chemistry form 5 lesson 18
TRANSCRIPT
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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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Transfer of electron (2)
B
G
Porouspot A
Metal ion
solution, A+ Metal ionsolution, B+
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G
Transfer of electron (3)
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.
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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
by reddish-brown solid
High LowSurface of nail was heavily covered
by reddish-brown solid
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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.