Some Important Commercial Salts

Chemistry Acids, Bases& Salts Some important Commercial Salts MORE ABOUT SALTS Salts are the ionic compounds consisting of two parts, one part carrying a positive charge called positive ion or cation and the other part carrying a negative charge called a negative ion or anion.  Family of Salts Salts of a strong acid and a strong base are neutral with pH value of 7. On the other hand, salts of a strong acid and weak base are acidic with pH value less than 7 and those of a strong base and weak acid are basic in nature, with pH value more than 7. Salts are generally formed by between an acid and base. Acid on the basis of kind of acid and base used they contain specific cation or anion. So we can divide salts in different families on the basis of cation or anion present. The salts having the same positive radical (or cation) or negative radical (or anion) are said to belong to the same family. Or we can say, salts can be classified into different families based on the common ion present. Activity: Aim: Write the formulae of the salts and to identify their acids and bases and the families. Method: (i) Write down the formulae of the salts given below: 1. Potassium Sulphate 2. Sodium Sulphate 3. Calcium Sulphate 4. Magnesium Sulphate 5. Copper Sulphate 6. Sodium Chloride 7. Sodium Nitrate 8. Sodium Carbonate 9. Ammonium Chloride 1 Chemistry (ii) Identify the acids and bases from which the above salts may be obtained. (iii) Salts having the same positive or negative radicals are said to belong to a family. For example, NaCl and Na2SO4 belong to the family of sodium salts. Similarly, NaCl and KCl belong to the family of chloride salts. How many families can you identify among the salts given in this activity? Observation and Conclusion: Families: (i) On the basis of common salts Sulphates = K2SO4, Na2SO4, CaSO4, MgSO4, CuSO4 Chlorides = NaCl, NH4Cl Carbonates = Na2CO3 (ii) On the basis of common bases Sodium Salts = Na2SO4, NaCl, NaNO3, Na2CO3 Potassium Salts = K2SO4 2 Chemistry Calcium Salts = CaSO4 Magnesium Salts = MgSO4 Copper Salts = CuSO4 Ammonium Salts = NH4Cl NaCl (sodium chloride) and Na2SO4 (sodium sulphate) belong to the family of sodium salts because both contain the same radical (or cation), that is Na+. These may be called sodium salts. Copper Sulphate (CuSO4) and Sodium Sulphate (Na2SO4) belong to the family of sulphates because both contain the same acid radical (or anion) that is sulphate (SO42–).  Formation of salts Salts can be prepared by many reactions some of these are (i) By neutralization of acids and bases. E.g.  NaOH (ii)   HCl (sodium hydroxide ) ( hydrochloric acid ) NaCl (sodium chloride )  H2O ( water ) By action of metals on acids. E.g. Zn  H2SO4 ( zinc ) (iii)   ZnSO4  (sulphuric acid ) ( zinc sulphate) H2 ( hydrogen) By action of acids on metal carbonates and bicarbonates. E.g. CaCO3  ( calcium carbonate) 2HCl   ( hydrochloric acid ) (iv) By action of metal on alkalis. E.g. 3 CaCl2  H 2O  CO2 ( calcium chloride ) ( water ) ( carbon dioxide ) Chemistry 2NaOH (sodium hydroxide )  Zn   Na 2ZnO 2  ( zinc ) (sodium zincate ) H2 ( hydrogen) pH of salts Depending upon the nature of acid and base which react to form the salt. OR The nature of acid and base produced when salt reacts with water, the salts can be classified into the following four types. (i) Salts of strong acid and strong base. E.g. NaCl NaCl (sodium chloride )  H 2O   ( water ) NaOH  (sodium hydroxide ) strongbase HCl ( hydrochloric acid ) strong acid Hence, the acid and the base produced neutralise each other completely. So, pH = 7. (ii) Salt of strong acid and weak base. E.g. NH4Cl NH 4Cl ( ammonium chloride )  H 2O   ( water ) NH 4OH ( ammonium hydroxide ) weak base  HCl ( hydrochloric acid ) strong acid Here, pH < 7. (iii) Salt of weak acid and strong base. E.g. Na2CO3 Na 2CO3 (sodium carbonate)  2H 2O   ( water ) 2 NaOH (sodium hydroxide ) strong base  H 2CO3 ( carbonic acid ) weak acid Here, pH > 7 (iv) Salt of weak acid and weak base. E.g. CH3COONH4 CH3COONH4  H 2O   CH3COOH  (ammonium acetate) ( water ) Here pH  7 4 (acetic acid ) NH 4OH ( ammonium hydroxide ) Chemistry  Common salt: A raw material for chemical The common salt thus obtained is an important raw material for materials of daily use, such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more. (i) Sodium Chloride (NaCl): By now we have learnt that the salt formed by the combination of hydrochloric acid and sodium hydroxide solution is called sodium chloride. This is the salt that we use in food it is a neutral salt. Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. These large crystals are often brown due to impurities. This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal. NaOH + HCl NaCl + H2O (ii) Sodium Hydroxide (NaOH): When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide process is called the chlor-alkali process because of the products chlor for chlorine and alkali for sodium hydroxide. 2NaCl (aq) + 2H2O (l) 2NaOH (aq) + Cl2 (g) + H2 (g) Chlorine gas is given off at the anode, and hydrogen gas at the cathode Sodium hydroxide solution is formed near the cathode. The products produced in this process are all useful. 5 Chemistry Important products from the chlor-alkali process (iii) Bleaching Powder (CaOCl2) It is a calcium salt of hypochlorous acid. It may be represented by CaOCl2 (Calcium Chlorohypochlorite, Chloride of lime). Actually it is a mixture of CaOCl2.4H2O and basic calcium chloride, CaCl2.Ca(OH)2.H2O chlorine is produced during electrolysis of aqueous sodium chloride (brine). This chlorine gas is use for the manufacture of bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2]. Bleaching power is represented as CaOCl2, though the actual composition is complex. Ca(OH)2 + Cl2 CaOCl2 + H2O 6 Chemistry Properties 1. Bleaching powder is a yellowish white powder which gives strong smell of chlorine. 2. It is soluble in water leaving behind a small residue of lime. 3. 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. CaOCl2 + CO2 CaCO3 + Cl2  4. When bleaching powder is treated with excess dilute acid, chlorine gas is produced. CaOCl2 + H2SO4 CaSO4 + H2O + Cl2  CaOCl2 + 2HCl CaCl2 + H2O + Cl2  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. Uses of Bleaching Powder. 1. 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. 2. It is strong disinfectant and is therefore, used for sterilization of water. 3. It is employed for making wool unshrinkable. 4. It is also used as an oxidizing agent in the manufacture of many chemicals. Bleaching of Cloth by bleaching powder The cloth to be bleached is initially treated with a very dilute solution of sodium hydroxide which removes any greasy matter. It is then dipped in bleaching powder solution followed by dilute hydrochloric solution taken in a separate tank. The acid also reacts with bleaching powder to evolve chlorine which bleaches the cloth. In order to remove the unreacted chlorine, it is then dipped in a dilute solution of sodium thiosulphate called antichlor. 7 Chemistry  Cl2  H2O   Na 2SO4  2HCl  S Na 2S2O3 (sodium thiosulphate) An antichlor is a substance which removes the unreacted chlorine. The cloth is thoroughly washed with water and is then dried. (iv) Baking Soda (NaHCO3) The soda commonly used in the kitchen for making tasty crispy pakoras is baking soda. Sometimes it is added for faster cooking. The chemical name of the compound is sodium hydrogen carbonate (NaHCO3). It is produced using sodium chloride as one of the raw materials. NaCl  H2O  CO2  NH3   NH 4Cl  NaHCO3 (ammonium chloride ) (sodium hydrogen carbonate) It can be used to neutralise an acid It is a mild non-corrosive base. The following reaction takes place when it is heated. Sodium hydrogen carbonate has got various uses in the household. NaHCO3 (sodium hydrogen carbonate) Heat   Na 2CO3  H2O  CO2  (sodium carbonate) Anhydrous sodium carbonate is generally called soda ash. Uses: (i) For making baking powder, which is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place NaHCO 3  H    CO2  H 2O  Sodium salt of acid ( from any acid ) Carbon dioxide produced during the reaction causes bread or cake to rise making them soft and spongy. (ii) Sodium hydrogen carbonate is also an ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief. 8 Chemistry (iii) It is also used in soda-acid fire extinguishers. (iv) It is used in medicines. It acts as mild antiseptic for infections. It is also present as an ingredient in ant-acids. Being alkaline it neutralises excess acid in the stomach.  Baking Soda as Antacid The acidity in the stomach is caused due to the formation of excess of hydrochloric acid (HCl). Sodium hydrogen carbonate (baking soda) reacts with the acid because of its alkaline nature and neutralises this effect. NaHCO3 + HCl NaCl + H2O + CO2 It therefore, acts as an antacid and is the major constituent of antacid medicines.  