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ELECTROLYSIS
In this chapter, you will learn to:
Differentiate between the electrolytes andnon-electrolytes
Understand the electrolysis of molten compounds
Understand the electrolysis of aqueous solutions
Analyse the voltaic cell
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Definition of electrochemistry:
Branch of chemistry dealing with theinterchange between chemical energy andelectrical energy.
Electrolysis:
A process during which an electrolyte is
decomposed by an electric current into itscomponent elements.
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Differences between electrolytes and non-electrolytes
Electrolyte Non-electrolyte
A chemical substance
which conductselectricity in the moltenor aqueous state.
Substance which cannot
conduct electricity in anystate.
Examples: dilute acids,alkalis, molten zincbromide &sodium
chloride
Examples: benzene,alcohol, naphthalene,tetrachloromethane
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Molten :a solid substance heated to melting point where
its turns liquid.
Electrolytes are able to conduct electricity
because there are free ions presents in themolten or aqueous state.
Aqueous solution :
A solution with water as the solvent. Contrast
with: non-aqueous solution.
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An ionic compound in the solid state cannotconduct electricity because its ions are held
together by strong ionic bonds which are notfree to move about.
But in the molten state, the ions can move
about freely.
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Electrolysis of Molten Compounds
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Exercise
1. State if each of the following is an electrolyte ornon-electrolyte.
(a) KOH solution(b) Na2O
(c) NH3 solution(d) Glucose solution(e) Ethanoic acid, CH3COOH(f) Acetone
(g) Kerosene(h) Petrol
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2. Which of the following substances can
conduct electricty? (i) potassium chloride crystals
(ii) Zinc sulphate solution
(iii) Iron (iv) Molten napthalene
Are all the above substances electrolytes?
Explain.
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Answer:
(ii) and (iii)No, only zinc sulphate solution is an
electrolyte as its ions can move about
freely and decomposition occurs at theelectrodes. Iron conducts electrolyte butcannot decompose as it is made up ofatoms and not ions. KCl is in the form of
solid. Its ion are held tightly by ionic bondsand cannot move freely. Moltennapthalene is in the form of molecules.
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6.2 Electrolysis for Molten Compounds
An electrolyte in the form of molten compoundsor aqueous solution can be broken down into its
elements.
Examples:Molten aluminium oxide(I) Aluminium(s) + Oxygen(g)
Copper(II) chloride solution(aq) Copper(s) + Chlorine(g)
Electrolysis - process during which an electrolyte is decomposed byan electric current into its component elements.
6.2 Electrolysis for Molten Compounds
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Ability to conduct electricity
Solid state -ions are not free to movebecause there are arranged in afixed crystal lattice.
-thus, cannot conduct electricity
(no free ions to move and carryelectrical charges).
Molten form or
aqueous solution
-ions move freely to carry
electrical charges.
-thus can conduct electricity
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Electrolytic cell and its component
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During electrolysis:
* Anion(-ve ions) will bepulled towards theanode(+ve electrode).
* Cation(+ve ions) will
be pulled towards thecathode(-veelectrode)
Anode Anion will
releasedelectrons tothe anode
Cathode Cations willaccept
electronsfrom cathode
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Method of writing balanced half equations(ion electronequation)
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6.3 Electrolysis of aqueous solutions
In aqueous solutions, the presence of waterresults in 2 types of cations(metallic cation &hydrogen ion) & 2 types of anions (non-metallic anion of salt & hydroxide ion).
This is because water in aqueous solutiondissociate to form H+ and OH- ions.
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Factors Which Influence The Discharge of
Ions
The factors affecting the selective dischargeof ions at the electrodes are:
(a) position of ions in electrochemical series
(b) concentration of ions in the solution
(c) types of electrode
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(a) Positions of Ions in The ElectrochemicalSeries
*The chart lists the ions in
order of difficulty of
discharge.
*The lower the ion in the ES,the easier it is to be
discharged
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Eg: Electrolysis of Lead(II) Nitrate,
Anode Cathode
and ions. isselected for discharged
(lower in ES)
and ions. isselected for discharged
(lower in ES)
Anode:Oxygen gas is released
Cathode:hydrogen gas is released
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(b) Types of Electrode Used In Electrolytes
Most common electrode: Carbon & Platinum
Eg: electrolysis of dilute sodium chloride using carbonelectrodes
Anode Cathode
ion present: , Ion present: ,
But if, mercury is used as the cathode, different reaction occurs at
cathode.
Sodium immediately forms an amalgam with mercury
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(c) Concentrations of ions in a solution
An ion will be selected for discharge if itsconcentration in the aqueous solution ishigh.
Especially noticeable in concentratedhalide solutions containing
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6.4 Electrolysis In Industries
Importance Applicationof electrolysis Extraction
Electroplating ofmetals
Purification
Chemical cells
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(a) Extraction of metals
Aluminium is extracted by electrolysis of a
mixture of bauxite( )with cryollite( )
(b) Metal Purification
Copper is often obtained by refining metalores and scrap copper. The anode is theimpure metal and the cathode is a small
amount of pure metal.
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(c) Electroplating
The cathode is the item to be plated and the
anode is the metal to be used as the plating.During electrolysis, the metal form the anode willdissolve and travel to the cathode, where theions will plate.
(d) Chemical cells
Consists of 2 metals of different types immersedin an electrolyte. The metals are connected by
an external wire to complete the circuit. Thisresult is an electric current across the wire whichsupply power to light bulbs.
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6.5 Voltaic cells
Characteristics Electrolytic cell Voltaic cell
Funct: Use DC to produce anon spontaneouschemical charge
Use spontaneouschange to produec adirect electric current
Structure: Consits of 2electrodes immersedin an electrolytewhich connected to abattery
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