BRIGHT LAND HIGH SCHOOL NAMAGOMA Oxidizing agents

An oxidizing agent is a substance which adds oxygen to another substance. Or It is a substance

SENIOR 4 CHEMISTRY
which removes hydrogen from another substance

SUMMARY NOTES Examples of oxidizing agents include oxygen (O2), concentrated sulphuric acid (H2SO4),
potassium permanganate (KMnO4) manganese (IV) oxide (MnO2), nitric acid (HNO3), potassium
1. REDOX REACTIONS
dichromate VII (K2CrO4), chlorine (Cl2) etc.…
A redox reaction is a reaction in which reduction and oxidation occur at the same time.
Therefore, in such a reaction, one substance is reduced and another one oxidized. Reducing agents

Oxidation A reducing agent is a substance which adds hydrogen to another substance.

 Oxidation is the addition of oxygen to a substance
Or it is a substance which removes oxygen from another substance.
 Oxidation is the removal of hydrogen from a substance
Examples of reducing agents include Hydrogen (H2), carbon (C), carbon monoxide (CO),
 Oxidation is the loss of electrons by a substance
hydrogen sulphide (H2S), sulphur dioxide (SO2) and ammonia (NH3).

Nugget: Use OIL RIG to remember

OIL: Oxidation is the loss (of electrons)

RIG: Reduction is the gain (of electrons)

Calculating oxidation number of an element in a compound

Example

Calculate the oxidation state of Mn in KMnO4-

Reduction
Step 1: Let the oxidation state of Mn be x
 Reduction is the removal of oxygen from a substance
Step 2: Add up the total oxidation state of all the other element and equate it to the total charge of
 Reduction is the addition of hydrogen to a substance
the compound. In this case the chare is -1. The charge that remains in their ionic state of other
 Reduction is the gain of electrons by a substance
elements the oxidation number of the other element.

x + 1+(-2x4) = -1

x = +6

Note: Oxidation increases the oxidation number (becomes more positive), reduction decreases
the oxidation number (becomes more negative) since the electrons gained are negatively charged.
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 Revenue generation through taxes levied on mining companies and exporting of the
extracted minerals eg gold.
Relevance of oxidation and reduction in extraction of metals
 Foreign investors are attracted who bring in foreign exchange and invest in different
Majority of the ores occur in their oxidized state as oxides e.g. hematite, iron III oxide Fe2O3, to mining and processing companies boosting economic growth and infrastructural
extract iron Fe from hematite, the ore is reduced removing oxygen from it. Two common development.
reduction processes are used. Chemical reduction or electrolytic reduction using electrolysis.  Availability of metals boosts the development of the local industries as it provides raw

Chemical reduction of hematite using caron as the reducing agent material rather than importing them e.g. steel production
 Growth and development of rural areas where theres mining activities eg roads,
Fe2O3 + 3C 2Fe + 3CO
electricity
The more reactive metals such as Aluminum are extracted by electrolysis. Aluminum ore bauxite
Challenges due to metal extraction
is first converted to aluminum oxide and then electrolysis is used to produce pure aluminum.
 Environmental degradation eg deforestation, soil erosion and water pollution from
Al3+ + 3e- Al
mining industries.
Ores mined in Uganda  Illegal and unregulated mining leading to loss of government revenue.
 Unsafe working conditions of miners leading to diseases.
An ore is a naturally occurring rock or mineral from which a metal is extracted.
 Lack of value addition especially industries leading to low revenue from the industry.
Ores that exist in Uganda include:  Fluctuating global prices which affects revenue
 Iron ore (Hematite and magnetite) extracted in Kabale district, Kisoro district and Tororo  Poor infrastructure such as poor roads affecting transportation of the ores
district.
Writing ionic equations
 Copper ore mined in Kilembe mines, Kasese district
Redox reactions involve the transfer of electrons from one specie, the reducing agent ti another
 Tin ore mined in Kabale, Kisoro and Rukungiri district
specie, the oxidizing agent. When writing ionic equations its essential to break down the reaction
 Cobalt ore mined in Kilembe mines, Kasese district.
into half equations, one for oxidation and one for reduction.
 Wolfram ore from which Tungsten is extracted, this is mined in Kabale and Kisoro
district Example: Zinc reacting with copper(II) sulfate solution
 Nickel ore mined in Kabale district.

Other minerals include; gold mined in Mubende, Karamoja, Bushenyi and Busia, Salt mined
in lake Katwe Kasese district

Contribution of metal extraction on Ugandas economy.

 Employment creation for Ugandans through different industries e.g. miners in the mines,
transportation, equipment supplies and processing.

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 NB: An electrode must be a good conductor of electricity and should not react with the
electrolyte.

A simple arrangement during electrolysis

ELECTROLYSIS

Electrolysis is the decomposition of an electrolyte in aqueous solution or molten state by passing

an electric current through it.

