Chapter 06: Electrolysis - Vardaan Master Notes

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ICSE CLASS 10 CHEMISTRY SYLLABUS OVERVIEW

(i) Electrolytes and non-electrolytes: Definitions and examples.
(ii) Substances containing molecules only, ions only, both molecules and ions: Relating composition with behavior as strong, weak, or non-electrolyte. Definitions of electrolysis, electrolyte, electrode, anode, cathode, anion, cation, oxidation and reduction (electronic concept).
(iii) Migration of ions & Selective Discharge: Factors influencing selective discharge illustrated by electrolysis of: (a) molten lead bromide, (b) acidified water with platinum electrodes, (c) aqueous copper(II) sulphate with copper electrodes. Electron transfer at electrodes.
(iv) Applications of Electrolysis: Electroplating with nickel and silver, electro-refining of copper, electrometallurgy of active metals (Aluminium). Choice of electrolyte, reasons, conditions, and electrode equations.
(v) Acids, bases and salts as electrolytes: Activity series indicating the tendency of metals to form ions.

6.1 INTRODUCTION & ELECTRICAL CONDUCTION

Electrolysis is a chemical process that establishes a fundamental relationship between electrical energy and chemical change. The word electrolysis is derived from 'electro' (flow of electrons or electricity) and 'lysis' (separating or breaking down chemically).

Electrolysis: The process of chemical decomposition of an electrolyte in its molten (fused) or aqueous solution state accompanied by a chemical change by the passage of direct electric current.

Metallic Conduction vs Electrolytic Conduction

Property	Metallic Conductors (e.g. Cu wire, Al, Fe)	Electrolytic Conductors (e.g. $CuSO_4$ aq, molten $PbBr_2$)
Flow of Current	Consists of a single stream of free mobile electrons moving from negative pole to positive pole.	Consists of two streams of free mobile ions: positive cations flowing towards cathode and negative anions flowing towards anode.
State of Matter	Conduct electricity in both solid and liquid (molten) states.	Conduct electricity only in aqueous solution or molten state (never in solid state).
Chemical Change	No chemical change occurs; no change in form or composition (only a physical process).	Chemical decomposition occurs; new chemical substances are formed at the electrodes.
Matter Transfer	No transfer of matter takes place during conduction.	Transfer of matter takes place in the form of ions moving towards electrodes.

6.2 IMPORTANT TERMINOLOGY & CLASSIFICATION

Fig. 6.1 A simple electrolytic cell or voltameter

Term	Definition & Key Concept
Electrolyte	A chemical compound which in aqueous solution or in molten state conducts electricity and undergoes chemical decomposition (e.g. $HCl, H_2SO_4, NaOH, NaCl, CuSO_4, PbBr_2$).
Non-electrolyte	A compound which neither in solution nor in molten state conducts electric current, and does not decompose at electrodes (e.g. distilled water, cane sugar, glucose, urea, alcohol, $CCl_4$, benzene).
Electrolytic Cell (Voltameter)	A non-conducting vessel (glass/silica) in which electrolysis is carried out. It converts electrical energy into chemical energy.
Electrochemical Cell	A device that converts chemical energy into electrical energy (e.g. Simple Voltaic cell, Daniel cell).
Electrodes	Graphite or metal rods/plates immersed in the electrolyte through which electric current enters or leaves the cell.
Anode	The electrode connected to the positive terminal of the battery. Anions migrate to anode and undergo oxidation (loss of electrons).
Cathode	The electrode connected to the negative terminal of the battery. Cations migrate to cathode and undergo reduction (gain of electrons).

Differences Between Anode and Cathode

Feature	Anode (+ Positive Electrode)	Cathode (- Negative Electrode)
Terminal Connection	Connected to positive terminal of battery.	Connected to negative terminal of battery.
Migrating Ions	Negative ions (Anions) migrate to anode.	Positive ions (Cations) migrate to cathode.
Electrode Reaction	Oxidation takes place (loss of electrons).	Reduction takes place (gain of electrons).
Nature of Electrode	Acts as an oxidising electrode.	Acts as a reducing electrode.

