Revision Notes

Double Displacement Reaction

– A chemical reaction in which two atoms or a group of atoms (ions) are exchanged between the reactants to form new compounds are called double displacement reactions.

– General formula for double displacement reaction: XY + ZA → XZ + YA

– 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.

-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.

• Any reaction in which an insoluble solid called precipitate is formed that separates from the solution is called a precipitation reaction.
• General formula for precipitation reaction:
  A + soluble salt B → precipitate + Soluble salt C

Oxidation and Reduction reactions:

• Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen.

– If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidized

– The substance which gives gains oxygen or loses hydrogen during a reaction, the substance is called the oxidizing agent.

– An oxidizing agent gets reduced

– Example:

Sulphur loses hydrogen and gets oxidized.

• Reduction is a chemical process in which a substance gains hydrogen or loses oxygen.

-If a substance gains hydrogen or loses oxygen during a reaction, it is said to be reduced.

– The substance (reactant) which is responsible for removing oxygen or gaining hydrogen for reduction during a reaction is called the reducing agent.

– A reducing agent gets oxidised.

– Examples:

(i) CuO + H₂ → Cu + H₂O

Copper loses oxygen and gets reduced

(ii)H₂S + Cl₂ → 2 HCl+ S

Sulphur loses hydrogen and gets reduced.

• Redox reaction: When oxidation and reduction takes place simultaneously in a reaction, it is known as redox reaction.

– Example:

ZnO + C → Zn + CO

Here, ZnO loses oxygen and gets reduced to Zn. Similarly, carbon gains oxygen and gets oxidized to CO

• 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:

• Corrosion of metals:

When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.

– The black coating on silver and the green coating on copper are other examples of corrosion.

Prevention:

– By galvanization

– By painting metal surfaces

– By regularly oiling or greasing the metal surfaces.

Galvanisation: Coating a thin layer of Zinc on metal surfaces by electroplating.

• Rancidity: When fats and oils are oxidised, they become rancid and their smell and taste change.

Prevention:

– Antioxidants are added to foods containing fats and oils.

– Keeping food in air tight containers help to slow down oxidation.

– By removing oxygen gas and filling nitrogen gas at the time of packing the food.

– Rancidity can be retarded to a certain extent by storing food away from heat and light.

– By keeping food in the refrigerator which will eventually slow down oxidation.