Redox Equilibrium Handbook Part 2

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LABORATORY ACTIVITY: ELECTROPLATING OF METALS BY ELECTROLYSIS

Aim To study the electroplating of an iron spoon with copper

Materials 1.0 mol dm−3 of copper(II) sulphate, CuSO4 solution, iron spoon, copper plate and
sandpaper.

Apparatus Beaker, battery, connecting wires with crocodile clips, ammeter, switch, and
rheostat.

Apparatus Set-
Up

Procedures 1. Clean the copper plate and iron spoon with sandpaper.
Observation
2. Pour 1.0 mol dm−3 of copper(II) sulphate, CuSO4 solution into a beaker until
half full.

3. Connect the iron spoon to the negative terminal of the battery and the
copper plate to the positive terminal of the battery using the connecting
wires.

4. Dip the iron spoon and copper plate into the 1.0 mol dm−3 of copper(II)
sulphate, CuSO4 solution as shown in Figure 1.33.

5. Turn on the switch and adjust the current flow to 0.2 A using the rheostat.

6. Turn off the switch after 30 minutes.

7. Take out the iron spoon from the electrolyte and leave to dry.

8. Record your observation and complete the table below.

Electrode Observation Inference Half equation
Cu → Cu2+ + 2e−
Anode Copper metal Copper atom
(Copper becomes smaller / ionises/oxidises and Cu2+ + 2e− → Cu
metal) become copper(II) ion
thinner
Cathode Copper(II) ion is
(Iron Thin layer of brown reduced and become
spoon) solid is deposited on
copper atom.
the iron spoon

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Iron spoon is
electroplated with

copper.

Discussion 1. Based on the half equations that you wrote for the reactions at the anode
and cathode:

(a) is electroplating a redox reaction?

Yes

(b) explain your answer in 1(a) in terms of electron transfer.

Electroplating is a redox reaction because oxidation and reduction process
occurs simultaneously during the process.

At anode, copper electrode is oxidised by releasing 2 electrons and
become copper(II) ion.

At cathode, copper(II) ion is reduced by receiving two electrons and
become copper atom.

2. Is there any colour change to the copper(II) sulphate, CuSO4 solution?

Explain your answer.

Blue colour of copper(II) sulphate solution remains unchanged because
concentration of Cu2+ ion remain unchanged.

The rate of ionisation (oxidation) of copper, Cu at the anode is the same as
the rate of discharged (reduction) copper(II) ions, Cu2+ at the cathode.

3. A good electroplating occurs if the layer of the plating on the metal is
uniform and long lasting. Suggest two ways to achieve good electroplating.

➢ Rotate iron spoon during the electroplating process to get uniform
layer of the plating metal.

➢ Use small current during the electroplating process to get strong
coating layer on the iron spoon.

4. Draw a labelled diagram of the set-up of apparatus for an experiment to
electroplate an iron ring with nickel metal.

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Complete the table below on electroplating metals.

Apparatus set-up Electroplating an iron spoon with silver Electroplating a key with copper metal
metal

Substance as Silver nitrate solution Copper(II) nitrate solution
electrolyte
Silver metal Copper metal
Substance act as
anode Ag → Ag+ + e- Cu → Cu2+ + 2e-
Silver metal becomes thinner and Copper metal becomes thinner and
Half equation at
anode and the smaller smaller
observation
Iron spoon Key
Substance act as
cathode Ag+ + e- → Ag Cu2+ + 2e- → Cu
A thin layer of grey solid is deposited on A thin layer of brown solid is deposited
Half equation at
cathode and the the iron spoon on the metal key

observation Ag + Ag+ → Ag+ + Ag Cu + Cu2+ → Cu2+ + Cu

Ionic equation

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PURIFICATIONS OF METALS

To determine whether a copper metal is pure, one must conduct the purification of metals
through electrolysis.
In the purification of copper, the impure copper is made to be the anode while the cathode is
a thin layer of pure copper.

Substance as electrolyte Copper(II) nitrate / Copper(II) sulphate
Substance act as anode
Half equation at anode Impure copper
and the observation
Impure copper (anode) becomes thinner / smaller.
Substance act as cathode Impurities will be settled at the bottom of the beaker at
Half equation at cathode the anode as anode sludge/sediment.
and the observation Cu → Cu2+ + 2e−

Ionic equation Pure copper
Colour changes of the
electrolyte Cu2+ + 2e- → Cu
Brown solid is deposited, and the copper cathode
becomes thicker.

Cu + Cu2+ → Cu2+ + Cu

Blue colour of copper(II) sulphate solution remains
unchanged because concentration of Cu2+ ion remain
unchanged.
The rate of ionisation (oxidation) of copper, Cu at the
anode is the same as the rate of discharged (reduction)
copper(II) ions, Cu2+ at the cathode.

