergents, paper and artificial silk. It is used for degreasing metals and for purification of bauxite. It is also used in the preparation of many other compounds such as sodium chlorate, sodium hypochlorite, etc. Hydrogen is used in the manufacture of important compounds such as ammonia, hydrochloric acid, methyl alcohol, etc. It is also used for hydrogenation of oils. Hydrogen is also used as a fuel. Chlorine is used for disinfection of drinking water and swimming pools. It is used in the preparation of insecticides and pesticides. It is also used in the preparation of many chloro compounds such as CCl 4, CHCl 3, sodium chlorate, hydro chloric acid, sodium hypochlorite, etc. BLEACHING POWDER, CaOCl 2 Bleaching powder is prepared on industrial scale by passing chlorine gas through dry slaked lime. Chemically it is calcium oxychloride and is represented by the formula, CaOCI 2. It is also known as chloride of lime. Ca (OH) 2 + Cl 2 CaOCl 2 + H 2 O Slaked lime Bleaching powder Properties Bleaching powder is a yellowish white powder which gives strong smell of chlorine. It is soluble in water leaving behind a small residue of lime. When exposed to air, bleaching powder deteriorates giving off chlorine. Decomposition of bleaching powder in air takes place due to its reaction with carbon dioxide gas. CaOCl 2 + CO 2 CaCO3 + Cl 2 To avoid decomposition, bleaching powder is packed in air-tight containers. When bleaching powder is treated with excess dilute acid, chlorine gas is produced. CaOC l2 + H 2 SO 4 CaSO 4 +H 2 O + Cl 2 Chlorine gas produced in this way is known as 'available chlorine'. It is this available chlorine which is responsible for the bleaching action of the bleaching powder

3 Uses of Bleaching Powder Bleaching powder is chiefly used for bleaching cotton and linen textiles, wood and paper pulp. Delicate articles like silk, wool, straw, etc., are not bleached by it, as these are likely to be damaged. It is a strong disinfectant and is therefore, used for sterilization of water. It is also used as an oxidizing agent in the manufacture of many chemicals. It is employed for making wool unshrinkable. WASHING SODA (Na 2 CO 3.0H 2 O) Washing soda is prepared from sodium chloride by Ammonia-soda process or solvay process. In this process carbon dioxide gas is bubbled through a brine solution saturated with ammonia. It results in the formation of sodium hydrogencarbonate. NaCl + H 2 O + CO 2 + NH 3 NH 4 Cl + NaHCO 3 Ammonium chloride Sodiumhydrogencarbonate Sodium hydrogencarbonate so formed precipitates out in the presence of excess of sodium chloride. The precipitated sodium hydrogencarbonate is filtered off and then ignited to get sodium carbonate (Na 2 CO 3 ). 2NaHCO 3 Na 2 CO 3 + CO 2 + H 2 O Sodium hydrogencarbonate Sodiumcarbonate ' Anhydrous sodium carbonate thus formed is called soda-ash. When soda-ash is dissolved in water and subjected to crystallisation, the crystals that separate out are of sodium carbonate decahydrate (Na 2 CO 3. 0H 2 O) which is also known as washing soda. Na 2 CO 3 + 0H 2 O Na 2 CO 3. 0H 2 O. Soda-ash Washing soda PROPERTIES OF SODIUM CARBONATE Sodium carbonate is readily soluble in water and its aqueous solution is alkaline in nature due to hydrolysis. Na 2 CO 3 + H 2 O NaHCO 3 + NaOH NaHCO 3 + H 2 O H 2 CO 3 + NaOH When aqueous solution of sodium carbonate is subjected to crystallization, the crystals obtained are decahydrate, Na 2 CO 3.0H 2 O. This is known as washing soda. The name 'washing soda' is derived because of its use in water softening and its detergent properties. Effl orescence. On exposure to air, the crystals of decahydrate lose water and become white opaque powder which is sodium carbonate monohydrate, Na 2 CO 3.H 2 O. Exposed to air Na 2 CO 3. 0H 2 O Na 2 CO 3. H 2 O+9H 2 O The loss of water by a crystalline solid to the atmosphere on exposure to air is called Effl orescence. USES OF SODIUM CARBONATE Large quantities of sodium carbonate are used in the manufacture of glass, borax, soap and caustic soda. It is used in paper, paints and textile industries. It is used for softening hard water. It removes temporary as well as permanent hardness. It is used for washing purposes in laundry. As an important laboratory reagent both in qualitative and quantitative analysis. BAKING SODA, SODIUM HYDROGENCARBONATE (NaHCO 3 ) PREPARATION Sodium hydrogen carbonate is obtained as primary product in Solvay's process for the manufacture of sodium carbonate. NaCl + NH 3 + CO 2 + H 2 O NH 4 C + NaHCO 3 It can also be prepared by passing carbon dioxide through a solution of sodium carbonate whereby sodium hydrogencarbonate being less soluble crystals out. Na 2 CO 3 + CO 2 + H 2 O NaHCO 3 PROPERTIES Sodium hydrogencarbonate is a white crystalline solid. It is sparingly soluble in water at room temperature. Aqueous solution of sodium hydrogencarbonate is weakly alkaline in nature due to hydrolysis. NaHCO 3 + H 2 O H 2 CO 3 + NaOH Thermal decomposition. On heating, sodium hydrogencarbonate decomposes to give carbon dioxide. Heat 2NaHCO 3 Na 2 CO 3 + CO 2 + H 2 O USES OF SODIUM HYDROGENCARBONATE

