The Colour of the status of liberty is changing! Is that MAGIC ?
You do notice the green colour formed on the statue of liberty, it is caused by redox reaction. The same phenomenon can be observed in rusting of nails or a cut apple turning brown, and burning magnesium which gives a bright white flame. These are examples of redox reactions.
To study redox reactions we need to understand the difference between oxidation and reduction :
Oxidation occurs when an atom loses electrons. For example a magnesium atom can lose two electrons to form a magnesium ion.
Reduction occurs when an atom gains electrons. For example an oxygen molecule can gain four electrons to form two oxide ions.
In a redox reaction oxidation and reduction occurs simultaneously. When magnesium burns two moles of magnesium atoms transform electrons to one mole of oxygen molecule to form one mole of magnesium oxide.
Consider the reaction between a strip of aluminium foil and a solution of copper II chloride, as the reaction proceed, the aluminium strip will disintegrate and metallic copper will deposit on the strip.
Two aluminium atom loses each one 3 electrons, as they are oxidized to form Aluminium ions (Al3+).
Three copper ions (Cu 2+) gain two electrons each, and got reduced to form copper metal (Cu).
Because oxidation and reduction occur in the same container, this is an example of a direct redox reaction. During this reaction the temperature rises, chemical energy is converted and released as heat energy.
Redox reactions can also be setup, so the oxidation and reduction reactions can occur in two different containers, this is called the indirect redox reaction. A galvanic cell is an example, the two solutions are connected by a salt bridge which allows ions to move between the two separated solutions.
As the aluminium in the first solution is oxidized, it loses electrons, copper ions in the second solution are reduced by gaining these electrons which move in the external wire and create electricity. Chemical energy is converted to electrical energy.
Oxidation Reaction- Reduction Reaction
Oxidation Reaction: 2Br– –> Br2 + 2 e- Loss of electrons / The oxidation number increases
Bromide ions are oxidized because they electrons
Br– is the reducing agent
Reduction: Cl2 + 2e- –> 2Cl– Gain of electron / The oxidation number decreases
Cl2 is the oxidizing agent
The reducing agent is oxidized
The oxidizing agent is reduced
Oxidation and reduction are complementary processes
Redox reaction = Oxidation + Reduction