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LABORATORY ACTIVITY: PURIFICATIONS OF METALS BY ELECTROLYSIS

Aim To study the purification of copper by electrolysis.

Materials 1.0 mol dm−3 of copper(II) nitrate, Cu(NO3)2 solution, impure copper plate and pure
copper plate.

Apparatus Beaker, battery, connecting wires with crocodile clips, ammeter and switch.

Apparatus Set-
Up

Procedures 1. Pour 1.0 mol dm−3 of copper(II) nitrate, Cu(NO3)2 solution into a beaker
Observation until half full.

2. Connect the pure copper plate to the negative terminal of the battery and
the impure copper plate to the positive terminal of the battery as shown in
Figure 1.34.

3. Turn on the switch and let the electricity flow for 30 minutes.

4. Record the changes at the anode and cathode in the following table and
complete the table.

Electrode Observation Inference Half equation
Cu → Cu2+ + 2e−
Anode Impure copper Copper atom
(Impure (anode) becomes ionises/oxidises and
copper) thinner / smaller. become copper(II) ion

Cathode Brown solid is Copper(II) ion is Cu2+ + 2e− → Cu
(Pure deposited, and the reduced and become

copper) copper cathode copper atom.
becomes thicker.

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1. Is purification process of copper a redox reaction? Explain your answer.

Yes, purification is a redox reaction because oxidation and reduction
process occur simultaneously during the process.

At anode, impure copper electrode is oxidised by releasing 2 electrons and
become copper(II) ion.

At cathode, copper(II) ion is reduced by receiving two electrons and
become copper atom.

2. Is there any colour change to the copper(II) nitrate, Cu(NO3)2 solution?
Explain your answer.

Blue colour of copper(II) sulphate solution remains unchanged because
concentration of Cu2+ ion remain unchanged.

The rate of ionisation (oxidation) of copper, Cu at the anode is the same as
the rate of discharged (reduction) copper(II) ions, Cu2+ at the cathode.

3. Impurities only form under the anode and not under the cathode. Give a
reason.

Anode is impure copper that contain impurities. When the impure copper
ionises and dissolve, the impurities form under the anode. Cathode is pure
copper that does not has impurities.

4. Write the conclusion for this experiment.

Purification of copper metal is carried out through electrolysis process

EXTRACTION OF METAL FROM ITS ORE

Metals usually exist as compounds or mixed with other substances, such as rocks and dirt.
Compounds containing metals are also known as ore or mineral and exist as metal oxides
metal sulphides or metal carbonate.

Non-reactive metals such as gold and silver do not need to be extracted because they exist as
metal elements.

Reactive metals like iron and aluminium require a particular way to be extracted from their
respective ores.

The methods for extracting reactive metals are based on the position of the metal in the
reactivity series of metal. Two of the most common ways to extract metals from their
respective ores are:

➢ Electrolysis : For metals that are more reactive than carbon

➢ Reduction by carbon : For metals that are less reactive than carbon.

EXTRACTION OF ALUMINIUM FROM ITS ORE, ALUMINIUM OXIDE
THROUGH ELECTROLYSIS.

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Metal to be extracted Aluminium metal
Purpose of process
To extract aluminium metals from their ores, bauxite (aluminium
oxide).

In the extraction of aluminium, Al, aluminium ore, or bauxite is firstly
purified to obtain aluminium oxide, Al2O3, which is then melted down
to enable the process of molten electrolysis to be conducted.

Al2O3 2Al3+ + 3O2-

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Melting point of Al2O3 2000 °C.

Function of cryolite, Na3AlF6 to lower the melting point of aluminium oxide

Name of electrolyte Molten aluminum oxide

Name of ions present Aluminium ion and oxide ion

Substance act as anode Blocks of carbon

Reaction occurs at anode Oxide ion, O2− donates electrons and undergoes the oxidation reaction,
and forms oxygen molecule, O2.

Half equation at anode 2O2- → O2 + 4e

Observation at anode Bubbles of colourless gas released

Substance act as cathode Steel tank lined with carbon

Aluminium ion, Al3+ is reduced to aluminium atom, Al by receiving
Reaction occurs at cathode

electrons and forms molten aluminium.

Half equation at cathode Al3+ + 3e− → Al

Observation at cathode Grey molten liquid deposited at the bottom of the tank

Overall half equation 6O2- + 4Al3+ → 3O2 + 4Al

Molten aluminium sinks to the bottom of the layer because its density
How is the metal collected?

is higher, and then is drained out through a designated channel.

Effect from the extraction 1. The entire process of aluminium extraction consumes a huge
of aluminium from its ore amount of electricity.

2. During the process of electrolysis for molten aluminium oxide,
Al2O3, oxygen gas produced reacts with carbon rod and form
carbon dioxide which negatively affects the environment.

3. The purification process of bauxite also creates a residue in a
form of red sludge that is toxic.

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EXTRACTION OF SODIUM FROM SODIUM CHLORIDE THROUGH
ELECTROLYSIS.