Baking soda in fire extinguishers Sodium hydrogen carbonate or baking soda is used in soda acid fire extinguishers. It is in the form of a conical metallic vessel. A strong solution of NaHCO3 is taken in a container. A glass ampoule containing H 2SO4 and provided with a knob is placed inside the container. When required, the ampoule can be broken by hitting the knob. As a result, the acid will come in contact with sodium hydrogen carbonate. The two will react to evolve CO2 gas. When enough pressure gets generated inside the container, the gas pushes the water solution which escapes out of the nozzle with force and extinguishes fire. 9 Chemistry Properties 1. Sodium hydrogen carbonate is a white crystalline solid. It is sparingly soluble in water at room temperature. 2. Aqueous solution of sodium hydrogen carbonate is weakly alkaline in nature due to hydrolysis. NaHCO3 + H2O H2CO3 + NaOH 3. Thermal decomposition: On heating, sodium hydrogen carbonate decomposes to give carbon dioxide. 2NaHCO3 Na2CO3 + CO2 + H2O 4. Reaction with acids: It reacts with acids to liberate carbon dioxide gas with brisk effervescence. 2NaHCO3 + H2SO4 Na2SO4 + 2CO2 + 2H2O NaHCO3 + HCl NaCl + CO2  + H2O (v) Washing Soda (Na2CO3.10H2O) The chemical formula of washing soda is Na2CO3.10H2O. Anhydrous sodium carbonate is generally called soda ash. (Na 2CO3). Sodium Carbonate is obtained on commercial scale by Solvay’s process. Raw materials used are Sodium chloride (NaCl) in the form of its concentrated solution called brine. (i) Ammonia (NH3). (ii) Lime Stone (CaCO3) The reactions taking place in this process are as follow : Step I: NaCl (aq) + NH3 (g) + H2O + CO2 (g) NaHCO3 (s) + NH4Cl (aq) Step II: 2 NaHCO3 Na2CO3 + H2O(l) + CO2 (g) Step III: Na2CO3 (s) + H2O (l) Na2CO3 (aq) Na2CO3 · H2O + 9H2O Na2CO3 · 10H2O is obtained by recrystallisation from a saturated solution of soda ash exposed to air. The loss of H2O by crystalline solid to the atmosphere on exposure to air is called efflorescence. 10 Chemistry Uses : (i) Used for washing clothes. (ii) Used for softening hard water. (iii) Sodium carbonate is used for the manufacture of detergents. (iv) Sodium Carbonate is used in paper and paint industry. Manufacturing of Washing Soda Washing soda is manufactured by Solvay Process, also called Ammonia-soda process. In addition to this, Le Blanc process and Electrolytic process are also available but are less popular. Solvay ammonia process for the manufacture of Na2CO3. (vi) Plaster of Paris Plaster of Paris is hemihydrate (hemi means half and hydrate means water) of calcium sulphate. Its molecular formula is or (CaSO 4)2 · H2O. In Plaster of Paris one molecule of water is shared by two formula units of CaSO4 as, CaSO4 – H CaSO4 CaSO4 – H – O or CaSO4 11 H2O Chemistry Preparation of Plaster of Paris Plaster of Paris is obtained by heating gypsum (CaSO4 · 2H2O) in a kiln at 373 K (or 100°C). K (100 C) 2  CaSO4 · 2H 2O  (s) 373     CaSO4 heat ( gypsum) 2 · H2O (s)  3H2O (g) ( Plaster of Paris ) 1 3 K (100 C ) CaSO4 · 2H 2O (s) 373    CaSO4 · H 2O (s)  H 2O (g) heat 2 2 ( gypsum) ( Plaster of Paris ) During the preparation of Plaster of Paris, temperature should be controlled carefully. Otherwise, anhydrous calcium sulphate (CaSO4) will be formed. Anhydrous calcium sulphate does not set into hard mass when mixed with water. So, if temperature is not controlled carefully, the Plaster of Paris obtained will have poor setting property. than CaSO4 · 2H 2O more   373 K CaSO4 ( dead burnt plaster) Properties of Plaster of Paris 1. Plaster of Paris is a white, odourless powder. 2. At ordinary room temperature, Plaster of Paris absorbs water and a large amount of heat is liberated. 3. When mixed with a limited amount of water (50% by mass), it forms a plastic mass, evolves heat and quickly sets to a hard porous mass within minutes. This is called the setting process. During setting, a slight expansion in volume occurs. It is due to this that it fills the mould completely and gives sharp impression. The reaction during process is 1 3 CaSO4 · H 2O (s)  H 2O (l)   CaSO4 · 2H 2O (s) 2 2 ( gypsum hard mass) ( Plaster of paris) ( water ) Uses of Plaster of Paris 1. 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. 1 1 CaSO4 · H 2O  H 2O   CaSO4 · 2H 2O 2 2 ( Plaster of paris) ( water ) 12 ( gypsum hard mass) Chemistry During the process of setting, it undergoes slight expansion (about 1%). Consequently, it produces a very sharp impression of the mould into which it is put. 2. It is used for setting fractured bones in right position in the body. or It is used in hospitals for immobilising the affected part in case of bone fracture or strain. 3. It is used for making statues, models and other decorative material. 4. It is used as a fire proofing material and for making chalks. 5. It is used in laboratories for sealing the air gaps in apparatus to make it airtight. It is also used to fill small gaps on walks & roofs. . 13