electrolysis 6.1

8/3/2019 Electrolysis 6.1

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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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electrolysis 6.1

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