Electrolytes And Electrolysis
Category : JEE Main & Advanced
(1) Definition : “The substances whose aqueous solution undergo decomposition into ions when electric current is passed through them are known as electrolytes and the whole process is known as electrolysis or electrolytic decomposition.”
Solutions of acids, bases, salts in water and fused salts etc. are the examples of electrolytes. Electrolytes may be weak or strong. Solutions of cane sugar, glycerine, alcohol etc., are examples of non-electrolytes.
(2) Electrolytic cell or Voltameter : The device in which the process of electrolysis or electrolytic decomposition is carried out is known as electrolytic cell or voltameter.
(i) Voltameter convert electrical energy into chemical energy.
(ii) The electrode on which oxidation takes place is called anode (or +ve pole) and the electrode on which reduction takes place is called cathode (or –ve pole)
(iii) During electrolysis in voltameter cations are discharged on cathode and anions on anode.
(iv) In voltameter, outside the electrolyte electrons flow from anode to cathode and current flow from cathode to anode.
For voltameter, \[{{\text{E}}_{\text{cell}}}=-\text{ve}\] and \[\Delta \text{G}=+\text{ve}\text{.}\]
(v) The anions on reaching the anode give up their electrons and converted into the neutral atoms.
At anode : \[{{\text{A}}^{\text{--}}}\xrightarrow{\,\,\,\,\,\,\,\,\,\,}\text{A}+{{e}^{-}}\] (Oxidation)
(vi) On the other hand cations on reaching the cathode take up electrons supplied by battery and converted to the neutral atoms.
At cathode : \[{{\text{B}}^{+}}+{{e}^{-}}\xrightarrow{\,\,\,\,\,\,\,\,\,\,}\text{B}\] (Reduction)
This overall change is known as primary change and products formed is known as primary products.
The primary products may be collected as such or they undergo further change to form molecules or compounds. These are called secondary products and the change is known as secondary change.
(3) Preferential discharge theory : According to this theory “If more than one type of ion is attracted towards a particular electrode, then the ion is discharged one which requires least energy or ions with lower discharge potential or which occur low in the electrochemical series”.
The potential at which the ion is discharged or deposited on the appropriate electrode is termed the discharge or deposition potential, (D.P.). The values of discharge potential are different for different ions.
The decreasing order of discharge potential or the increasing order of deposition of some of the ions is given below,
For cations : \[L{{i}^{+}},{{K}^{+}},N{{a}^{+}},C{{a}^{2+}},M{{g}^{2+}},A{{l}^{3+}},Z{{n}^{2+}},\] \[F{{e}^{2+}},\]\[N{{i}^{2+}},{{H}^{+}},C{{u}^{2+}},H{{g}^{2+}},A{{g}^{+}},A{{u}^{3+}}.\]
For anions : \[SO_{4}^{2-},NO_{3}^{-},O{{H}^{-}},C{{l}^{-}},B{{r}^{-}},{{I}^{-}}.\]
Products of electrolysis of some electrolytes
|
Electrolyte |
Electrode |
Product at cathode |
Product at anode |
|
Aqueous NaOH |
Pt or Graphite |
\[2{{H}^{+}}+2{{e}^{-}}\to \]\[{{H}_{2}}\] |
\[2O{{H}^{-}}\to \frac{1}{2}{{O}_{2}}+{{H}_{2}}O+2{{e}^{-}}\] |
