Chptr 2 Lecture 3

7/23/2019 Chptr 2 Lecture 3

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CHE142

Inorganic & physicalchemistry

Faculty of Chemical Engineering

Chapter 2: Redox & ElectrochemistryLecture 3 : Electrolytic Cell



Electrolytic Cell

Spontaneous!!! NOT

Spontaneous!!!



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• Instead of the reactions that would occur spontaneously at

the electrodes, the external power supply causes the

reverse reactions to occur.• Thus, for reduction, electrons are forced to the cathode in

an electrolytic reaction.

The cathode is still the site of reduction, but in electrolysis, it is

negatively charged because the external power supply forceselectrons (and their negative charges) into the location of the

reduction reaction.

• Anode is still the site of oxidation, but in electrolysis, it is

positively charged.In forcing oxidation of the material, the external power supply

pulls the electrons away from the region of the oxidation

reaction.



• In a voltaic cell, electrons are generated at the anode, so it

is negative, and electrons are consumed at the cathode, so

it is positive.

• In an electrolytic cell, the electrons come from the external

power source, which supplies them to the cathode, so it is

negative, and removes them from the anode, so it ispositive.

Electrolytic CellElectrolytic Cell



• The redox process which happens in an electrolytic

cell is known as electrolysis.

• Electrolysis is the process of passing an electriccurrent through an ionic solution or molten salt to

produce a chemical reaction.

• Electrolytic cells can be divided into two categoriesbased on the nature of the electrodes used.

If the electrodes are chemically inert (unreactive) materials

that simply provide a path for electrons, the process is called

passive electrolysis. Examples of passive electrodes: carbon,

graphite, platinum.

If the electrodes are part of the electrolytic reaction, we have

active electrolysis.



The electrolysis of molten sodium chloride (NaCl) ….




The Downs Cell …

Industrial method electrolytic cell of molten NaCl electrolysis

The iron screen prevent Na to spontaneously react with Cl2

Most practical, highly effective method of obtaining metallic Na, but operating cost

is very high (construction cost, electricity cost, heating cost)




The electrolysis of aqueous sodium chloride (NaCl) ….



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Faraday’s Law of Electrolysis• A quantitative unit of electricity is called the faraday.

• A smaller electrical unit commonly used is the coulomb (C).

• One coulomb is defined as the amount of charge that passes a

given point when 1 ampere (A) of electric current flows for 1

second.

FARADAY’S LAW OFFARADAY’S LAW OF

ELECTROLYSISELECTROLYSIS



• Problems based on Faraday's law often ask you to calculate

current, mass of material, or time.

FARADAY’S LAW OFFARADAY’S LAW OF

ELECTROLYSISELECTROLYSIS


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Example:

How long does it take to produce 3.0g of Cl2 (g) byelectrolysis of aqueous NaCl using a power supply with acurrent of 12A?

Solution:From half-reaction, we loss of 2 mol of electrons produces 1 mol of chlorine

gas:

So, we find the total charge:

Then, use relationship between charge and current to find time needed:

FARADAY’S LAW OF ELECTROLYSISFARADAY’S LAW OF ELECTROLYSIS

Chptr 2 Lecture 3

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