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188 chemistry formation of NO 2nd thus, the brown colour Similarly, in manufacture of sulphuric intensifies. acid by contact process, 26 Effect of temperature can also be seen in 2SO 2(g) + 2 (g) 2SO 3g); K c 1.7 × 10 an endothermic reaction, though the value of K is suggestive of reaction 3+ – 2– [Co(H 2) 6 (aq) + 4Cl (aq) [CoCl 4 (aq) + going to completion, but practically the 6H2O(l) oxidation of SO 2 to SO 3s very slow. Thus, platinum or divanadium penta-oxide (V O ) pink colourless blue 2 5 At room temperature, the equilibrium is used as catalyst to increase the rate of the 2– reaction. mixture is blue due to [CoCl 4 . When cooled in a freesing mixture, the colour of the mixture Note: If a reaction has an exceedingly small turns pink due to [Co(H O) ] .+ K, a catalyst would be of little help. 2 6 6.9 IONIC EQUILIBRIUM IN SOLUTION 6.8.5 Effect of a Catalyst A catalyst increases the rate of the chemical Under the effect of change of concentration on the direction of equilibrium, you have reaction by making available a new low energy pathway for the conversion of reactants to incidently come across with the following products. It increases the rate of forward equilibrium which involves ions: 3+ – 2+ and reverse reactions that pass through the Fe (aq) + SCN (aq) [Fe(SCN)] (aq) same transition state and does not affect There are numerous equilibria that involve equilibrium. Catalyst lowers the activation ions only. In the following sections we will energy for the forward and reverse reactions study the equilibria involving ions. It is well by exactly the same amount. Catalyst does known that the aqueous solution of sugar not affect the equilibrium composition of does not conduct electricity. However, when a reaction mixture. It does not appear in common salt (sodium chloride) is added the balanced chemical equation or in the to water it conducts electricity. Also, the equilibrium constant expression. conductance of electricity increases with an Let us consider the formation of NH 3 increase in concentration of common salt. from dinitrogen and dihydrogen which is highly exothermic reaction and proceeds Michael Faraday classified the substances into two categories based on their ability with decrease in total number of moles to conduct electricity. One category of formed as compared to the reactants. Equilibrium constant decreases with increase substances conduct electricity in their aqueous solutions and are called electrolytes in temperature. At low temperature rate while the other do not and are thus, referred to decreases and it takes long time to reach at equilibrium, whereas high temperatures give as non-electrolytes. Faraday further classified electrolytes into strong and weak electrolytes. satisfactory rates but poor yields. Strong electrolytes on dissolution in water German chemist, Fritz Haber discovered are ionized almost completely, while the weak that a catalyst consisting of iron catalyse electrolytes are only partially dissociated. the reaction to occur at a satisfactory rate For example, an aqueous solution of at temperatures, where the equilibrium sodium chloride is comprised entirely of concentration of NH 3is reasonably favourable. sodium ions and chloride ions, while that Since the number of moles formed in the of acetic acid mainly contains unionized reaction is less than those of reactants, the acetic acid molecules and only some acetate yield of NH 3can be improved by increasing ions and hydronium ions. This is because the pressure. there is almost 100% ionization in case Optimum conditions of temperature of sodium chloride as compared to less and pressure for the synthesis of NH using than 5% ionization of acetic acid which is 3 catalyst are around 500°C and 200 atm. a weak electrolyte. It should be noted Reprint 2026-27