Chapter6 Electrochemistry

7/29/2019 Chapter6 Electrochemistry

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ELECTROCHEMISTRY


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CHEMICALSUBSTANCES

ELECTROLYTES NON-ELECTROLYTES

Substances that can conduct

electricity when they are inmolten state or aqueous and

undergo chemical changes

Substances that cannot

conduct electricity either inmolten state or aqueous

Example:

Ionic substances

(i) Dilute acid solution

(ii) Dilute alkaline solution

(iii) Molten salts

(iv) Aqueous salt solution

Example:

Covalent substances

(i) Naphthalene

(ii) Sugar solution

(iii) Latex

(iv) Hydrogen chloride in organic

solvents
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Heat

Molten lead (II) iodide

Bulb

Carbon

electrodes

Heat

Solid lead (II) iodide

Carbon

electrodes

Bulb

.. .. ... .

.... ... .

....

switchswitch


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Do not conductelectricity

This is because the

ions are held in a

fixed crystal lattice.

They are held with

the strong

electrostatic forcesof attraction

Do not freely move

I-

Pb2+

Pb2+

Pb2+

I-

I-

I-

I-

I-

Pb2+

Pb2+

Pb2+

Pb2+

Solid lead (II) iodide

IN A SOLID STATE


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Molten lead (II) iodide

I -Pb

2+

Pb2+ Pb

2+

I-

I-

Can conduct

electricity

This is because the

presence of moving

ions. The ions are

free to move in themolten or aqueous

solution

IN A MOLTEN STATE


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ELECTROLYTES AND NON

ELECTROLYTES

Classify the following chemicals into electrolytes or non-

electrolytes

Potassium chloride, KCl solution Molten lead (II) bromide, PbBr2

Glucose solution

Trichloromethane

Molten zinc oxide, ZnO Sodium chloride, NaCl solution

Ammonia in tetrachloromethane


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A process of breaking down the chemical

compounds into their constituent elements using

electric currentExample:

Molten aluminium oxide (I) aluminium (s) + oxygen (g)

Copper (II) chloride (aq) copper (s) + chlorine (g)

Electric current

Electric current

ELECTROLYSIS


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ELECTROLYTIC CELL

Electrolyte

e-

e- e-

e-

ElectrodeElectrode

- (cathode)+ (anode)

..

. .. .

.. . ..

...

- +

Anion Cation


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The set of apparatus needed to conductelectrolysis

It consists of a battery, an electrolyte, and twoelectrodes (anode and cathode)

Anode - the electrode which is connected to thepositive terminal of an electric source in theelectrolytic cell

Cathode - the electrode which is connected tothe negative terminal of an electric source in theelectrolytic cell

ELECTROLYTIC CELL


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ANODE

(POSITIVE ELECTRODE)

CATHODE

(NEGATIVE ELECTRODE)

Anions (negative ions)

will be pulled towards

the anode (positiveelectrode)

Cations (positive ions) will

be pulled towards the

cathode (negativeelectrode)

At the anode, anions

will be discharged by

releasing the electrons

to the anode

At the cathode, cations will

be discharged by

receiving the electrons

from the cathode

During electrolysis:


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ELECTROLYSIS OF MOLTEN

COMPOUNDS


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QUESTIONS

(a) Molten zinc chloride (I)

(b) Molten aluminium (III) bromide

(c) Molten zinc (II) iodide

(d) Molten lead (II) oxide Electric current

Electric current

Electric current

Electric current


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Example 1

Molten Potassium

Iodide, KI

e-

e- e-

e-

Carbon

electrode Carbonelectrode


..

. .. .

.. . ..

...

The ions that presents in electrolyte are:

(i) Anion:

(ii) Cation:


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Example 2

e-

e- e-

e-

Carbon



..

. .. .

.. . ..

...


(i) Anion:

(ii) Cation:

Molten zinc oxide,

ZnO


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Example 3

e-

e- e-

e-

Carbon



..

. .. .

.. . ..

...


