Chemistry Chapter 6 Form 4

8/10/2019 Chemistry Chapter 6 Form 4

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6.1 Electrolytes and Non-electrolytes

Electrolytes Non-electrolytes

•Substances that

can conductelectricity when they

are in molten state

or aqueous solution and undergo

chemical changes.

•Substances that

cannot conductelectricity either in

molten state or

aqueous solution.





•Example:

Sodium chloride,NaCl solution

Molten Lead (II)

bromide, PbBr 2

All ionic compounds

•Example:

Naphthalene

Glucose solution

All covalent

compounds





• Contain freely

moving ions.

• Contain neutral

molecules•Do not contain freely

moving ions.



6.2 Electrolysis of Molten Compound

What is electrolysis?

• A process whereby the ionic

compounds in molten or aqueousstate are broken down intoconstituent elements by passing

electricity through them.



WHAT IS ELECTROLYTIC CELL?



BATTERY

ANODECATHODE

ELECTROLYTE

Electrode whichis connected to

the positive

terminal

Electrode which

is connectedto the negative

terminal

Electrodes



ANODE CATHODE

PbBr 2

Pb 2+ Br -

Release electron Accept electron

2Br- Br2

+ 2e- Pb2+ + 2e- Pb

Pb2+

+ 2Br- Pb + Br2

2Br- Br2 + 2e-

Pb2+ + 2e- PbHalf equation

Overall ionic equation



Factors affecting the products of

electrolysis for aqueous solution

A. Position of ions in the

Electrochemical SeriesB. The Concentration of the solution

C. The type of Electrodes



A. Position in the Electrochemical Series

The lower positionof the ion in theElectrochemical

Series, the easierthe ion to bedischarged duringelectrolysis

http://wiki.one-school.net/index.php/Image:ElectrochemicalSeries.png



Copper(II)

sulphate

solution

The anions, OH- and SO4

2- move

to the anode. OH- ions are

selectivelydischarged.

Bubbles of

colourless gas isreleased.

Anode Cathode

4OH- (aq) 2H2O(l) + O2 (g) + 4e-

The cations,Cu2+.and H+

move to thecathode.

Cu2+ ions accepttwo electrons.

Brown solid is

formed at thecathode.

Cu2+ (aq) + 2e- Cu (s)

The overall ionic equation :

2Cu2+ (aq) + 4OH-(aq) 2Cu (s) + 2H2O(l) + O2 (g)

CuSO4 solutionCu2+, H+,SO4

2- OH-.

A. Position in the Electrochemical Series



Concentrated

hydrochloric

acid

B. The Concentration of the solution

The anions,OH- and Cl- move tothe anode.

Cl- ions areselectivelydischargedbecauseconcentration of

Cl-

is higher thanOH- Bubbles of

yellowish gas isproduced.

2Cl- →

Cl2+ 2e

Anode Cathode

The cations,H+ ionsmove to the

cathode. H+ ions

acceptelectron.

Bubbles of

Colourlessgas isproduced.

2H+ + 2e →H2


2H+

(aq) + 2Cl-

(aq) Cl2 (g) + H2 (g)

Example : Concentrated hydrochloric acid consists ofH+, Cl-,OH-.



Observation :The blue copper(II)sulphate solution becomesfaded/paler because the

concentration ofcopper(II)ions decreases.

Cu2+

.and H+

move to thecathode.

Cu2+ ions selectivelydischargedbecause Cu2+islower than H+ in ECS

Browndeposit/solid is

producedCu2+ + 2e → Cu

Carbon Carbon

C. The Type of Electrodes (Carbon electrodes)

OH- andSO4

2- move tothe anode.

OH- ions areselectivelydischarged

due to thelower positionin ECS.

Bubbles ofcolourless gas

is released atthe anode.

4OH- 2H2O + O2 +4e-

Anode CathodeUsing Carbon Electrode


2Cu2+

(aq) + 4OH-

(aq) 2Cu (s) + 2H2O(l) + O2 (g)



The main source of aluminium is bauxite ore (Aluminium Oxide),

the melting point of aluminium oxide is very high (over 2 000°C),

Cryolite (Na3AIF6) is added to lower the melting point to 980oC.

A. Extraction of Aluminium

Anode : 2O2- O2 + 4e

Cathode : Al3+ + 3e Al

INDUSTRIAL APPLICATIONS OF ELECTROLYSIS



Coating with a Thin Protective Layer of Metal.

C. Electroplating

Anode : Cu Cu2+ + 2e Cathode : Cu2+ + 2e Cu

Corrodes/

becomes

thinner

Coated/

deposit layer of

brown solid

Blue

solutionremainsunchanged

Copper

http://wiki.one-school.net/index.php/Image:Electroplating.png



















Voltaic Cells

[Galvanic Cell]

A simple voltaic cell is a cell with two different

metals being immersed into an electrolyte and

connected by wire

It converts chemical energy to electrical energy.



Mechanism of simple voltaic cellAnode (-ve) Cathode(+ve)

more electropositive metal

in ECS

Less electropositive metal in

ECSRelease electrons Accept electrons

Electrons are flow through the external circuit towards the

copper plate.



- Daniell cell is a voltaic cell.

- Electrodes - Zinc (anode) and copper (cathode)

- The two solutions are separated by the porous pot/salt bridge-A salt bridge is made from a filter paper soaked in a saturated solution

that will not react with the two electrolytes.

