electrochemistry lesson 25

7/28/2019 Electrochemistry Lesson 25

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Dissociation equation :

Setup of the apparatus

Electrodes Positive terminal Negative terminal

Ions gathered

Observation

Halfequation

Process occur

Anion : I

PbI2 Pb2+ + 2 I

Cation : Pb2+

2 I I2 + 2 e Pb2+ + 2 e Pb

Dark purplish solid with

some pungent brown gas isreleased.

Grey solid is deposited at thenegative electrode.

Oxidation Reduction


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Ions gathered

Observation

Halfequation

Process occur

Anion : OH ; Br

KBr K+ + Br

Cation : H+ ; K+

4 OH O2 + 2 H2O + 2e 2 H+ + 2 e H2

Bubbling is observed.

Colourless gas evolvedignite the glowing wooden

splinter.


Colourless gas evolved

resulted a pop sound to be

heard when put close to a

burning wooden splinter

Oxidation Reduction

H2O H+ + OH


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Ions gathered

Observation

Halfequation

Process occur

Anion : OH ; Cl

FeCl2 Fe2+ + 2 Cl

Cation : H+ ; Fe2+

Reduction

H2O H+ + OH



resulted a pop sound to be

heard when put close to a

burning wooden splinter

2 H+ + 2 e H24 OH O2 + 2 H2O + 2e



ignite the glowing wooden

splinter.

Oxidation


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Ions gathered

Observation

Halfequation

Process occur

Anion : OH ; Cl

CuCl2 Cu2+ + 2 Cl

Cation : H+ ; Cu2+

4 OH O2 + 2 H2O + 2e Cu2+ + 2 e Cu



splinter.

Brown colour solid deposited

around the electrode,resulting the blue colour of

CuSO4 faded with time.

Oxidation Reduction

H2O H+ + OH


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Ions gathered

Observation

Halfequation

Process occur

Anion : OH ; conc. NO3

Pb(NO3)2 Pb2+ + 2NO3

Cation : H+ ; conc. Pb2+

4 OH O2 + 2 H2O + 2e Pb2+ + 2 e Pb



splinter.

Oxidation Reduction

H2O H+ + OH

Grey solid is deposited

around the electrode.


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Ions gathered

Observation

Halfequation

Process occur

Anion : OH ; NO3- Cation : H+ ; Ag+

Oxidation Reduction

H2O H+ + OH

Silver electrode corroded

and become thinner.

Ag+ + e AgAg Ag+ + e

AgNO3 Ag+ + NO3

Silver colour solid deposited

around the electrode,


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6.4 Application of Electrolysis in Industrial

6.4.1 Extraction of aluminium from its ore (bauxite)

When aluminium oxide is dissolved in molten cryolite

Al2O3 (l) 2 Al3+ (l) + 3 O2 (l)

Electrolyte mixture is then placed in carbon-lined iron vat (cathode).

The heating effect of the electric current melts the electrolyte

mixture, producing Na+, Al3+, O2- and F- ions


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During electrolysis, aluminium is preferentially discharged at

the cathode.

Cathode : Al3+ + 3 e Al

At the anode, oxygen is released instead of fluorine.Anode : 2 O2 O2 + 4 e


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In industrial process, chlorine gas, together with sodium

metal, is prepared using molten sodium chloride (brine) using

mercury-cathode cell.

Mercury is specially used to attract the sodium formed in

cathode and form an alloy named amalgam

This method is not environment friendly as the mercury used

is poisonous.

Half equation occur at cathode

Half equation occur at anode

Na+ + e- Na

2Cl- Cl2 + 2 e-


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6.4.2 Purification of copper in industries

Pure copper can only be obtained under such method. The set-up of

purifying copper is shown in diagram below.


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Ions gathered

Observation

Halfequation

Process occur

Anion : OH ; SO42- Cation : H+ ; Cu2+

Oxidation Reduction

Copper electrode

corroded at anode andbecome thinner.

Cu2+ + 2 e CuCu Cu2+ + 2 e

Brown colour solid

deposited around theelectrode,

The blue colour of CuSO4

remain unchanged.