Strong vs Weak Electrolytes

Property	Strong Electrolytes	Weak Electrolytes
Degree of Ionisation ($\alpha$)	Almost completely dissociated in aqueous/molten state ($\alpha \approx 100\%$).	Partially dissociated in aqueous/molten state ($\alpha < 5\%$).
Particles Present in Solution	Contains (almost) only free mobile ions.	Contains both ions AND unionised molecules.
Electrical Conductivity & Bulb Test	High conductivity; causes testing bulb to glow brightly.	Poor conductivity; causes testing bulb to glow dimly.
Strong Acids	$HCl, H_2SO_4, HNO_3$	$CH_3COOH, H_2CO_3, H_3PO_4$, Oxalic acid
Strong Bases	$NaOH, KOH$ (molten or aq)	$NH_4OH, Ca(OH)_2, Mg(OH)_2$
Salts	$NaCl, PbBr_2, CuCl_2, CuSO_4, AgNO_3$	$Na_2CO_3, KHCO_3, (CH_3COO)_2Pb$

Exam Tip — Non-Electrolytes: Non-electrolytes are purely covalent compounds. They do not possess ions even in solution. Examples: Pure distilled water, alcohol, kerosene, carbon disulfide ($CS_2$), cane sugar, glucose, urea, benzene.

6.3 OXIDATION AND REDUCTION (ELECTRONIC CONCEPT)

ELECTRONIC DEFINITIONS

Oxidation: A chemical process in which an atom or an ion loses electron(s).

$Zn - 2e^- \rightarrow Zn^{2+}$ | $Na - e^- \rightarrow Na^+$
$Fe^{2+} - e^- \rightarrow Fe^{3+}$ | $S^{2-} - 2e^- \rightarrow S$

Reduction: A chemical process in which an atom or an ion gains electron(s).

$Cu^{2+} + 2e^- \rightarrow Cu$ | $Fe^{3+} + e^- \rightarrow Fe^{2+}$ | $S + 2e^- \rightarrow S^{2-}$

Classical Concept vs Electronic Concept

Oxidation involves: (i) Addition of oxygen ($C + O_2 \rightarrow CO_2$, $2Mg + O_2 \rightarrow 2MgO$), (ii) Removal of hydrogen ($H_2S + Cl_2 \rightarrow 2HCl + S$), or (iii) Loss of electrons.

Reduction involves: (i) Removal of oxygen ($CuO + H_2 \rightarrow Cu + H_2O$, $ZnO + C \rightarrow Zn + CO$), (ii) Addition of hydrogen ($Cl_2 + H_2S \rightarrow 2HCl + S$), or (iii) Gain of electrons.

Oxidising Agents: Substances that accept electrons or supply oxygen (Solids: $MnO_2, PbO_2$; Liquids: $H_2O_2$, conc. $HNO_3$, conc. $H_2SO_4, Br_2$; Gases: $O_2, O_3, Cl_2, SO_2$).

Reducing Agents: Substances that donate electrons or supply hydrogen (Solids: $C, Zn, Al, Cu, Na, SnCl_2$; Liquids: $H_2O_2, HI, HBr$; Gases: $H_2S, CO, SO_2$).

6.4 THEORY OF ELECTROLYTIC DISSOCIATION

Fig. 6.2 & Fig. 6.3 Electrostatic attraction in solid state vs free mobile ions in water/molten state

Svante Arrhenius (1887) proposed the ionic theory of electrolytic dissociation:

An electrolyte on dissolving in water dissociates into free mobile positive ions (cations) and negative ions (anions).
All ions carry electric charge and are responsible for the flow of current.
The total number of positive charges equals the total number of negative charges (electrical neutrality).
In solution, a dynamic equilibrium exists between unionised molecules and ions formed.
Non-electrolytes like sugar/urea do not form ions in water.

Arrhenius vs Modern Concept of Dissociation:

Arrhenius Concept: Assumed water ionises electrolytes.

Modern Concept: Electrovalent compounds are ionic even in solid state, held by strong electrostatic forces making ions immobile. Water (having high dielectric constant $\approx 81$) breaks electrostatic attraction, setting ions free to move!

