Topic 11 Electrolysisnew

5/26/2018 Topic 11 Electrolysisnew

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ELECTROLYSIS

Mohamad nasir bin othman


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INTRODUCTION

In contrast to spontaneousredox reaction, which result inthe conversion of chemicalenergy into electrical energy,electrolysis is the process in

which electrical energy is usedto cause a non-spontaneouschemical reaction to occur.

An electrolytic cell is anapparatus for carrying outelectrolysis. The same

principle underlie electrolysisand the processes that takeplace in galvanic cells.


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Electrolyte cell

The electrolyte cell contains a pair of

electrode connected to the battery.

The battery serves as an electron pump,

driving electrons to the cathode, where

reduction occurs, and withdrawing electrons

from the anode, where oxidation occurs.


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The reactions at the electrodes are,

Anode (oxidation): 2Cl-(l) Cl2(g) + 2e

Cathode (reduction) 2Na

+

(l) + 2e 2Na(l)Overall: 2Na

+(l) + 2Cl

_(l) 2Na(l) + Cl2(g)


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Electrolysis of Water

19.8

Anode: 2H2O (l) O2(g)+ 4H+(aq)+ 4e-

Cathode: 4H+(aq) + 4e 2H2(g)

2H2O (l) 2H2(g) + O2(g)


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Electrolysis

Electrolysis of moltencompounds

Metals are formed at the cathode

Non-metals are formed at theanode

Electrolysis of aqueoussolution

Selective discharge of ions determined by:

Position of ions in the electrochemical series

Concentration of ions

Types of electrodes


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Electrolysis of molten compounds

Example 1 : PbBr2

Anode: ?

Katode: ?


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a. Position of ions in the electrochemical

series

The lower the position

of the ion in the

electrochemical series,

the easier the ion willbe discharged.

Example: Electrolysis of

aqueous sodium sulphate

(Na2SO4).

Na2SO4 2Na+ + SO4

2-

H2O = H+ + OH

-

Anode : ?

Cathode: ?

Cation Anion

K+

Na+

Mg2+

Al3+

Zn

2+

Fe3+

Sn2+

Pb2+

H+

Cu

2+

Hg+

Ag+

F-

SO42-

NO3-

Cl-

Br

-

I-

OH-

Tendency

to

discharge

increases


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b. Effect of concentration of ions in the

solution, CuCl2(aq)

CuCl2 Cu2+

+ 2Cl-

H2O H+

+ OH-

Anode: ?

Cathode: ?


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c. Effect of types of electrode used

Electrolysis of CuSO4 solution

CuSO4 Cu2+

+ SO42-

H2O H+

+ OH-

(a) If carbon is used as the electrodes, OH-

ions aredischarged at the anode because of the position of OH-ion in the electrochemical series.

(b) if Cu is used as the anode,both SO42-

and OH- ions

are not discharged. Cu acts as an active electrode here

because it takes part in the chemical reaction duringelectrolysis.


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Quantitative aspects of electrolysis

The quatitative treatmentof electrolysis wasdeveloped primarily byFaraday.

He observed that the massof product formed (orreactant consumed) at anelectrode is proportional toboth the amount of

electricity transferred at theelectrode and the molarmass of the substanceinquestion.


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The amount of electricity transferred is measuredin the unit of coulomb (C).

One coulomb is the quantity of electric charge

transferred by a current of one ampere (A) in onesecond (c). Thus,

Q = It

Q : quantity of electric charge (C)

I :current (A)

t : time (s)


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Faraday constant (F)

Faraday constant (F) is the charge on one mole

of electrons, that is

1 F = 96500 C


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Faradays First Law

Faradays First Law states that the mass of a

substance produced at an electrode during

electrolysis is proportional to the quantity of

electricity (in coulomb) passed.


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Example 1

An aqueous solution of copper(II)sulphate is

electrolysed using a current of 0.150A for 5

hours. Calculate the mass of copper deposited

at an electrode at the cathode.

Q = it = 0.150 x 5 x 3600 = 2700 C

Number of moles of electrons = 2700/96500 = 0.028 mol

Number of moles of copper deposited= x 0.028 = 0.014 mol

Mass of copper deposited = 0.014 x 63.5 g = 0.889 g


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Faradays Second Law

If the same quantity ofelectricity is passedthrough differentelectrolytes, the mass ofthe substance liberated atthe electrode is inverselyproportional to thecharges on the ions.

Thus, 1F of electricity will

discharge 1 mole of Ag+

ions, but mole of Cu2+ions, 1/3 mole of Al3+ ionsand mole of oxygen gas.

We can determine the mass of a substance

deposited at an electrode by weighing the

electrode before and after the electric current

Is supplied. The apparatus used for this type of

experiment is called coulometer.

The apparatus on the right is called a voltameter.A voltameter is

an electrolyte cell which is set up for collecting and measuringthe volume of any gas liberated during the electrolysis.

Figure 1


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Example 2

With reference to Figure 1,calculate

(a) the mass of copperdeposited

(b) the volume (in dm3)ofhydrogen liberated

(c) the volume (in cm3 )of oxygen

collected at room temperatureand pressure, if 0.32 gof silver isdeposited in the silvercoulometer.

Answer: (a) 0.094g (b) 0.0355dm3(c) 17.8 cm3

Figure 1


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(a) Number of moles of silver deposited = 0.32/108 = 2.96 x 10-3 mol

1F = 1mol e = 1 mol Ag = mol Cu2+Number of moles of Cu deposited = x 2.96 x 10-3

= 1.48 x 10-3 mol

Mass of Cu deposited = 1.48 x 10-3 mol x 63.5 g/mol

= 0.094 g

(b) 1 F = Ag+= H2Number of moles of H2liberated = x 2.96 x 10

-3

= 1.48 x 10-3 mol

Volume of H2liberated = 1.48 x 10-3

mol x 24

= 0.0355 dm3


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(c) 2O2- O2 + 4e

Number of moles of O2liberated = x 2.96 x 10-3

= 7.40 x 10-4 mol

Volume of H2liberated = 7.40 x 10-4 mol x 24 x 1000

= 17.8 cm3


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Miskonsepsi Pelajar 1

Anion sudah mencapai keadaan oktet; sudah

stabil, mengapa masih berlaku tindakbalas

pada anod sel elektrolisis?

Miskonsepsi: Anion stabil tetapi bersifat

nukleofilik (kaya elektron) dan juga reaktif; ia

gemar untuk mendermakan elektronnya bagi

membentuk kestabilan baru melalui ikatankovalen.


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Miskonsepsi Pelajar 2

Kenapa dalam sel elektrolisis kation pergi ke

katod; tidak ke anod?

Jawapan: Kation bercas positif dan katod

bercas negatif. Cas +ve danve akan saling

tarik menarik.

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Topic 11 Electrolysisnew