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After completing this lesson you should be able to :
• The dichromate and permanganate ions are strong
oxidising agents in acidic solutions. Hydrogen peroxide is
an example of a molecule which is an oxidising agent.
• Carbon monoxide is an example of a gas that can be used
as a reducing agent.
Remember
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Something is oxidised if
• It gains oxygen
• It loses electrons (O.I.L.)
• Its oxidation state increases (advanced higher)
Something is reduced if
• It loses oxygen
• It gains electrons (R.I.G.)
• Its oxidation state decreases (advanced higher)
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Molecules and group ions can act as
oxidising and reducing agents
• Compounds can also act as oxidising or reducing
agents.
• The electrochemical series contain a number of
ions and molecules.
•
Oxidising and reducing agents can be selected
using an electrochemical series.
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Iron ore, coke
(carbon) and
limestone enter
the furnace from
the top of the
furnace.
Iron is extracted from its ore using carbon. The
reaction requires a lot of heat. To provide the
necessary heat the reaction is carried out in a
BLAST FURNACE.
The extraction of the iron happens in 3 stages.
STAGE 1: The coke (carbon) burns to produce
carbon dioxide CO2.
C(s) + O2(g)
CO2(g)
STAGE 2: The CO2 from stage 1 reacts with more
carbon to form carbon monoxide CO.
CO2(g) + C(s)
Molten iron
flows to the
bottom of the
furnace.
2CO(g)
STAGE 3
STAGE 2
STAGE 1
STAGE 3: The CO from stage 2
reacts with iron(III) oxide Fe2O3 in
the iron ore removing the oxygen.
Fe2O3(s) + 3CO(s)
2Fe(l) + 3CO2(g)
Fe(l)
+
+
(
The
each
reduction.
)2(
)3+
carbon
3C
3C
heat
heat
heat
is changed to an
has to
+
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+ carbon
2
+
3C
2
+
3C
. To do this
. This change is
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AND
The terms
and
originates from the extraction of
metals industry. These terms were used for centuries before atoms and
electrons were discovered.
An
An
reaction is a reaction where a reactant
.
reaction is a reaction where a reactant
.
The extraction of iron from
these original definitions of
+
+
The
been
.
in the
and
heat
carbon
heat
3C
has
The carbon
has
. It has been
.
+
2
carbon
+
to produce
.
an
shows
3C
. It has
to produce carbon
1. Permaganate MnO4- is a strong
oxidising agent
Oxidising agents such as potassium permaganate (KMnO4) are
used in explosives and fire works
MnO4-(aq) + 8H+ +5e- → Mn2+(aq) + 4H2O(l)
Mn(VII)
Mn(II)
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When pure glycerol is poured onto a pile of potassium permanganate,
white smoke is produced in the beginning. Oxidation of glycerol with
potassium permanganate is an exothermic process, heat released in the
reaction being enough to cause glycerol to evaporates. As the reaction
proceeds, more and more heat is released in the reaction and glycerol
is eventually ignited. Because of presence of potassium ions (from
potassium permanganate), the flame is violet in colour. Although there
are lot of reaction pathways and lot of products formed during the
combustion, the following equation shows the main process in which
glycerol is completely oxidized to carbon(IV) oxide.
The black residue after the reaction is mostly a mixture of
manganese(III) oxide and/or manganese(IV) oxide and potassium
carbonate.
Redox chemistry
At advanced level, the redox nature of the reaction can
be explored. Do this by allowing the residue to cool down
and then dissolving it in water. This produces a green
solution suggesting the presence of a Mn(Vl) species, as
well as a brown solid, manganese(lV) oxide. This confirms
the reduction of the manganate(Vll) ion; the glycerol has
been oxidised to water (hence the steam) and carbon
dioxide.
The most stable is +2,
which is a pale pink color in
aqueous solutions. Also
important is
+4, brown/black, which is
found in manganese
dioxide; and +7 found in
the purple permanganate
anion MnO4–. Manganese's
+6 oxidation state is green.
2. Dichromate and permanganate in acidic
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solutions are strong oxidising agents.
Cr2O72-(aq) + 14H+(aq) + 6e- → 2Cr3+(aq) + 7H2O(l)
Orange
green
• Oxidation of alcohols –revisited
• REDOX GUIDE
MnO4-(aq) + 8H+ + 5e- → Mn2+(aq) + 4H2O(l)
Purple
colourless
Both half equations contain H+ ions showing they
are strong oxidising agents in the presence of acid
Demonstration
The action of CuO as an oxidising agent for alcohols can be
shown by placing a small amount of ethanol in an evaporating
basin. A 2p coin is then heated to just below red heat in a
Bunsen and then wafted in the air allowing the surface to
become coated in black copper(II) oxide. The warm coin is
then carefully lowered into the ethanol. Instantly the
bright shiny surface of the coin is restored as the CuO is
reduced to copper. The process of heating the coin —
forming the oxide — and reducing the oxide in alcohol is
repeated until little liquid is left in the dish. At this point,
if pH indicator is added, it shows that an acid has formed
whilst the distinctive smell of ethanal can be detected.
Unit 2 re-visited
3. An Oxidation and Reduction
reaction – Blue Bottle experiment
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Glucose (an aldohexose) in an alkaline solution is slowly oxidised by oxygen,
forming gluconic acid:
CH2OH-(CHOH)4-CHO + ½O2 CH2OH-(CHOH)4-CO2H
In the presence of sodium hydroxide, gluconic acid is converted to sodium
gluconate. Methylene blue speeds up the reaction by acting as an oxygen
transfer agent. As glucose is oxidised by the dissolved oxygen, methylene
blue itself is reduced, forming the colourless methylene white, and the blue
colour of the solution disappears.
What to do
1. Put some water in the conical flask. Put in the stopper. Shake
vigorously to check for leaks. If there are none, pour the
water away and proceed.
2. Put 100 cm3 of potassium hydroxide solution into a conical
flask.
3. Add 3.3 g dextrose.
4. Add 3–4 drops of methylene blue indicator.
5. Put a stopper on the flask.
6. Shake vigorously.
7. When the solution clears, repeat the process.
8. It is necessary periodically to remove the stopper.
Methylene blue is reduced by the alkaline dextrose solution
to produce a colourless solution.
When the solution is shaken, it is oxidised by the oxygen in
the flask to produce the blue dye.
The stopper must be taken off to allow more oxygen in.
4. Potassium chlorate is a strong
oxidising agent
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Potassium chlorate is a very strong oxidizing agent that oxidizes the sugar in the
jelly baby almost instantaneously. When the experiment is performed, the jelly
baby bursts into giant flames and a screaming sound can be heard as rapidly
expanding gases are emitted from the test tube in which the experiment is
performed. The smell of candy floss is also given off.
2KClO3  2KCl(s) + 3 O2
C12H22O11(s) + 12O2(g)  12CO2(g) + 11H2O(g)
5. Hydrogen peroxide is a strong
oxidising agent
Oxidising agents such as hydrogen peroxide are used in many
bleaching products
H2O2(aq) + 2H+(aq) + 2e- → 2H2O(l)
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Elephant Toothpaste Chemistry
The overall equation for this reaction
is:
2 H2O2(aq)  2 H2O(l) + O2(g)
The catalyst is potassium iodide. Some of the iodide ions are
converted to I2 molecules by the hydrogen peroxide
H2O2(aq) + 2H+(aq) + 2e- → 2H2O(l)
2I-(aq) → I2(s) + 2e-
H2O2(aq) + 2H+(aq) + 2I-(aq) → I2(s) + 2H2O(l)
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