Ionisation vs Electrolytic Dissociation

Feature	Ionisation	Electrolytic Dissociation
Definition	Process by which polar covalent molecules are converted into ions in aqueous solution.	Process by which electrovalent (ionic) compounds separate into pre-existing ions in fused or aqueous state.
Original State	Molecules were not initially in ionic state.	Ions were already present in solid crystal lattice.
Examples	$HCl + H_2O \rightarrow H_3O^+ + Cl^-$ $NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$	$NaCl \xrightarrow{\text{water}} Na^+ + Cl^-$ $PbBr_2 \xrightarrow{\text{heat}} Pb^{2+} + 2Br^-$

6.5 CHARACTERISTICS OF ELECTROLYSIS

The passage of electricity causes cations to migrate to cathode and anions to migrate to anode.
The number of electrons gained at cathode equals the number of electrons lost at anode.
Products of electrolysis are liberated at electrode surfaces only.
Hydrogen and metals are liberated at cathode (called electro-positive elements).
Non-metals and oxygen are liberated at anode (called electro-negative elements).
Electrolysis is a Redox process (reduction at cathode + oxidation at anode).
Direct Current (D.C.) is mandatory! Alternating current (A.C.) causes no net chemical change because electrode polarity alternates rapidly!

6.6 ELECTROCHEMICAL (ACTIVITY) SERIES

Activity Series of Metals (Cations)

Metal Atom	Cation Formed	Ease of Discharge at Cathode
Potassium ($K$)	$K^+$	Most Electropositive (Difficult to discharge)
Calcium ($Ca$)	$Ca^{2+}$
Sodium ($Na$)	$Na^+$
Magnesium ($Mg$)	$Mg^{2+}$
Aluminium ($Al$)	$Al^{3+}$	$\downarrow$ Increasing ease of gain of electrons & discharge at Cathode $\downarrow$
Zinc ($Zn$)	$Zn^{2+}$
Iron ($Fe$)	$Fe^{2+} / Fe^{3+}$
Lead ($Pb$)	$Pb^{2+}$
Hydrogen ($H$)	$H^+$	Reference Metal / Non-metal
Copper ($Cu$)	$Cu^{2+}$	Least Electropositive (Easiest to discharge)
Mercury ($Hg$)	$Hg^{2+}$
Silver ($Ag$)	$Ag^+$
Gold / Platinum ($Au/Pt$)	$Au^{3+} / Pt^{4+}$

Activity Series of Anions

Anion Symbol	Anion Name	Ease of Discharge at Anode
$SO_4^{2-}$	Sulphate ion	Most difficult to oxidize/discharge
$NO_3^-$	Nitrate ion	Difficult
$Cl^-$	Chloride ion	$\downarrow$ Increasing ease of loss of electrons & discharge at Anode $\downarrow$
$Br^-$	Bromide ion	Easier
$I^-$	Iodide ion	Very easy
$OH^-$	Hydroxyl ion	Easiest to oxidize/discharge

6.7 PREFERENTIAL OR SELECTIVE DISCHARGE OF IONS

When an electrolyte contains multiple cations or anions competing for discharge at the same electrode, selective discharge depends on three critical factors:

Relative Position in Electrochemical Series: Lower position in series = preferential discharge.
Relative Concentration of Ions: Higher concentration can override position in series (e.g. concentrated $NaCl$ brine).
Nature of Electrodes: Inert electrodes (Graphite, Pt) do not react; Active electrodes ($Cu, Ni, Ag$) dissolve into electrolyte.

