Acids, Bases & Salts Salts & its Types

Class-X Chemistry Acids, Bases& Salts Salts & its Types  Salts A salt is a compound formed by the reaction of an acid with a base in which the hydrogen of the acid is replaced by the metal. In polybasic acids, more than one hydrogen atoms are present in a molecule. The hydrogen atoms can be replaced partially or completely. So, two kinds of salts are possible. Here, partial replacement of hydrogen atoms from H2SO4 has resulted in the formation of sodium hydrogen sulphate. Here, complete replacement of hydrogen atoms from H2SO4 has resulted in the formation of sodium sulphate. NaHSO4 and Na2SO4 represent two kinds of salts. Types of Salts: The different types of salts are: normal salt, acid salt, basic salt and double salt. 1.Normal salt: A salt that does not contain any replaceable hydrogen atoms or hydroxyl groups is called a normal salt. Examples: Na2SO4 obtained in the reaction between H2SO4 and NaOH is a normal salt because it is formed by the complete replacement of both the H atoms of H 2SO4. Similarly, calcium sulphate (CaSO4), sodium phosphate (Na3PO4) and potassium phosphate (K3PO4) are also normal salts. 2.Acid salt: When a polybasic acid is not completely neutralized by a base, the salt produced will contain replaceable hydrogen atoms. Hence, it may further take part in the reaction with the base as an acid. Such a salt is called an acid salt. For example, the salt NaHSO4 produced in the reaction between NaOH and H2SO4 is an acid salt because it is capable of further reaction with the base NaOH to produce the normal salt Na2SO4. Thus, “A salt that contains replaceable hydrogen atoms is called an acid salt”. 1 Class-X Chemistry Examples: NaHSO4, NaH2PO4 and Na2HPO4 3.Basic salt: When a polyacidic base reacts with lesser amount of acid than is necessary for complete neutralization, the salt produced contain hydroxyl group(s) (OH) also. Such a salt is called a basic salt. Examples: 1 mole of Pb (OH)2 requires 2 moles of HCl for complete neutralization. But when 1 mole of Pb(OH)2 is made to react with 1 mole of HCl, some Pb(OH)2 is left unreacted. The salt produced is not PbCl2 , but Pb(OH)Cl. Pb(OH) 2  HCl  Pb(OH)Cl  H 2O lead hydroxy chloride Similarly, when one mole of Bi(OH)2 is reacted with 1 mole of HNO3, the salt Bi(OH)2NO3 is formed. Bi(OH)3  HNO3  Bi(OH) 2 NO3  H 2O Salts like Pb(OH)Cl and Bi(OH)2NO3 contain the OH group. These salts are called basic salts, because they can further react with the acids to form H 2O and the corresponding normal salts. Thus, a basic salt is formed when a polyacidic base reacts with a lesser amount of an acid than is necessary for the formation of a normal salt. 2 Class-X Chemistry 4.Double salt: In a double salt, there are two different negative ions and / or positive ions. For example, the mineral dolomite, CaCO3.MgCO3, contain both Ca2+ and Mg2+ ions. Hence, it is a double salt. Potash alum, K2SO4.Al2(SO4)3. 24H2O, also is a double salt. Double salts exist only in the solid state. When dissolved in water, they break up into a mixture of two separate salts. For example, when potash alum is dissolved in water, it breaks up as follows. 2K   SO42 K 2SO4 2Al3  3SO42 Al2 (SO 4 )3 Preparation of Salts: 1.By the reaction between metal and acid: Certain metals (for example, Zn and Mg) react with HCl or H 2SO4 to form salt and water. Zn  2HCl  ZnCl2  H2  2.By the reaction between an acid and a base: All acid-base reactions (neutralization reactions) produce salts. NaOH  HCl  NaCl  H 2O CuO  2HCl  CuCl2  H 2O 3.By direct union of a metal and a nonmetal: Sodium and chlorine combine directly to form sodium chloride. 2Na  Cl2  2NaCl Similarly, when sulphur is heated with iron filings, ferrous sulphide (FeS) is formed. Fe  S  FeS 4.By the union between an acidic oxide and a basic oxide: CO 2  CaO  CaCO3 carbon calcium dioxide oxide 3 calcium carbonate Class-X Chemistry 5.By the reaction between a metal and a base: When zinc is heated with an aqueous solution of NaOH, sodium zincate (salt) is formed with the evolution of hydrogen gas. Zn  2NaOH  Na 2 ZnO2  H 2  General Properties of Salts: 1.Reaction with an acid: When a salt reacts with an acid, another salt and acid are formed. For example, when sodium chloride is heated with sulphuric acid, sodium hydrogen sulphate (at low temperature) and then sodium sulphate (at high temperature) are produced and hydrogen chloride gas is evolved. 2.Reaction with a base: A salt reacts with a base to produce another salt and base. (NH 4 ) 2 SO 4  2NaOH  Na 2SO 4  2NH 4 OH 3.Reaction with a metal: Sometimes, a salt solution may react with a metal. For example, when an iron nail is dipped into an aqueous solution of copper sulphate, copper gets deposited on the surface of the nail and the ferrous sulphate formed remains in the solution. CuSO4  Fe  FeSO4  Cu  This reaction shows that iron is more reactive than copper. 4 Class-X Chemistry Thus, a more reactive metal can displace a less reactive metal from a solution of its salt. Uses of Salts: The following table gives uses of some salts. 5