|
Fused NaOH |
Pt or Graphite |
\[N{{a}^{+}}+{{e}^{-}}\to Na\] |
\[2O{{H}^{-}}\to \frac{1}{2}{{O}_{2}}+{{H}_{2}}O+2{{e}^{-}}\] |
|
Aqueous NaCl |
Pt or Graphite |
\[2{{H}^{+}}+2{{e}^{-}}\to {{H}_{2}}\] |
\[2C{{l}^{-}}\to C{{l}_{2}}+2{{e}^{-}}\] |
|
Fused NaCl |
Pt or Graphite |
\[N{{a}^{+}}+{{e}^{-}}\to Na\] |
\[2C{{l}^{-}}\to C{{l}_{2}}+2{{e}^{-}}\] |
|
Aqueous CuSO4 |
Pt or Graphite |
\[C{{u}^{2+}}+2{{e}^{-}}\to Cu\] |
\[2O{{H}^{-}}\to \frac{1}{2}{{O}_{2}}+{{H}_{2}}O+2{{e}^{-}}\] |
|
Aqueous CuSO4 |
Cu electrode |
\[C{{u}^{2+}}+2{{e}^{-}}\to Cu\] |
\[Cu\] oxidised to \[C{{u}^{2+}}\]ions |
|
Dilute H2SO4 |
Pt electrode |
\[2{{H}^{+}}+2{{e}^{-}}\to {{H}_{2}}\] |
\[2O{{H}^{-}}\to \frac{1}{2}{{O}_{2}}+{{H}_{2}}O+2{{e}^{-}}\] |
|
Conc. H2SO4 |
Pt electrode |
\[2{{H}^{+}}+2{{e}^{-}}\to {{H}_{2}}\] |
Peroxodisulphuric acid\[({{H}_{2}}{{S}_{2}}{{O}_{8}})\] |
|
Aqueous AgNO3 |
Pt electrode |
\[A{{g}^{+}}+{{e}^{-}}\to Ag\] |
\[2O{{H}^{-}}\to \frac{1}{2}{{O}_{2}}+{{H}_{2}}O+2{{e}^{-}}\] |
|
Aqueous AgNO3 |
Ag electrode |
\[A{{g}^{+}}+{{e}^{-}}\to Ag\] |
\[Ag\] oxidised to \[A{{g}^{+}}\] ions |
(4) Application of electrolysis : Electrolysis has wide applications in industries. Some of the important applications are, as follows,
(i) Production of hydrogen by electrolysis of water.
(ii) Manufacture of heavy water \[({{D}_{2}}O)\].
(iii) The metals like \[Na,\]K, Mg, Al, etc., are obtained by electrolysis of fused electrolytes.
(iv) Non-metals like hydrogen, fluorine, chlorine are obtained by electrolysis.
(v) In this method pure metal is deposited at cathode from a solution containing the metal ions \[Ag,\ Cu\]etc.
(vi) Compounds like NaOH, KOH, \[N{{a}_{2}}C{{O}_{3}},\]\[KCl{{O}_{3}},\]white lead, \[KMn{{O}_{4}}\]etc. are synthesised by electrosynthesis method.
(vii) Electroplating : The process of coating an inferior metal with a superior metal by electrolysis is known as electroplating. The aim of electroplating is, to prevent the inferior metal from corrosion and to make it more attractive in appearance. The object to be plated is made the cathode of an electrolytic cell that contains a solution of ions of the metal to be deposited.
|
For electroplating |
Anode |
Cathode |
Electrolyte |
|
With copper |
Cu |
Object |
\[CuS{{O}_{4}}+\,\text{dilute}\,{{H}_{2}}S{{O}_{4}}\] |
|
With silver |
Ag |
Object |
\[K[Ag{{(CN)}_{2}}]\] |
|
With nickel |
Ni |
Object |
Nickel ammonium sulphate |
|
With gold |
Au |
Object |
\[K[Au{{(CN)}_{2}}]\] |
|
With zinc |
Zn |
Iron objects |
\[ZnS{{O}_{4}}\] |
|
With tin |
Sn |
Iron objects |
\[SnS{{O}_{4}}\] |
Thickness of coated layer : Let the dimensions of metal sheet to be coated be \[(a\,cm\times b\,cm).\]
Thickness of coated layer \[=c\,cm\]
Volume of coated layer\[=(a\times b\times c)\,c{{m}^{3}}\]
Mass of the deposited substance \[=\text{ Volume}\times \text{ density}\] \[=(a\times b\times c)\times dg\]
\[\therefore \] \[(a\times b\times c)\times d=\frac{I\times t\times E}{96500}\]
Using above relation we may calculate the thickness of coated layer.
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