6.8 DETAILED EXPERIMENTAL EXAMPLES OF ELECTROLYSIS

I. Electrolysis of Molten Lead Bromide ($PbBr_2$)

Fig. 6.4 Electrolysis of molten lead bromide

Electrolyte: Molten Lead Bromide ($PbBr_2$ maintained molten above $380^\circ C$ by continuous heating).
Electrolytic Cell: Crucible made of Silica (Silica is non-reactive, withstands high heat, non-conductor).
Electrodes: Both Cathode and Anode are Inert Graphite plates (Graphite is unaffected by corrosive bromine vapours).
Ionisation Equation: $PbBr_2(l) \xrightarrow{\text{heat}} Pb^{2+} + 2Br^-$
Reaction at Cathode (-ve): $Pb^{2+} + 2e^- \rightarrow Pb(s)$
(Silvery grey metal lead deposited at bottom)
Reaction at Anode (+ve): $2Br^- \rightarrow Br_2(g) + 2e^-$
(Dark reddish-brown bromine vapors evolved)

II. Electrolysis of Acidified Water (Hoffman's Voltameter)

Fig. 6.5 Electrolysis of acidified water using platinum electrodes

Electrolyte: Water acidified with dilute $H_2SO_4$ (Dilute $H_2SO_4$ acts as a catalyst because pure water is a non-conductor).
Why $H_2SO_4$ instead of $HNO_3$ or $HCl$? Dilute $H_2SO_4$ is non-volatile, whereas $HNO_3$ and $HCl$ are volatile acids.
Electrodes: Inert Platinum foils.
Ionisation Equations:
$H_2O \rightleftharpoons H^+ + OH^-$
$H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$
Reaction at Cathode (-ve): $2H^+ + 2e^- \rightarrow H_2\uparrow$
(Colourless hydrogen gas, burns with a pop sound)
Reaction at Anode (+ve): $4OH^- \rightarrow 2H_2O + O_2\uparrow + 4e^-$
(Colourless oxygen gas, relights a glowing splint)
Volume Ratio: Volume of Hydrogen collected at cathode : Volume of Oxygen collected at anode = 2 : 1.

III & IV. Electrolysis of Aqueous $CuSO_4$ Solution

Fig. 6.6 & Fig. 6.7 Electrolysis of aqueous copper sulphate solution using platinum electrodes vs copper electrodes

Case III: Using Platinum (Inert) Electrodes

Electrolyte: Aqueous Copper Sulphate solution ($CuSO_4 + H_2O$).
Ions Present: Cations ($Cu^{2+}, H^+$), Anions ($SO_4^{2-}, OH^-$).
Reaction at Cathode (-ve): $Cu^{2+} + 2e^- \rightarrow Cu$
($Cu^{2+}$ lower than $H^+$ in series $\implies$ Reddish-brown Cu deposited)
Reaction at Anode (+ve): $4OH^- \rightarrow 2H_2O + O_2\uparrow + 4e^-$
($OH^-$ lower than $SO_4^{2-}$ in series $\implies O_2$ gas evolved)
Observations:
1. Reddish-brown copper coats the cathode.
2. Oxygen gas bubbles evolve at anode.
3. Blue color of solution fades and becomes colorless because $Cu^{2+}$ ions are removed. Solution becomes acidic due to accumulation of $H^+$ and $SO_4^{2-}$ ($H_2SO_4$).

Case IV: Using Active Copper Electrodes

Electrolyte: Aqueous Copper Sulphate solution.
Electrodes: Active Copper cathode and Copper anode.
Reaction at Cathode (-ve): $Cu^{2+} + 2e^- \rightarrow Cu$
(Copper deposited, cathode gains mass)
Reaction at Anode (+ve): $Cu \rightarrow Cu^{2+} + 2e^-$
(Copper anode dissolves, anode loses mass)
Observations:
1. Blue color of solution remains UNCHANGED because for every $Cu^{2+}$ discharged at cathode, an equal $Cu^{2+}$ enters solution from copper anode!
2. Anode gradually diminishes in mass while cathode increases in mass.

6.9 INDUSTRIAL APPLICATIONS OF ELECTROLYSIS

1. Electroplating

Fig. 6.8 & Fig. 6.9 Electroplating of a brass spoon with silver and an iron article with nickel

Electroplating: Process of depositing a thin, smooth, and firm layer of a superior metal (gold, silver, nickel, chromium) on an inferior metallic article with the help of electricity.

Reasons: (i) Decoration / aesthetic appeal, (ii) Protection from rusting and corrosion.

