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ICSE Notes 2016 : Chemistry (Bishop Cotton Boys School (BCBS), Bangalore)

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Give reasons: 1. Electrolysis of acidulated water is considered to be an example of catalysis. Ans. Pure water is almost a non-electrolyte and a polar solvent. It can be electrolytically decomposed by addition of traces of dilute acid which dissociates as ions and help in dissociating water into hydrogen and hydroxide ions. 2. Copper is good conductor of electricity but is considered to be nonelectrolyte. Ans. Copper being a metal contains free electrons and conduction of electricity is carried out by the movement of free electrons, but copper does not dissociate into ions and hence, is a non-electrolyte. 3. Sodium chloride in solid state does not conduct electricity but in molten state or solution state conducts electricity. Ans. (i) solid sodium chloride contains ions, but ions are not free, they are hold together by an electrostatic force of attraction. (ii) when in solution state or in molten state, the ions becomes free by overcoming the electrostatic force of attraction. Hence ---------4. A solution of ionic compounds is an electrolyte while covalent compound is a non-electrolyte. Ans. An ionic compound, being a electrovalent dissociates forming ions which conduct electricity while a covalent compounds contains molecules which do not conduct electricity. Hence ------5. During electrolysis of copper sulphate using copper electrode the blue colour solution does not fade. Ans. For every copper ion discharged at the cathode as neutral atom- a copper ion is added to the solution at the anode. Hence total number of copper ions- remains the same. Hence---- 6. Silver nitrate is not used an electrolyte in electroplating of silver or sodium argento cyanide is used as a electrolyte in electroplating of silver Ans. In aq. solution of sodium argento cyanide, silver ions migrate slowly as compared to silver nitrate thereby ensuring even deposition of metal silver on the article to be electroplated. 7. Metals like sodium, potassium and calcium are obtained by electrolytic reductions. Ans. These metals are highly electropositive and possess strong affinity towards oxygen; their oxides are difficult to reduce using reducing agents. 8. Mentions any two ways to ensure even electroplating. Ans. (i) a low current and for longer time ensures smooth, firm, uniform and long lasting deposition (ii) a direct current and alternating current should be used. 9. Electrolysis of molten lead bromide is considered a redox reaction. Ans. Red ox reaction is reaction in which reduction and oxidation occurs simultaneously. At cathode lead ions gain electrons to form neutral atom thereby undergo reduction and simultaneously at anode bromide ion ions lose electrons to form neutral atom thereby undergo oxidation. Hence-------10. During electrolytic dissociation of sodium chloride, sodium ions discharge at the cathode. Ans. Sodium being a metal donates electron and forms positively charged ion i.e. Cation which migrates to negatively charged electrode cathode where it gains electrons to form neutral atom. 11. in the electrolysis acidified water, dilute sulphuric acid is preferred to dilute nitric acid. Ans. Since dilute nitric acid is a volatile acid, may decompose and the nitrate radical may tend to interfere with the electrolytic reaction. 12. Lead is not used for welding purpose but its alloy solder is used. Ans. Its alloy consist of lead and tin , tin lowers the melting point of alloy and enhances expansion capacity on solidification hence---------- 13. Zinc is used as cathode in the dry cell. Ans. Due to its strong electropositive nature and ease to form zinc ions. 14. Fused cryolite and fluorspar is added to the electrolytic mixture of alumina. Ans. (i) addition of cryolite and fluorspar lowers the melting point from 2050 to 950 degree Celsius. (ii) it enhances the conductivity of the mixture. (iii) it enhances the mobility of the mixture. 15. A layer of powdered coke is sprinkled over the electrolytic mixture of alumina. Ans. (i) it prevents the burning of the carbon electrode in air. (ii) it also prevents or minimizes- heat loss by radiation. 16. Graphite anodes are continuously replaced during the electrolysis of aluminum. Ans. (i) The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode. (ii) The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. 17. Copper displaces silver from a silver nitrate solution. Ans. Copper is placed at higher position in reactivity series as compared to silver. So copper displaces silver from silver nitrate solution forming blue colour solution of copper sulphate. ****************************************************************** ****

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