Electrolysis Flashcards
define electrolytes
compounds which conduct electricity when dissolved in water or in the molten state, and undergo chemical decomposition due to flow of current through it
define electrolytic cell
a non conducting vessel containing the electrolyte
define electrolysis
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
strong electrolytic salts
NaCl, Na2SO4, CuCl2, PbSO4
weak electrolytic salts
carbonate, bicarbonate, oxalate, formate
electrodes
allow the electric current to enter or leave the electrolytic solution
what happens to anions at anode
donate excess electrons to the anode and are oxidised to neutral atoms
what happens to cations at cathode
gain excess electrons from the cathode and are reduced to neutral atoms
preferential discharge of ions depends on
position in electrochemical series
define electrolytic dissociation
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
define electrochemical series
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
define selective discharge of ions
the preferential discharge of ions present in an electrolyte at the respective electrodes is also known as selective discharge of ions
factors affecting selective discharge
- relative position of anion or cation in electrochemical series
- concentration of ions in the electrolyte
- nature of the electrode
relative position of cation or anion affecting selective discharge
lower the position of the ion in electrochemical series, greater the tendency to be liberated at respective electrode
concentration of ions in electrolyte affecting selective discharge
higher the concentration of ion- greater the probability of it being discharged at respective electrodes
nature of electrode affecting selective discharge
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)
electrolysis of lead bromide
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)
during electrolysis of lead bromide, why is electrolytic cell made of silica
non reactive
withstands high temperature
non conductor of electricity
why are electrodes inert in electrolysis of lead bromide
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
electrolysis of acidified water using platinum electrodes
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)
example of catalysis
electrolysis of acidulated water
why is dil. H2SO4 preferred over dil. HNO3 during electrolysis of water
nitric acid is volatile, may decompose and the nitrate radical may tend to interfere with the electrolytic reaction
why is current passed for prolonged time during electrolysis of water
for accurate comparison
this ensures saturation of the gases in the electrolyte since solubility of oxygen differs from that of hydrogen
what happens due to presence of excess hydrogen and sulphate ions at anode
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
ratio of H and O librerated at cathode and anode
2:1
electrolysis of aq. Cu2SO4 using copper electrodes
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
why is electrolyte of aq. Cu2SO4 acidified with dil. sulphuric acid
enhances conductivity of the electrolyte
prevents hydrolysis of the electrolyte
what happens to colour of aq. copper sulphate soln. during electrolysis
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
applications of electrolysis
- electroplating
- electro refining
- electrometallurgy
define electroplating
electrolytic process of deposition of a superior metal on the surface of a baser metal
reasons for electroplating
- prevents corrosion or rusting of base metal
- makes article attractive and gives it an expensive appearance
during electroplating, why must the electrolyte contain ions of the metal used for plating on the article
electrolyte dissociates the ions of the metal which migrate towards the cathode and are deposited as neutral metallic atoms on cathode
electroplating with nickel
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
electroplating with silver
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
why is sodium argentocyanide preferred over silver nitrate during electroplating
migration of Ag ions from sodium silver cyanide is slower, and an even deposition of the metal silver is obtained on the article
how to electroplate an iron nail with copper
iron nail placed at cathode, copper sheet placed at anode, electrolyte is aq. copper sulphate soln
define electrorefining
process by which metals containing impurities are purified electrolytically to give a pure metal
electro refining of copper
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
what happens to impurities during electrorefining
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
Electrolysis of water is why aka catalysis
Can be electrolytically decomposed by adding traces of sulphuric acid which dissociate as H ions and SO4 ions and help in dissociating water into its respective ions, water being a polar solvent
