Revision Notes

A chemical equation represents a chemical reaction. So reactants are on the left hand side (LHS) of arrow and there is written (+) Sign between them. Similarly, Products are on the Right hand side (RHS) of arrow with a plus (+) sign between them.

Chemical equation can be divided into two types,
(i) Balanced chemical equation
(ii) Unbalanced chemical equation

Balanced chemical equation-

• A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.

For example,
Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.

As there are an equal number of atoms of different elements in the reactants and products, so the above chemical equation is a balanced equation. Complete balanced chemical equation gives us information about the reactants, products and the conditions under which a reaction has taken place.

• Unbalanced chemical equation – An equation which has an unequal number of atoms of one or more elements in reactants and products is called unbalanced chemical equation.
• An unbalanced chemical equation is called a skeletal equation.

For example,
Hydrogen reacts with oxygen to form water

In the above chemical equation, there are an unequal number of oxygen atoms in reactants and products, so it is an unbalanced equation.

A chemical equation is said to be balanced when the number of atoms of each elements on the reactant side is equal to the number of atoms of corresponding elements on the product side. Unbalanced chemical equation has to be made balanced to justify the law of conservation of mass.

The Law of Conservation of Mass states that when a chemical reaction occurs, the mass is conserved. In other words, the mass of reactants and products are equal and mass is neither generated nor destroyed during a chemical reaction.

Balancing of chemical Equations

The process of making the number of different types of atoms equal on both the sides of an equation is called balancing of equation.

TYPES OF CHEMICAL REACTIONS

Chemical reactions involve the breaking and making of bonds between atoms to produce new substances

• Combination reactions:
  Those reactions, in which two or more substances combine to form a single substance, are called combination reactions.
• In a combination reaction, two or more elements can combine to form a compound; two or more compounds can combine to form a new compound; or an element and a compound can combine to form a new compound.
• General formula is A + B    →   AB
  Examples:
  Calcium oxide (CaO) reacts vigorously with water and form a single product, calcium hydroxide (slaked lime).
  CaO(s) + H₂O (l)   →   Ca(OH)₂ (s)  + Heat
• Decomposition reaction: In a decomposition reaction, a single substance decomposes to two or more simpler substances.
  – General formula is AB   →  A + B
  – Decomposition reactions are the opposite of combination reactions.
  – Decomposition reaction requires energy in the form of heat, light or electricity

Ferrous sulphate crystals (FeSO₄.7H₂O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃). Ferric oxide is a solid, while SO₂ and SO₃ are gases.

TYPES OF DECOMPOSITION REACTION

– Decomposition reaction always occurs with the inputs of energy and this energy can be in the form of heat, electric current or light.

• Thermal decomposition- When a decomposition reaction is carried out by heating, it is called ‘thermal decomposition’.
  
  Example: Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction
  
  
• Electrolytic decomposition- When a decomposition reaction is carried out by using electric current, it is called electrolytic decomposition.
  
  Example: When electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas. electricity
  
  This decomposition reaction takes place by the action of electricity. It is called electrolysis of water.
• Photolytic decomposition- When a decomposition reaction is carried out by using light energy, it is called photolytic decomposition
  
  Example: When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.
  
  – white silver chloride turns grey in sunlight.

Displacement Reaction

– Displacement reaction occurs when a more reactive element displaces a less reactive element from its compound.

– General formula for displacement reaction

• Displacement reaction is also called substitution reaction.
  Example:
  When as strip of zinc metal is placed in copper sulphate solution, then zinc sulphate solution and copper are obtained.
  
  – The reactivity series is a list of metals arranged in the decreasing order of reactivity.
  – The most reactive metal is placed at the top and the least reactive metal is placed at the bottom.