Electrolysis

Question 1

define electrolytes

Answer 1

compounds which conduct electricity when dissolved in water or in the molten state, and undergo chemical decomposition due to flow of current through it

Question 2

define electrolytic cell

Answer 2

a non conducting vessel containing the electrolyte

Question 3

define electrolysis

Answer 3

the decomposition of a chemical compound in the aqueous or fused state by the passage of a direct electric current resulting in the discharge of ions as neutral atoms at the respective electrodes

Question 4

strong electrolytic salts

Answer 4

NaCl, Na2SO4, CuCl2, PbSO4

Question 5

weak electrolytic salts

Answer 5

carbonate, bicarbonate, oxalate, formate

Question 6

electrodes

Answer 6

allow the electric current to enter or leave the electrolytic solution

Question 7

what happens to anions at anode

Answer 7

donate excess electrons to the anode and are oxidised to neutral atoms

Question 8

what happens to cations at cathode

Answer 8

gain excess electrons from the cathode and are reduced to neutral atoms

Question 9

preferential discharge of ions depends on

Answer 9

position in electrochemical series

Question 10

define electrolytic dissociation

Answer 10

the process due to which an ionic compound in the fused state or in aqueous solution state dissociates into ions by passage of electric current through it is called electrolytic dissociation

Question 11

define electrochemical series

Answer 11

depending on the ease with which the metals lose their electrons and form ions- they are arranged in a series known as metal activity series or electrochemical series

at the top: metals which lose electrons easily, cations which gain electrons difficulty
at the bottom: metals which lose electrons difficultly, cations which gain electrons easily

Question 12

define selective discharge of ions

Answer 12

the preferential discharge of ions present in an electrolyte at the respective electrodes is also known as selective discharge of ions

Question 13

factors affecting selective discharge

Answer 13

1. relative position of anion or cation in electrochemical series
2. concentration of ions in the electrolyte
3. nature of the electrode

Question 14

relative position of cation or anion affecting selective discharge

Answer 14

lower the position of the ion in electrochemical series, greater the tendency to be liberated at respective electrode

Question 15

concentration of ions in electrolyte affecting selective discharge

Answer 15

higher the concentration of ion- greater the probability of it being discharged at respective electrodes

Question 16

nature of electrode affecting selective discharge

Answer 16

determines preferential ion- which will be discharged at electrode
1. inert electrodes: do not take part in electrolytic reaction (Iron, graphite, platinum)
2. active electrodes: take part in electrolytic reaction (Cu, Ni, Ag)

Question 17

electrolysis of lead bromide

Answer 17

temp: above 380 C
current: 3 A
product at cathode- lead metal (silvery grey deposit, reduction reaction)
product at anode- bromine vapours (reddish brown fumes, oxidation)

Question 18

during electrolysis of lead bromide, why is electrolytic cell made of silica

Answer 18

non reactive
withstands high temperature
non conductor of electricity

Question 19

why are electrodes inert in electrolysis of lead bromide

Answer 19

they should not take part in the reaction, otherwise they would prevent the discharge of the respective ions
graphite anode is preferred since it is unaffected by bromine vapours

Question 20

electrolysis of acidified water using platinum electrodes

Answer 20

cathode and anode: Pt foil (inert)
temp: ordinary
current: 3 A
cathode: hydrogen gas
anode: oxygen (OH discharged because it is lower in electrochemical series- the unstable radical forms water and oxygen)

Question 21

example of catalysis

Answer 21

electrolysis of acidulated water

Question 22

why is dil. H2SO4 preferred over dil. HNO3 during electrolysis of water

Answer 22

nitric acid is volatile, may decompose and the nitrate radical may tend to interfere with the electrolytic reaction

Question 23

why is current passed for prolonged time during electrolysis of water

Answer 23

for accurate comparison
this ensures saturation of the gases in the electrolyte since solubility of oxygen differs from that of hydrogen

Question 24

what happens due to presence of excess hydrogen and sulphate ions at anode

Answer 24

concentration of H2SO4 increases at the anode and decreases at the cathode- but the total conc. of H22SO4 molecules remains the same since it is the hydrogen and hydroxyl ions that are discharged at the electrodes respectively

Question 25

ratio of H and O librerated at cathode and anode

Answer 25

2:1

Question 26

electrolysis of aq. Cu2SO4 using copper electrodes

Answer 26

temp: ordinary
current: 3 A
cathode: copper metal
anode: sulphate and hydroxyl ions are NOT discharged since Cu being an active electrode, itself takes part in reaction and forms Cu ions. Cu anode hence shows a loss in mass

Question 27

why is electrolyte of aq. Cu2SO4 acidified with dil. sulphuric acid

Answer 27

enhances conductivity of the electrolyte
prevents hydrolysis of the electrolyte

Question 28

what happens to colour of aq. copper sulphate soln. during electrolysis

Answer 28

blue colour remains unchanged during electrolysis using Cu electrodes- because Cu ions amount remains same
colour fades when Pt is used- blue Cu ions discharged at cathode are not replaced at anode

Question 29

applications of electrolysis

Answer 29

1. electroplating
2. electro refining
3. electrometallurgy

Question 30

define electroplating

Answer 30

electrolytic process of deposition of a superior metal on the surface of a baser metal

Question 31

reasons for electroplating

Answer 31

1. prevents corrosion or rusting of base metal
2. makes article attractive and gives it an expensive appearance

Question 32

during electroplating, why must the electrolyte contain ions of the metal used for plating on the article

Answer 32

electrolyte dissociates the ions of the metal which migrate towards the cathode and are deposited as neutral metallic atoms on cathode

Question 33

electroplating with nickel

Answer 33

electrolyte: aq. soln. of NiSO4
cathode: cleaned article to be electroplated
anode: plate or block of Ni
reaction at cathode: Ni ions deposited on article
reaction at anode: anode loses electrons to give Ni ions in soln. hence anode diminishes in mass

Question 34

electroplating with silver

Answer 34

electrolyte: sodium argentocyanide
cathode: article
anode: plate or block of silver
reaction at cathode: Ag ions deposited as neutral Ag atoms
reaction at anode: silver anode loses electrons to give silver ions in soln. hence anode diminishes in mass

Question 35

why is sodium argentocyanide preferred over silver nitrate during electroplating

Answer 35

migration of Ag ions from sodium silver cyanide is slower, and an even deposition of the metal silver is obtained on the article

Question 36

how to electroplate an iron nail with copper

Answer 36

iron nail placed at cathode, copper sheet placed at anode, electrolyte is aq. copper sulphate soln

Question 37

define electrorefining

Answer 37

process by which metals containing impurities are purified electrolytically to give a pure metal

Question 38

electro refining of copper

Answer 38

electrolyte: aq. copper sulphate soln. acidified
cathode: pure thin sheet of copper
anode: impure block of copper
reaction at cathode: Cu ions discharged as neutral copper atoms, pure Cu deposited on thin sheet of pure copper placed at cathode
reaction at anode: anode loses electrons to give Cu ions in soln. hence anode diminishes in mass

Question 39

what happens to impurities during electrorefining

Answer 39

settle down as anode mud or slime which contains insoluble impurities like gold and silver
impurities such as iron and zinc (higher in electrochemical series) ionise and dissolve in electrolytic copper sulphate soln