Chemical reactions, balanced and unbalanced chemical equations

May 10, 2018 Sushil Humagain Chemistry 0

Chemical reaction:

  • Physical Change: temporary & usually reversible change in which no new substance are formed
  • e.g. Melting of ice or wax, magnetizing or demagnetizing of iron, preparation of solution, making of different objects from wood, soil, paper etc.
  • Chemical Change: permanent & usually irreversible change in which new substances with different properties are formed

e.g. formation of curd from milk, rusting of iron, burning of candle, electrolysis of water etc.

 

  Physical change   Chemical change
1 Temporary change 1 Permanent Change
2 Usually a reversible change 2 Usually an irreversible change
3 New substance is not formed 3 New substance is formed
4 Only physical properties e.g. color, taste, physical properties are changed. 4 The physical properties, chemical properties and chemical composition are changed.
5 Change in energy is not usually seen. 5 Usually, energy is absorbed or evolved in it.
6 Change in mass (or amount of substance) is not seen e.g. dissolving salt in water 6 Change in mass (or amount of substance) is seen e.g. rusting of iron

 

Chemical Reaction:

  • The combination, decomposition or exchange that takes place in the molecules of matter during a chemical change
  • The substance that take part in a chemical reaction are called reactants.
  • The substance which are formed after a chemical reaction are called products.
  • A chemical reaction can be expressed in word equation and chemical equation.
  1. Word Equation:
  • The chemical reaction expressed in terms of full names of reactants and products is called word equation.

e.g. Hydrogen + Oxygen —–>Water (with electric sparks)

  • Reactants are written at first and then an arrow (à) is written means “forms or gives” and then products are written. The two way arrows are used to show that the reaction is reversible.
  • If reactants or products are two or more they are joined by the (+) sign.
  • The required conditions for the chemical reaction can be written above the arrow to make the equation more informative.

e.g. Potassium Chlorate —–>Potassium chloride + Oxygen (with heat and MnO2)

Demerits of Word equation:

  • It takes more space and time.
  • The total number of atoms and molecules of reactants and products cannot be counted.
  • The equation cannot be balanced.
  1. Chemical or Formula Equation:
  • The chemical reaction expressed by writing symbols and molecular formulae of reactants and products is called chemical equation or formula equation.
  • It is more informative, easy and practicable way to express a chemical reaction.

e.g. 2KClO3 —–>2KCl + 302 (in the presence of heat and MnO2)

  • It can be further classified into two types. They are:
  1. Skeletal or Unbalanced Chemical equation:
  • The chemical equation in which the total number of atoms of each element in reactants and products are not equal.

Demerits of Skeletal or Unbalanced Chemical equation:

  • It does not follow the law of conservation of mass.
  • It does not tell us about the ratio of reactant and product molecules.
  • It does not give information about the total number of each element in reactants and products.

2. Balanced Chemical equation:

  • The chemical equation written by balancing total number of atoms of each element in reactants and products
  • It is more informative than the unbalanced chemical equation.
  • It is based on the law of conservation of mass & Dalton’s atomic theory.
Unbalanced Chemical Equation Balanced Chemical Equation
Na + Cl2 —–> NaCl

 

 2Na + Cl2 —–> 2NaCl

 
Cl2 + KI —–> KCl + I2

 

 Cl2 + KI —–> 2KCl + I2

 
N2+ H2 —–> NH3 N2+ 3H2 —–> 2NH3

 

Balancing of Chemical Equation

  • “Hit & Trial Method” is the simplest method to balance a chemical equation.

Steps:

  1. The chemical change is written correctly in the form of word equation.
  2. The word equation is written correctly in the form of formula equation using symbols and molecular formula.
  3. The number of atoms of each element are counted on each side of the chemical equation. They are balanced by adding suitable coefficients without changing the molecular formula of reactants and products.

e.g. i. Potassium Chlorate —–>Potassium chloride + Oxygen [Word Equation]

ii. KClO3 —–> KCl+ 302   [Formula Equation]

iii. 2KClO3 —–>2KCl + 302 [Balanced Equation]

  • To make a chemical equation more informative, states of reactants and products can be expressed by following signs in brackets.
(s) => solid (aq.) =>aqueous(solution in water)
(g) or ↑ => gas (ppt.) or ↓ =>precipitate
(l) => liquid (conc.) => concentrated
∆ => heat (dil.) => dilute

 

Information obtained from a balanced chemical equation:

  1. The symbol and molecular formulae of reactants & products.
  2. The total number of atoms or molecules of reactants & products.
  3. The ratio of molecular weight of reactant & product molecules.
  4. The type of chemical reaction.

Limitations of a balanced chemical equation:

  • A balanced chemical equation may not provide information about:
  1. The physical state of reactants and products.
  2. Concentration of reactants
  3. Conditions required for the reaction e.g. heat, light, pressure, catalyst etc.
  4. The rate of chemical reaction
  5. The time taken for the reaction to complete

Reversible & Irreversible Reactions:

  1. Reversible Reaction: chemical reaction which occurs both in forward as well as backward direction
  • A two way arrow (↔) is used to show such reaction

e.g. N2 +3H2 <—–> 2NH3

  1. Irreversible Reaction: chemical reaction which occurs only in one direction i.e. forward direction
  • A single headed arrow (→) is used to show such reaction.

e.g. CaCO3 —–> CaO +CO2

Author: Sulaksha Purna Shrestha

Chemical reactions, balanced and unbalanced chemical equations