OXIDATION - REDUCTION, METAL
Oxidation and Reduction
Classical concepts: According to classical concepts oxidation has been defined as a reaction of either of the following types.
(i) Reactions involving addition of oxygen or of any non-metal element (except hydrogen), e.g.
2C + O2 2CO
4Fe + 3O2 2Fe2O3
H2 + F2 2HF
(ii) Reactions involving removal of hydrogen or of any metal, e.g.
2H2O 2H2 + O2
(iii) Reactions involving increase of valency of a metal, e.g.
2FeCl2 + Cl2 2FeCl3
A substance which can bring out an oxidation reaction is known as an oxidizing agent.
Conversely, reduction has been defined as a reaction of either of the following types.
(i) Reactions involving removal of oxygen or of any non-metal element (other than hydrogen), e.g.
CuO + H2 Cu + H2O
(ii) Reactions involving addition of hydrogen or of any metal, e.g.
O2 + 2H2 2H2O
(iii) Reactions involving decrease of valency of a metal, e.g.
2FeCl3 + H2S 2FeCl2 + HCl + S
A substance which can bring out a reduction reaction is known as a reducing agent.
Modern concepts: According to electronic interpretation oxidation is a process which involves the loss of one or more electrons while reduction is a process which involves the gain of one or more electrons.
Examples of oxidation-
Na Na+ + e
H2 2H+ + 2e
Al Al+3 + 3e
Fe+2 Fe+3 + e
2Cl- Cl2 + 2e
In the above examples, Na, H2, Al, Fe+2, Cl- are said to be oxidized. Such equations in which electrons are lost are known as oxidation half-reactions.
Examples of reduction-
Cl2 + 2e 2Cl-
Fe+3 + e Fe+2
2H+ + 2e H2
In the above examples, Cl2, Fe+3, H+ are said to be reduced. Such equations in which electrons are lost are known as reduction half-reactions.
Oxidation-Reduction Reactions (Redox Reactions)
An oxidation reaction is always accompained by an equivalent amount of reduction and vice versa. Such overall reactions which involve a simultaneous oxidation and reduction are called redox reactions. Thus a redox reaction is made up of two half reactions- oxidation half reaction and reduction half reaction.
For example- formation of sodium chloride from sodium and chlorine.
2Na(s) + Cl2(g) 2NaCl(s)
Two half-reactions are:
2Na 2Na+ + 2e (oxidation half reaction)
Cl2 + 2e 2Cl- (reduction half reaction)
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2Na + Cl2 2NaCl
The electrons lost by metallic sodium are gained by chlorine, thus sodium is said to be oxidised to Na+ and chlorine is said to be reduced to Cl-. Sodium is an reducing agent and chlorine is a oxidising agent.
2Na + Cl2 2NaCl
reducing agent oxidising agent
A substance which can accept one or more electrons is called oxidising agent and a substance which can donate one or more electrons is called reducing agent.
Other examples-
H2 + Cl2 2HCl
reducing agent oxidising agent
H2 + CuO Cu + H2O
reducing agent oxidising agent
Zn + H2SO4 ZnSO4 + H2
reducing agent oxidising agent
Zn + CuSO4 ZnSO4 + Cu
reducing agent oxidising agent
2KI + Cl2 2KCl + I2
reducing agent oxidising agent
Metals
Elements have been classified into- metals and non-metals.
According to the modern views, an element which forms positively charged ion (cation) by loss of electrons from its atom is called a metal, while the element which forms negatively charged ion (anion) by gain of electrons from its atom is called non-metal. For example,
Metals Cations Non-metals Anions
Na Na+ + e Cl + e Cl-
Ca Ca+2 + e O + 2e O-2
Al Al+3 + e S + 2e S-2
Although, there is no sharp line of demarcation between metals and non-metals, most of the metals are hard, malleable, ductile and god conductors of heat and electricity with shining luster. A summary of characteristic features of metals and non-metals is given below.
However, certain elements exhibit the properties of metal as well as non-metals. Such elements are named as metalloids. For example, boron, silicon, arsenic and antimony behave as metals in their properties but as non-metals in their chemical properties.
Occurrence of metals
Metals occur in nature in two forms depending upon their electropositive character.
1. In free or native state- A few metals particularly noble metals having least electropositive character occur in nature in free state. Examples are silver, gold, platinum etc.
2. In combined state- Most of the metals occur in nature as their compounds. These metallic compounds occur in the earth’s crust along with a number of rocky and other impurities are known as minerals.
Physical Properties of Metals
The general physical properties of metals are:
• are hard and strong.
• solids (Only mercury is liquid at room temperature).
• shiny luster (when polished).
• good conductors of heat.
• good conductors of electricity.
• dense
• malleable and ductile.
• sonorus (having loud, full or deep sound)
Chemical Properties of Metals
Many substances on the Earth's surface will at sometime come in contact with air, water or acids. The shiny surface of most metals becomes dull in time. This is due to a slow chemical reaction between the surface of the metal and oxygen in the air. The reaction forms only one product; a surface coating of the metal oxide. The general word equation is:
Metal + oxygen → metal oxide
For example: The dull appearance of the metal lead is due to a coating of lead oxide. If the surface is scratched then the shiny lead metal can be seen underneath.
Lead + oxygen → lead oxide
Heating can speed up the reaction with oxygen. If a piece of copper is heated it quickly becomes coated in black copper oxide. The word equation is:
copper + oxygen → copper oxide
Use of Metals
The five most used metals in order of use are: iron, aluminum, copper, zinc, and manganese.
Copper, gold and silver are good conductors of electricity and are ductile. Therefore copper is used for electrical cables and gold and silver are used for electrical connections.
Iron and steel are both hard and strong. Therefore they are used to construct bridges, buildings and motor cars. The disadvantage of using iron is that is tends to rust.
Aluminium is a good conductor of heat and is malleable. It is used to make saucepans and thin foil. It is also used to make aeroplane bodies as it is a low density metal.
Gold and silver are very malleable ductile and very unreactive. They are used to make intricate jewellery which does not tarnish.
Alloys
An alloy is a metal mixed with other elements. The properties of an alloy is different from pure metals. Some alloys are listed below.
• steel: contains iron and carbon plus other metals. It is stronger than pure iron. It is used as girders for buildings and car bodies.
• solder: it is a mixture of tin and lead. It has a lower melting point than pure metals. It is used for connecting electrical circuits.
• duralumin: contains aluminium and copper. It is lighter than copper and stronger than aluminium. It is used in aeroplanes and alloy wheels.
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