-The common electrolyte used are or a solution of sodium or potassium

salts.

Daniell cell using a porous

pot

Daniell cell using a salt

bridge



Functions of

salt bridge or porous pot :

(a) To prevents the two

electrolytes from mixing.

(b) To allow the flow of ions so

that the electric circuit is

completed.



Mechanism of Daniell Cell

Anode Cathode

Electrode becomes

thinnerElectrode becomes thicker

Metal releases electronsZn Zn2+ + 2e- Metals accept electronsCu2+ + 2e- Cu

The concentration of copper(II) sulphate solution

decreases. The blue colour of the solution becomes

paler.

The overall ionic equation in Daniell cell is:

Zn + Cu2+ Zn2+ + Cu



Various types

of

voltaic cell

Lead acid

Accumulator Dry cell Alkaline cellNickel-cadmium

cell

•Portable

•Cheap

•Stable voltage

of 1.5 v

Mercury cell

Portable

•Portable

•Voltage of

1.5 V

•More long

lasting

•Portable

•Rechargeable

up to hundreds

of times

•Rechargeable

•Can be made

to have

higher voltage

Advantages

•Heavy

•Expensive

•

Spilled easily

• Not

rechargeable

• Not rechargeable

•Produces voltage

of 1.3 V only

•Poisonous

• Not

rechargeable

•May leak

•Produce low

voltage of 1.25

•Expensive

•

Heavy

Disadvantages



SIMILARITIES BETWEEN AN ELECTROLYTIC CELL AND AVOLTAIC CELL

• Consists of an electrolyte, an anode and acathode

• Electrons flow from the anode to the cathode

in the external circuit



DIFFERENCES BETWEEN AN ELECTROLYTIC CELL AND A VOLTAIC CELL:Type of cell

onversion of

energy

Flow of

electrons

Negative

terminal

Positive

terminal

Types of

electrodes

Voltaic cell

Chemical energy

electrical energy

From positive terminal

to negative terminal

From negative terminal

to positive terminal

Cathode:

Accepts electrons

Anode:

Release of electrons

Anode :Releases electrons

Cathode:Accepts electrons

Two similar

(usually carbon) or

different metals

Two different metals

Electrolytic cell

Electrical energy Chemical energy



The Electrochemical Series ~

An arrangement of metals based on the abilityof each metal atom to release electrons.

• The higher the position of a metal in theelectrochemical series, the greater the ability of

the metal atoms to release electrons, and themore electropositive is the metal.



1) The potential difference between two

metals in a voltaic cell.

2) The ability of a metal to displace another

metal from its salt solution.

THE CONSTRUCTION OF THE ELECTROCHEMICAL

SERIES IS BASED ON :



1) THE POTENTIAL DIFFERENCE BETWEEN TWO

METALS

• In a voltaic cell, two different metals

are used to create a potential difference,

shown by the reading of the voltmeter.

• Thefurther apart

the two metals are in

the electrochemical series, thegreater

the voltage produced by the cell.



•The metal which is

higher

in theelectrochemical series will be the

negative

terminal.

• The metal which is lower in the electrochemical

series will be the positive terminal

• Electrons flow from negative to positive metal

through the external circuit.



Magnesium strip

Copper strip

V

Copper(II) sulphate,CuSO4 solution

Voltmeter

Electron flow

Mg Mg2+ + 2eCu2+ + 2e Cu

Overall ionic equation : Mg + Cu2+ Cu + Mg2+

Showing the flow

of electrons



2) The ability of a metal to displace another

metal from its salt solution

• A metal which is higher in theelectrochemical series is able to displacea metal below it in the

electrochemical series from its saltsolution.

• Example :

Zinc can displace copper from copper(II)sulphate solution.

Zn + CuSO4 Cu + ZnSO4



The importance of the electrochemical

series

1. To determine the terminals of voltaic cells

2. To compare the standard voltage of the

voltage cell3. To predict the ability of metal to displace

another metal from its salt solution

Example :



Example :

Figure below shows the cell voltages of a few voltaic cells using different metals

as electrodes.

Predict the negative terminal, positive terminal and cell voltage of the following

voltaic cells.

Voltaic cells Negative terminal Positive terminal Cell voltage/V

Mg/Ag

Mg/Zn

Zn/Pb

Magnesium

Magnesium

Zinc

Zinc

Lead

Silver 3.0 + 0.5 = 3.5

3.0 – 1.1 = 1.9

1.1 – 0.4 = 0.7



The importance and effects of

electrochemical industries

• Theimportance

of electrochemicalindustries in our daily life is as follows:

Oxygen gas, chlorine gas can be produced byelectrolysis.

Electroplating are widely used to preventcorrosion and enhance the appearance of thematerials.

The industrial manufacture of batteries produces various types of cells and batteriesin different shapes and sizes.



• The pollution caused by theelectrochemical industries are as follows:

The electroplating industries producepollutants such as poisonous heavy metalions and poisonous cyanide ions.

The use of mercury electrodes inelectrolysis can cause air and water pollution.

• Thus, the waste should be treatedand disposed of in a safe andorderly manner.

Chemistry Chapter 6 Form 4

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Transcript of Chemistry Chapter 6 Form 4