Chemistry Form 5 Lesson 18

7/28/2019 Chemistry Form 5 Lesson 18

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CHEMISTRY FORM 5

LESSON 18


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From the experiment above, it can be summarised that, in adisplacement of halogen, a electronegative halogendisplaced a . electronegative halide ion.

The halide ions of a electronegative halogen act as ...agent. They lose their electrons and are .. to form halogen

molecules. The electrons are accepted by the . electronegative halogen

which act as .. agent. This will result the halogen to

more

less

less reducing

readily

more

oxidising

.

In short, theres an electron transfer from a halide ions of ..electronegative halogen to a . electronegative halogen

13.5 Transfer of electrons at a distance

There are a few chemical cells that is usually used to study thepotential of the cell. Through the cell, the flow of electron can bededuced using a galvanometer

less

more


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Transfer of electron (1)

B

G

A

abridge

Metal ion

solution, A+Metal ion

solution, B+


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B

G

Porouspot A

Metal ion

solution, A+ Metal ionsolution, B+


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G


Metal ion Metal ion

Dilute

sulphuricacid

so ut on,+

solution, B+


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For all chemical cell above

The electrode which release electrons are .. terminal(.) agent release electrons, thus thesubstance is

The electrode which receive electrons are . terminal

(.) agent receive electrons, thus thesubstance is

Salt bridge ; porous pot ; sulphuric acid all have the same

negative

anode Reducingoxidised

positive

cathode Oxidisingreduced

, ..

to separate the 2 electrolytes used


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To study the transfer of electron at a distance.

G

Sulphuric acid

Iron (II) sulphateAcidified potassiummanganate (VII)

Graphite electrode


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- Galvanometer point to the direction

of acidified potassium manganate(VII) arm.

- Green colour of iron (II) sulphate

turned to brown

- Purple colour of acidified potassium

manganate (VII) decolourised- When added FeSO4, KSCN turned

solution to red blood colour.

- Electron transfer from

Fe2+ to MnO4-

- Iron (II) ion is oxidised

to iron (III) ion.

- MnO4- is reduced to

Mn2+- Iron (III) ion confirmed

presence.

-

of acidified potassium dichromate(VI) arm.

- Colourless solution of KI turned to

brown

- Orange colour of acidified

potassium dichromate (VI) turnedgreen

- When starch solution is added to KI

solution, the solution turn from

brown to dark blue

-

I- to Cr2O72-

- Iodide ion is oxidised

to iodine solution

- Cr2O72- is reduced to

Cr3+

- Iodine confirmed

presence


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Fe2+

Fe3+ + e-

8 H+ + MnO4- +

5 e-

Mn2+ + 4 H2O

8 H+ +

MnO4- +

5 Fe

2+

Mn2+ +

4 H2O +

5 Fe3+

Acidified

potassium

manganate

(VII)

Iron (II) ion

14 H+ +

2 I I2 + 2 e-

14 H+

+ Cr2O72-

+ 6 e-

2 Cr3+ +

7 H2O

Cr2O72-

+6 I-

2 Cr3+ +

7 H2O +

3 I2

c e

potassium

dichromate

(VI)

Iodide ion

To test the presence of iron (III) ion, Fe3+

To test the presence of iodine, I2


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Nature of the product of corrosion

Some oxide of metal surprisingly gives extra protection to

the metal. For example, aluminium. Natural aluminium is

always coated by a layer of oxide which is highly resistant

to reaction and corrosion. So the layer prevented

aluminium from corroded, despite its highly electropositivemetal

In the other hand, some oxide of metal is easily

.

example, iron. The corrosion in iron allows more ironmetal to corrode underneath it. This phenomenon is also

known as

2 main important components for rusting are.. and

A possible mechanism for rusting, consistent with the known

facts, is illustrated in Figure below

rusting

Oxygen (air) water


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Fe (s) Fe2+ (aq) + 2e-

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

2 Fe (s) + O2 (aq) + 2 H2O 2 Fe2+ + 4 OH [or 2 Fe(OH)2]

4 Fe(OH)2 (aq) + O2(aq) + 2H2O(l) 4 Fe(OH)3 (aq)

Forming rust : 2 Fe(OH)3 (s) Fe2O3.x H2O + (3 x) H2O


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3. Prevention of rusting

Various methods are used to prevent / slowing down rusting.

Methods Explanation

Alloying

Iron is alloyed with nickel and chromium to form .

The chromium forms an impervious oxide layer on the surface of

iron increasing its resistance to corrosion. Chromium at the same

time .. the steel

Alloy of Iron

decorated / coated

Barrier

Use grease / oil to coat the moving parts of machine

Coating ironwith chromium (plating) or zinc (galvanising)

Sacrificial

Also known as protection

Metal with a . position in electrochemical series is

connected to iron. Under such way, ... Will be

oxidised first before iron.

anodic

higher

reactive metal


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To study the effect of the contact of other metals on rusting of iron

When a more electropositive metal is in contact with iron, the

metal inhibits rusting whereas a less electropositive metal is in

contact with iron speed up rusting.

Type of metal in contact with iron nail

Rusting of iron

Iron nails / temperature / medium in which irons kept


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If theres presence of Fe2+ Show blue colouration (due to

presence of Fe2+

If there presence of OH- Show pink colouration (due to

presence of OH-)

Surface of nail was artiall covered

Low

one

by reddish-brown solid

None High Surface of nail remain unchanged

None High Surface of nail remain unchanged

Moderate LowSurface of nail was almost covered


High LowSurface of nail was heavily covered



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Magnesium and zinc

Copper and tin

Magnesium and zinc are more electropositive than iron, so they are more

readily to lose electron, while copper and tin are less electropositive than

iron, so iron is more readily to lose electrons.

The less electropositive the metal that iron is in contact with, the faster

the rusting of iron occur

To detect the presence of iron (II) ions, Fe2+

To detect the presence of hydroxide ion, OH

As a control in this experiment.

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Chemistry Form 5 Lesson 18