An electrolyte is an ionic compound which conducts an electric current in aqueous solution or in

molten state and is decomposed by it.

Electrolytes are composed of ions. In the solid state, the ions are rigidly held in regular positions Ions

and are not able to move freely. Melting the solid breaks the forces between the ions and An ion is a charged particle. Types of ions include:
therefore the ions are free to move in a molten electrolyte. Dissolving a solid in water or any
Cation: This is a positively charged ion that will move to the cathode during electrolysis e.g. all
other polar solvent, causes the breakdown of the lattice setting the ions free in aqueous state.
metallic ions e.g. Na+, NH4+, H+, Cu2+, Pb2+ etc.
Electrodes
Anion: This is a negatively charged ion that moves to the anode during electrolysis e.g. all non-
These are poles of carbon (graphite) or metal where current enters and leaves the electrolyte. The metal ions and radicals e.g. Cl-, SO42-, OH-, NO3-, Br- etc.
types of electrodes include;
Theory of electrolysis (Ionic theory)
Cathode: This is a negative electrode at which electrons enter the electrolyte or leave the
This states that electrolytes consist of ions which are positively and negatively charged particles
external circuit.
that move to different electrodes during electrolysis.
Anode: This is the positive electrode at which the electrons leave/the electrolyte or enter the
In ionic compounds, these charges are held together by electrostatic forces but in solution or
external circuit.
molten state, these ions are free to move. The positive ions move to the cathode and the negative
to the anode.

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What happens to anions at the anode? Electrolysis of dilute sulphuric acid in an apparatus called a voltammeter

When an electric current is applied to the electrolyte, the negatively charged ions called anions
move to the positively charged electrode called the anode. Once there, they lose electrons to
become atoms and are said to be discharged i.e. 2Xn-(aq or l) → X2 (g) + ne

What happens to cations at the cathode?

The positively charged ions called cations move to the negatively charged electrode called
cathode where they gain electrons and become atoms which are then said to be discharged i.e.
Mn+(aq or l) + ne→ M (s)

Electrolysis of dilute sulphuric acid

This is commonly called electrolysis of water. Electrolysis of copper(II) sulphate solution (using copper electrodes – active electrodes)

Ions present:

From sulphuric acid are H+ and SO42-

From water are H+ and OH-.

Reaction at cathode:

The hydrogen ions migrate to the cathode, gain electrons and become hydrogen gas.

2H+(aq) + 2e- →H2(g)

Reaction at the anode:

The hydroxide ions and sulphate ions migrate to the anode. The hydroxide ions being less
reactive than sulphate ions are discharged and oxygen gas is formed.
Ions present:
4OH-(aq) →2H2O(l) + O2(g) + 4e-
From copper (II) sulphate: Cu2+ and SO42-
Overall equation
From water: H+ and OH-
4H+(aq) + 4OH-(aq) → 2H2(g) + O2(g)+ 2H2O(l)
Reaction at cathode:
Note: Using the carbon electrodes made of graphite gives you the same results.

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Copper (II) ions and hydrogen ions migrate to cathode. Copper (II) ions are discharged because Overall equation: 2Cu2+(aq) + 4OH- (aq) → 2Cu(s) + 2H2O(l) + O2 (g)
they are less reactive than hydrogen ions. Copper (II) ions gain electrons from the cathode and
Note: Using the carbon electrodes made of graphite gives you the same results.
copper is deposited. A brown layer of copper is deposited at the cathode and thus the mass of the
cathode increases.

Cu2+(aq) + 2e- → Cu(s)

Reaction at anode:

Both the sulphate and hydroxide ions migrate to the anode but none loses its electrons. Instead
the copper anode itself loses electrons and as it does so, it becomes copper (II) ions which
dissolves in solution. The anode electrode dissolves and its mass decreases.

Cu(s) → Cu2+(aq) + 2e-

Electrolysis of copper (II) sulphate solution (using copper cathode and platinum anode)

Ions present: From copper (II) sulphate: Cu2+ and SO42-

From water: H+ and OH-.

Copper (II) ions and hydrogen ions move to the cathode. Copper (II) ions being less reactive than
hydrogen ions are discharged. Copper (II) ions gain electrons and copper is deposited. The blue
colour of the electrolyte (copper (II) sulphate solution) fades as copper is deposited because
copper (II) ions are removed from the solution.

Cu2+(aq) + 2e- → Cu(s)

Sulphate ions and hydroxide ions move to the anode. Hydroxide ions being less reactive than
sulphate ions are discharged by giving up their electrons. Bubbles of a colourless gas (oxygen)
are formed at the anode.

4OH- (aq) → 2H2O(l) + O2(g) + 4e-

The overall equation is obtained by adding the two equations after multiplying the first equation
by 2, to obtain the same number of electrons in both equations.

2Cu2+(aq) + 4e- → 2Cu(s)

4OH-(aq) → 2H2O(l) + O2(g) + 4e-

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