4 It is used as a component of baking powder. In addition to sodium bicarbonate, baking powders contain mild edible acid such as tartaric acid or some other similar acidic compound. When mixed with water and heated, sodium hydrogencarbonate reacts with the acidic component of the baking powder producing carbon dioxide which causes bread or cake to swell and become light. Tartaric acid present in baking powder neutralizes sodium carbonate which would otherwise impart bitter taste to the food item. NaHCO 3 + H + CO 2 + H 2 O + Sodium salt of acid From acid It is used in soda-acid fire extinguishers. Soda acid fire extinguishers contain a solution of sodium hydrogencarbonate and sulphuric acid. These two chemicals are brought into contact by pressing a knob or by inverting the extinguisher. Carbon dioxide is produced which forces a stream of effervescing liquid on the fire. CO 2 surrounds the combustible substance and cuts off the supply of air. Thus, the fire gets extinguished. It is used in medicines. It acts as mild antiseptic for skin infections. It is also present as an ingredient in antacids. Being alkaline it neutralizes excess acid in the stomach. EFFLORESCENCE Certain hydrated crystalline salts when exposed to atmosphere lose their water of crystallisation spontaneously and change into amorphous powder. The spontaneous loss of water of crystallisation, wholly or partly, when crystals with water of crystallisation are exposed to air is called efforescence and the substances exhibiting efflorescence are called efflorescent substances. For example, washing soda (Na 2 CO 3. 0 H 2 O), Gluber s salt (Na 2 SO 4. 0 H 2 O). Blue vitriol (CuSO 4. 5 H 2 O) and Epsom salt (MgSO 4. 7 H 2 O). DELIQUESCENCE Certain crystalline substances when EXOPOSED to atmosphere absorb moisture and change into solution. The absorption of moisture from air by crystals to form a solution is called deliquescence. Sodium hydroxide, potassium hydroxide, calcium chloride etc. are deliquescent substances. HYGROSCOPIC SUBSTANCES Certain substances absorb water from the atmosphere without undergoing change in physical state. Such substances are known as hygroscopic substances and the phenomenon is known as hygroscopy. Anhydrous sodium carbonate, anhydrous copper sulphate, concentrated sulphuric acid are examples of hygroscopic substances. Water of Crystallization Water of crystallization is the definite number of water molecules which are present in loose chemical combination with one formula unit of the ionic solid in crystalline form. For example, copper sulphate crystals are CuSO 4. 5H 2 O. Thus, one formula unit of copper sulphate is in combination with five molecules of water. Water of crystallization can be removed by heating crystals to about 00 C. A salt which contains certain fixed number of water molecules in loose chemical combination with its one formula unit is called hydrated salt. For example, CuSO 4. 5H 2 O is a hydrated salt. A hydrated salt which completely loses its water of crystallization is called anhydrous salt. For example, CuSO 4 is anhydrous copper (II) sulphate. The anhydrous substances left when hydrated salts are heated are not crystalline. Some examples of hydrated salts are given in Table Chemical Name Common Name Formula Sodium carbonate decahydrate Washing Soda Na 2 CO 3.0H 2 O Sodium sulphate decahydrate Glauber's Salt Na 2 SO 4.0H 2 O Copper (II) sulphate Blue Vitriol CuSO 4.5H 2 O pentahydrate Iron (II) sulphate heptahydrate Green Vitriol FeSO 4.7H 2 O Zinc sulphate heptahydrate White Vitriol ZnSO 4.7H 2 O Magnesium sulphate Epsom Salt MgSO 4.7H 2 O heptahydrate Calcium sulphate dihydrate Gypsum CaSO 4.2H 2 O

5 PLASTER OF PARIS, Calcium Sulphate Hemihydrate, CaSO 4. 2 H 2O Preparation Plaster of Paris is calcium sulphate hemihydrate (CaSO 4./2H 2 O). It is prepared by heating gypsum (CaSO 4.2H 2 O) to 393 K. 393 K CaSO 4.2H 2 O CaSO 4. 2 H 2O + 2 H 2O The temperature should not be allowed to rise above 393 K because above this temperature, the whole of water of crystallization is lost. The resulting anhydrous calcium sulphate (CaSO 4 ) is called dead burnt plaster because it loses the properties of setting with water. Uses of Plaster of Paris It is used for producing moulds for industries such as pottery, ceramics. On mixing with water it changes into plastic mass and solidifies due to rehydration. This is called setting of plaster of Paris. CaSO 4. 2 H 2O + 2 H 2O CaSO 4.2H 2 O Plaster of Paris Gypsum (Hard mass) It is used for setting fractured bones in right position in the body. It is used for making statues, models and other decorative material. It is used as a fire-proofing material. It is used in laboratories for sealing the air gaps in apparatus to make it air tight

6 BASANT S SCIENCE Class-x