5 Essential Rules for Electroplating:

Article to be plated is ALWAYS made the Cathode (metal cations move to cathode to gain electrons & deposit).
Block of pure plating metal is ALWAYS made the Anode (continuously dissolves to replenish electrolyte ions).
Electrolyte MUST be a soluble salt solution containing ions of the plating metal.
Low D.C. Current for a longer duration must be used (high current causes uneven, rough, flaky deposition).
Direct Current (D.C.) ONLY must be used (A.C. causes alternating deposition and stripping).

Electroplating Process	Anode (+ve)	Cathode (-ve)	Electrolyte Used & Key Equations
Silver Plating (e.g. Brass spoon)	Block of pure Silver ($Ag$)	Cleaned article (Brass spoon)	Sodium Argentocyanide solution $Na[Ag(CN)_2]$ Preparation: $AgNO_3 + NaCN \rightarrow AgCN\downarrow + NaNO_3$ $AgCN + NaCN \rightarrow Na[Ag(CN)_2]$ Cathode: $Ag^+ + e^- \rightarrow Ag$ Anode: $Ag - e^- \rightarrow Ag^+$ Why not $AgNO_3$ directly? $AgNO_3$ causes fast, rough & non-uniform deposit!
Nickel Plating (e.g. Iron key)	Block of pure Nickel ($Ni$)	Cleaned iron article	Nickel Sulphate solution ($NiSO_4$ acidified with dil. $H_2SO_4$) Cathode: $Ni^{2+} + 2e^- \rightarrow Ni$ Anode: $Ni - 2e^- \rightarrow Ni^{2+}$

2. Electro-refining of Metals (Purification of Copper)

Objective: Obtaining 99.99% pure commercial copper for electrical transmission wires.

Anode: Slabs of Impure Crude Copper.
Cathode: Thin strips of Pure Copper.
Electrolyte: Acidified Copper Sulphate solution ($CuSO_4 + dil. H_2SO_4$).
Reaction at Cathode: $Cu^{2+} + 2e^- \rightarrow Cu \quad (\text{Pure copper deposits on cathode strip})$
Reaction at Anode: $Cu \rightarrow Cu^{2+} + 2e^- \quad (\text{Impure copper anode dissolves})$
Anode Mud: Insoluble noble impurities like Gold ($Au$) and Silver ($Ag$) settle down at the bottom below the anode as Anode Mud.

3. Electrometallurgy (Extraction of Metals)

Highly electropositive metals ($K, Na, Ca, Mg, Al$) have strong affinity for oxygen and cannot be reduced by carbon or $CO$. They are extracted by electrolysis of their fused (molten) halides or oxides.

Why fused state instead of aqueous solution? In aqueous solution, $H^+$ ions are discharged at cathode preferentially over $Na^+, K^+, Al^{3+}$, so no metal would be obtained!

Extraction of Aluminium (Hall-Héroult Process):

Electrolyte: Fused Alumina ($Al_2O_3$) + Cryolite ($Na_3AlF_6$) + Fluorspar ($CaF_2$).
Cathode: Inner gas carbon lining of steel tank ($2Al^{3+} + 6e^- \rightarrow 2Al$).
Anode: Thick graphite rods ($3O^{2-} - 6e^- \rightarrow 3[O]; 3[O]+3[O] \rightarrow 3O_2$).

6.10 ACIDS, BASES AND SALTS AS ELECTROLYTES SUMMARY

Class of Compound	Strong Electrolytes (Ionise completely)	Weak Electrolytes (Ionise partially)	Non-Electrolytes (Do not ionise)
Acids	$HCl, HNO_3, H_2SO_4$	$CH_3COOH, HCOOH,$ $H_2CO_3, H_3PO_4$	—
Bases	$NaOH, KOH, LiOH$	$NH_4OH, Ca(OH)_2,$ $Mg(OH)_2$	—
Salts	$NaCl, PbBr_2,$ $CuSO_4, AgNO_3$	$Na_2CO_3, KHCO_3,$ $(CH_3COO)_2Pb$	Organic covalent compounds ($CCl_4$, Benzene, Sugar)