(i) Anion:

(ii) Cation:

Molten lead (II)

bromide, PbBr2


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Example 4

e-

e- e-

e-

Carbon



..

. .. .

.. . ..

...


(i) Anion:

(ii) Cation:

Molten lead (II) oxide,

PbO


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Example 5

e-

e- e-

e-

Carbon



..

. .. .

.. . ..

...


(i) Anion:

(ii) Cation:

Molten zinc (II)

chloride, ZnCl2


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COMPOUND

e-

e- e-

e-

Carbonelectrode

Carbonelectrode


..

. .. .

.. . ..

...


(i) Anion:

(ii) Cation:

Molten lead (II)

bromide, PbBr2


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Molten lead (II)

bromide

e-

e-

e-

e-

Carbon

electrodeCarbon

electrode


..

. .. .

.. . ..

...

Pb2+

Pb2+

Pb2+

Pb2+

Br-

Br-

Br-

Br-


COMPOUND


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Pb2+

are attracted to the cathodeBr-are attracted to anode

Br-discharge by releasing electrons to the

anode

CATHODE

(NEGATIVE ELECTRODE)

ANODE

(POSITIVE ELECTRODE)

Br-

Br + e-

Neutral atoms form and combine to formneutral bromine molecules

Br + Br Br2 (g)

The half reaction at anode can berepresented by the half equation

2Br-(l) Br2 (g) + 2e

-

Pb2+

discharge by receiving electrons

(provided by the battery) at the cathode

to form neutral lead atoms

Pb2+

(l) + 2e-

Pb (s)

The half reaction at cathode can be

represented by the half equation


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The Half Equation:

Cathode:

Anode:

Overall Equation:

2Br-(l) Br2 (g) + 2e

-

Pb2+(l) + 2e- Pb (s)

Pb2+

(l) + 2Br-(l) Pb(s) + Br2 (g)


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Write the half equation and the overall

equation for the electrolysis of these

molten compounds :

(A) Molten potassium iodide, KI(B) Molten zinc (II) chloride, ZnCl2

(C) Molten lead (II) oxide, PbO


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ELECTROLYSIS OF AQUEOUS

SOLUTIONWater molecules in an aqueous solution can slightly dissociate to

produce H+

and OH-

H2O H+

(aq) + OH-(aq)

So, aqueous solution contains H+

and OH-and electrolyte ions

Examples:

Aqueous Solution

Ions present

Anion CationNaCl solution Cl- , OH

-Na

+, H

+

CuSO4 solution

HNO3 solution


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If they are more than one cations or anions

attracted, the choice of ion to be discharged

depends on :

Positions of the ions

in the electrochemical series

Types ofelectrodes

Concentration of ions

in the electrolyte
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Electrochemical Series (pg.101)

CATIONS

K+

Na+

Ca2+

Mg2+

Al3+

Zn2+

Fe2+

Sn

2+

Pb2+

H+

Cu2+

Ag+

Tendency to

discharge

increases

ANIONS

F-

SO42-

NO3-

Cl-

Br-

I-

OH-


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Copper (II) chloride

solution, CuCl2

e-

e-

e-

e-

Carbon

electrodeCarbon

electrode


H+

Cu2+

Cu2+

H+

Cl-

Cl-

OH-

OH-


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Observation

and

Product

H+

, Cu2+

Cu2+

are preferred to be

discharged

(Cu2+

is below H+

in the ES)

OH-, Cl

-

OH-are preferred to be

discharged

(OH-is below Cl

-in the ES)

Ions that are

attracted

CathodeAnode

Bubbles of gas, O2

is releasedBrown solid, Cu is formed

Half equation4OH

-(aq) 2H2O (l) + O2 (g)

+ 4 e-

Cu2+

(aq) + 2e-

Cu (s)


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Concentration of ions in

electrolyte solutions

Concentration of ions in electrolyte solution also

can affects the choice of ions to be discharged

An ion that is more concentrated is preferably

discharged Anode the ions to be discharged is

determined by the concentration of ions

Cathode

the ions to be discharged isdetermined by the position of the ion in the

E.S


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Concentrated

copper (II) chloride

solution, CuCl2

e-

e-

e-

e-

Carbon

electrodeCarbon

electrode


Cu2+H+Cl

-OH

-


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Observation

and

Product

H+

, Cu2+

Cu2+

are preferred to bedischarged

(Cu2+

is below H+

in the ES)