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6.4.3 Electroplating method.

Electroplating method is one way of applying these finishes. Forks,

spoons, and jewellery are often electroplated to give the objects the

appearance of silver or gold while still keeping the cost of the objects

low.

At anode

Observation : silver plate is

corroded.

Half equation : Ag Ag+ + e

At cathode

Observation : silver is coated

around the bracelet

Half equation : Ag+ + e Ag


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6.5 Chemical Cell

As mentioned early in the topic, there are 2 types of conversion of

energies. In electrolytic cell, the conversion energy takes place

where . energy is converted to . energy.

In this sub-topic, the conversion takes place where CHEMICALenergy to ELECTRICAL energy. This conversion of energy is

known as CHEMICAL CELL (sometimes called as voltaic cell)

The following experiment shows basically how a simple chemical

cell works.

chemical electrical


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Electrical energy

produced. Voltagedecrease as reactant

decrease with time

Magnesium oxidise

to form magnesium

ion, while hydrogen

ion reduced to H2

No chemical

reaction takeplace

No chemical

reaction take

place


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Discussion :

A chemical cell basically worked when there is a difference between

the electrodes potential of 2 different metals. Each metal

possessed its own electrode potential. When 2 different metals

contact each other, they shall be a potential difference betweenthe metals in the form of electromotive forces (e.m.f.) in the form of

voltage.

In fact, electrochemical series is build base on the electrode

potential possessed by each metal. Table below shows the value of

the electrode potential of some metals in the electrochemical series

If both are the same metal, theres no difference in electrode

potential, thus theres no e.m.f. [Exp 2]

Metal Ag+ Cu2+ H+ Pb2+ Sn2+ Fe2+ Zn2+ Al3+ Mg2+ Na+ K+

Eo (V) +0.80 +0.34 0.00

0.14

0.23

0.44

0.76

1.66

2.38

2.71

2.91


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Higher the position in electrochemical series, the more stable it is as

a positive ion. Since magnesium has higher position than copper, so

magnesium tends to donate electron to form a more stable

magnesium ion.

Half equation : Mg (s) Mg2+ (aq) + 2 e- [Magnesium corroded]

This is also supported by the deflection of needle where the

deflection move to right (where electron is

donated by magnesium). Since magnesium donate electrons,

it is the of the cell. In the solution now contain Mg2+ (from Mg) ; Na+ (from salt)

and H+(from aqueous solution), so theres a selection of ions

to be discharge in copper electrode, which act as

. of the cell as it tend to receiveelectron from magnesium plate. In cathode, one with lower

position in ECS will be selected, so hydrogen will be selected.

negative terminal (anode)

positive terminal (cathode)


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Half equation : 2 H+ (aq) + 2e- H2 (g)

[hydrogen gas evolve at copper electrode]

In chemical cell, when the anode and cathode are recognised, an

overall equation can be written by

Anode : Mg (s) Mg2+ (aq) + 2 e- [electrons are donated]

Cathode : 2 H+ (aq) + 2e- H2 (g) [electrons are received]

Overall :

In short, when there are 2 different metals, the higher position metal

in ECS will be the of the cell (donate electrons) while the

metal with the lower position in ECS will be the of the

cell (receive electrons)

Mg (s) + 2 H+ (g) Mg2+ (aq) + H2 (g)

anode

cathode


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6.6 Daniel Cell

A Daniel cell is typical chemical cell that is builds by 2 different

metals immerse in each of its aqueous solution separately

connected by wires and salt bridge or porous pot as shown in the

diagram.V

V


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Salt bridge contains inert ion or salts that does not react with

electrolyte such as sodium chloride, potassium chloride or

sulphuric acid. The function of the salt bridge / porous pot in

the Daniel cell

Similar to the simple chemical cell, the electrodes are divided

to anode and cathode where

Electrode Anode Cathode

Function

Terminal

To complete the chemical cell

To separate between the 2 chemical cells.

To donate electrons To receive electrons

Negative terminal Positive terminal

electrochemistry lesson 25

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