Metal to be extracted Sodium metal

Purpose of process To extract sodium metals from sodium chloride
NaCl → Na+ + Cl-

Substance act as anode Carbon

Substance act as cathode Ferum / iron

Name of electrolyte Molten sodium chloride

Name of ions present Sodium ion and chloride ion

Half equation at anode 2Cl- → Cl2 + 2e-

Observation at anode Greenish-yellow bubbles are released

Half equation at cathode Na+ + e- → Na

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Observation at cathode Grey solid formed at the surface and channel out
Overall half equation 2Na+ + 2Cl- → 2Na + Cl2
CaCl2 added at molten NaCl to lower the melting point of NaCl
Others

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EXTRACTION OF METAL FROM ITS ORE THROUGH THE PROCESS OF
CARBON REDUCTION

Iron metal, which is less reactive than carbon, C can be extracted by carbon, C reduction
process.

This process is carried out in a blast furnace by heating up iron ore or hematite (Fe2O3) with

coke, C and limestone, CaCO3.

Iron ore, coke and calcium carbonate, CaCO3

Waste gas Waste gas
carbonate, carbonate,

CaCO3 CaCO3

Hot air Hot air
carbonate,
carbonate,
CaCSOla3g
CaCO3 carbonate,
Molten iron
CaCO3
carbonate,
Blast furnace.
CaCO3

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Zone Reaction Explanation
Zone 1
Zone 2 Coke, C reacts with oxygen, O2 from the Type of reaction : redox exothermic
hot air to form carbon dioxide, CO2. Heat released to the surrounding causing
Zone 3 Equation: temperature in the blast furnace will
C(s) + O2(g) → CO2(g) increase and until 1600 ⁰C.

Carbon dioxide, CO2 reacts with the Type of reaction : endothermic reaction
remaining coal. Heat is absorbed from the surrounding.
Equation: Temperature lower.
C(s) + CO2(g) → 2CO(g) Reducing agent: Carbon monoxide, CO

Coke, C and carbon monoxide, CO Iron(III) oxide, Fe2O3 is reduced by carbon
function as reducing agents and reduce monoxide, CO.
iron(III) oxide, Fe2O3 or iron ore to molten
iron. Iron deposits are formed at the bottom of
the blast furnace.
Equation:
2Fe2O3(s) + 3C(s) → 4Fe(l) + 3CO2(g)
Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)

Calcium carbonate, CaCO3 decomposes to Calcium oxide, CaO is a base and can
form calcium oxide, CaO (quicklime) and neutralise the acidic silicone(IV) oxide,
carbon dioxide, CO2 at high temperature. SiO2.

Equation: This reaction is important to separate the
CaCO3(s) → CaO(s) + CO2(g) impurities from molten iron.
Impurities in the iron ore like silicon(IV)
oxide, SiO2 react with calcium oxide to The difference in density causes the slag
form slag or calcium silicate, CaSiO3. to be at the top layer of the molten iron,
which makes the separation process
Equation: easier
CaO(s) + SiO2(s) → CaSiO3(l)

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EXTRACTION USING A MORE REACTIVE METAL

Metal Ways to extraction

K

Very reactive metals.

Na The best way to extract the metal

from its ore is by electrolysis

Mg

Al

C

Moderately reactive metals.

Zn The best way to extract the metal

from its ore is reduction by

Fe carbon.

Sn

Pb

Cu Less reactive metal.

Metal is extracted from its ore by

Hg direct heating in air

Ag Unreactive metal.

Exists as an element.

Au

The more reactive metal can extract the less reactive metal from its metal oxide when metals
are heated together.

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Example 1:
Thermite reaction : Reaction of Aluminium power with iron(III) oxide, Fe2O3.
For example, in a thermite reaction, aluminium powder, Al is heated together with iron(III)
oxide powder, Fe2O3.

Equation Fe2O3 + 2Al → Al2O3 + 2Fe
Explanation
Aluminium is more reactive than iron.
Aluminium can reduce iron(III) oxide, Fe2O3 to produce molten iron, Fe.
Aluminium is oxidised to aluminium oxide and act as the reducing
agent.
Iron(III) oxide is reduced to iron and act as the oxidising agent.

Example 2:
The redox reaction between zinc and lead(II) oxide

Equation Zn + PbO → ZnO + Pb

Explanation Zinc is more reactive than lead.
Zinc can reduce lead(II) oxide, PbO to produce molten lead, Pb.
Zinc is oxidised to zinc oxide and act as the reducing agent.
Lead(II) oxide is reduced to lead and act as the oxidising agent.

Example 3:
Reaction between iron and aluminium oxide

Equation Reaction does not occur.

Explanation Iron is less reactive than aluminium.
So iron is not able to reduce aluminium oxide to aluminium.

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