OH-, Cl

-

Cl-

are preferred to bedischarged

(Cl-

is more concentrated)

Ions that are

attracted

CathodeAnode

Yellow bubbles gas, Cl2

is released

Brown solid, Cu is formed

Half equation 2Cl-(aq) Cl2 (g) + 2 e

-Cu

2+(aq) + 2e

-Cu (s)


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Types of Electrodes

Silver nitrate, AgNO3

solution

e-

e-

e-

e-

Silver plate Carbon

electrode


Ag+Ag

Ag+


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Types of Electrodes

Ag+

(aq) + e-

Ag (s)

Observation

and

Product

Ag+

receives one electron to

form metal atoms

Ag atom releases one

electron to form Ag+

Ions

CathodeAnode

The silver plate corrodes Grey silver solid, Ag is

formed

Half equation Ag (s) Ag+

(aq) + e-


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Exercises

Electrolysis of copper (II) sulphate, CuSO4, solution

using carbon electrodes

Anode Cathode

Ions that areattracted

Ion that are

prefer to be

dischargedObservation

Product

Half equation


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Exercises

Electrolysis of concentrated potassium chloride, KCl,

solution using carbon electrodes

Anode Cathode

Ions that areattracted

Ion that are

prefer to be

discharged

Observation

Product

Half equation


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Exercises

Electrolysis of nickel (II) sulphate, NiSO4, solution

using nickel plate as anode and cathode

Anode CathodeHalf equation

Observation

Product


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Factors that influence electrolysis ofaqueous solutions

TYPES OF ELECTRODES

POSITION OF IONS

IN THE ELECTROCHEMICALSERIES

CONCENTRATION OF IONS

IN THE ELECTROLYTE SOLUTION


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USES OF ELECTROLYSIS

IN INDUSTRIES

Reacts with a carbonate

metal to form carbon

dioxide gas (CO2),water

(H2

O) and salt

ELECTROPLATING

OF METALS

EXTRACTION OF METALS

PURIFICATION OF

METALS
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Extraction of Metals

Electrolysis can be used to extract metals fromtheir ores

Reactive metals that are more reactive thanC such as (K, Na, Ca, Mg, Al) cannot be

extracted through heating of their metaloxides with C

They need to be extracted from their moltenores using the electrolysis process

In this process: Electrolyte The molten ore

Anode Carbon electrode

Cathode Carbon electrode


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Example:

Extraction of aluminium from electrolysis of

molten aluminium oxide, Al2O3


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Purification of Metals

Metals that have been extracted from theirores are normally not pure. They contains

impurities which need to be removed

These metals can be purified by electrolysis

process

In the purification process

Electrolyte The solution that contains

the metal ions

Anode The impure metal plate

Cathode The pure metal plate


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Example:

Purification of copper


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Electroplating of Metals

Many types of metals can be plated with othermetals through electrolysis.

The aim of metal plating through electrolysis

includes:

(i) making the metal more resistant to corrosion

(ii) making the metal appear more attractive

In the electroplating process

Electrolyte The solution which contains

ions of plating metal

Anode Plating metal

Cathode Metal to be plated


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Example:

Electroplating of a iron spoon with zinc metal


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Example:

Electroplating of a iron spoon with argentum metal


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ELEKTROKIMIA

VOLTAIC CELL


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Copper (II)

sulphate

solution

ELECTROLYTIC CELL

e-

e- e-

e-

Carbon

electrode

Carbon

electrode

- (cathode) + (anode)


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VOLTAIC CELL

Cu

Copper (II)

sulphate

V

e-

e-

e-

e-

- (anode) + (cathode)

Zn

Zn (s) Zn2+

(aq) + 2e-

Cu2+

(aq) + 2e-

Cu (s)


