Chemical Reaction[part=3]

Displacement reactions:

• Those reactions, in which one element takes the place of another element in a compound, are known as displacement reactions.
• In general, a more reactive element displaces a less reactive element from its compound.
  Examples:
  1. When as strip of zinc metal is placed in copper sulphate solution, then zinc sulphate solution and copper are obtained.
      In this reaction, zinc displaces copper from copper sulphate compound so that copper is free. This displacement reaction takes place because zinc is more reactive than copper.
  2. When a piece of iron metal (or iron nail) is placed in copper sulphate solution, then iron sulphate solution and copper metal are formed.
      In this reaction, iron displaces copper from copper sulphate solution. The deep blue color of copper sulphate solution fades due to the formation of green solution of iron sulphate. A red-brown coating (or layer) of copper metal is formed on the surface of iron metal (or iron nail).
• This displacement reaction takes place because iron is more reactive than copper.

Double Displacement reactions:

• Those reactions, in which two compounds react by an exchange of ions to form two new compounds, are called double displacement reactions.
  Example:
  When barium chloride solution is added to sodium sulphate solution, then a white precipitate of barium sulphate is formed along with sodium chloride solution.
   In this reaction, two compounds barium chloride and sodium sulphate react to form two new compounds, barium sulphate and sodium chloride. An exchange of ions takes place in this reaction.
  In this reaction, barium sulphate is formed as a white, insoluble solid called precipitate which separates out suddenly from the solution.
  Note: any reaction in which an insoluble solid called precipitate is formed that separates from the solution is called a precipitation reaction.

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Oxidation and Reduction reactions:

Oxidation:

The addition of oxygen to a substance is called oxidation.

The removal of hydrogen from a substance is called oxidation.

Reduction:

The addition of hydrogen to a substance is called reduction.

The removal of oxygen from a substance is called reduction.

• The oxidation and reduction reactions are also called redox reactions. Example:
  When copper oxide is heated with hydrogen, then copper metal and water are formed.
   In the above reaction, copper oxide (CuO) is changing into copper (Cu), so copper oxide is being reduced to copper.
  Hydrogen is changing into water (H2O), so hydrogen is being oxidized to water.
  Copper oxide is giving oxygen required for the oxidation of hydrogen, therefore, copper oxide is oxidizing agent and hydrogen is reducing agent.
• Effect of oxidation reactions in everyday life:
  Oxidation has damaging effect on metals as well as on food. There are two common effects of oxidation reactions which we observe in daily. These are:
  1. Corrosion of metals:
     Corrosion is the process in which metals are eaten up gradually by the action of air, moisture or a chemical (such as an acid) on their surface.
     • Corrosion is caused mainly by the oxidation of metals by oxygen of air. Rusting of iron metal is the most common form of corrosion.
     • During the corrosion of iron (rusting of iron), iron metal is oxidized by the oxygen of air in the presence of water (moisture) to form hydrated iron (III) oxide called rust.
       
     • Corrosion weakens the iron and steel objects and structures such as railings, car bodies, bridges and ships, etc., and cuts short their life.
  2. Rancidity
     • When the fats and oils present in food materials get oxidized by the oxygen (of air), their oxidation products have unpleasant smell and taste.
     • The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
     • Rancidity spoils the food materials prepared in fats and oils which have been kept for a considerable time and make them unfit for eating.
     • The development of rancidity of food can be prevented or retarded (slowed down) in the following ways:
       1. Rancidity can be prevented by adding anti-oxidants to foods containing fats and oils:
          Anti-oxidant is a substance (or chemical) which prevents oxidation. Anti-oxidants are actually reducing agent.
          The two common anti-oxidants used in foods to prevent the development of rancidity are BHA (Butylated Hydroxy – Anisole) and BHT (Butylated Hydroxy-Toluene).
       2. Rancidity can be prevented by packaging fats and oils containing foods in nitrogen gas:
          When the packed is surrounded by unreactive gas nitrogen, there is no oxygen to cause its oxidation and make it rancid.
          The manufacturers of potato chips fill the plastic bags containing chips with nitrogen gas.
       3. Rancidity can be retarded by keeping food in a refrigerator.
       4. Rancidity can be retarded by storing food in air-tight containers.
       5. Rancidity can be retarded by storing food away from light.