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VOLTAIC CELL

Also known as galvanic cell

Has two different metals which are immersed

into an electrolyte and connected by wire

Produces electrical energy from the chemicalreactions occurring inside the cell

Example : Daniell cell

Chemical energy electrical energy


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DANIELL CELL

V

Dilute

H2SO4

(Salt bridge)

Zn (-) Cu (+)

CuSO4

solution

ZnSO4

solution

(Anode) (Cathode)

e-

e-

e-

e-

Zn (s) Zn2+

(aq) + 2e-

Cu2+

(aq) + 2e-

Cu (s)


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SALT BRIDGE

Can be made from any electrolyte that does notreact with the electrodes in Daniell Cell

Function :

(i) to allow the flow of the ions so the electriccurrent is completed

Example:

(i) diluted H2SO4

(ii) sodium nitrate, NaNO3 solution

(iii) Potassium chloride solution, KCl


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DANIELL CELL

V

Porous

pot

contains

ZnSO4

Zn (-) Cu (+)

CuSO4solution

(Anode)(Cathode)

e- e

-

e- e

-


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Reactivity Series

K

Na

Ca

Mg

Al

Zn

Fe

Sn

Pb

H

Cu

Ag

METALS MORE

ELECTROPOSITIVE

(negative terminal)

MORE

ELECTRONEGATIVE

(positive terminal)


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Anode and Cathode

Anode the electrode where the process ofdonation of electrons takes place

Cathode the electrode where the process of

receiving of electrons takes place

Negative terminal Cathode

Positive terminal - Anode

Negative terminal Anode

Positive terminal - Cathode

ELECTROLYTIC CELL VOLTAIC CELL


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Sel Volta


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Sel Volta danSel Elektrolisis


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ELECTROCHEMICAL SERIES


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Electrochemical Series (pg.101)

CATIONS

K+

Na+

Ca2+

Mg2+

Al3+

Zn2+

Fe2+

Sn2+

Pb2+

H+

Cu2+

Ag+

Tendency to

discharge

increases

ANIONS

F-

SO42-

NO3-

Cl-

Br-

I-

OH-


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ELECTROCHEMICALSERIES

An arrangement of metals, based on the

tendency of each metal atom to donate electrons


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The higherthe position of a metal in the

electrochemical series:

ELECTROCHEMICAL SERIES

More electropositive The greaterthe tendency

of the metal atoms to

donate electrons


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PRINCIPLES DURING CONSTRUCTING

THE ELECTROCHEMICAL SERIES

Potential differences

between two metals in the

voltaic cell

Ability of a metal to

displace another metalfrom its salt solution

Potential differences between two


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The value of the potential difference of a simple voltaic

cell is affected by the position of the metals in theelectrochemical series

When two different metals are immersed in an electrolytesolution, a potential difference is generated

The larger the value of the cell voltage is produced

The further the two metals in the electrochemical series


metals in the voltaic cell



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The metal that is more

electropositive

Located higherin the

electrochemical series

Act as the negative terminal

The metal that is less

electropositive

Located lowerin the

electrochemical series

Act as the positive terminal



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Mg

Zn

Pb

Cu

Ag

3.0 V

1.1 V 0.4 V

0.5 V

Predict the negative terminal, positive terminal and cellvoltage of the following voltaic cell

(a) Magnesium and silver (c) zinc and lead

(b) Magnesium and zinc (d) zinc and silver


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Metal Displacement

A metal that is more electropositive can displaceanother metal which is less electropositive from

its salt solution

This reactiondisplacement reaction

A metal that is located higherin the electrochemical

series can displace another metal which is located

lowerin the electrochemical series from its salt

solution


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IMPORTANCE OF THE

ELECTROCHEMICALSERIES

Reacts with a carbonate

metal to form carbon

dioxide gas (CO2),water

(H O) and salt

It can determined the terminals of voltaic cells

It can determined the

cell voltage for a pair of

metals

It can predicted the

potential of a metal to

displace another metal

from its solution
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Chapter6 Electrochemistry

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