Chemical Reactions: Representation, Type of Equations & Formation

20 Dec 2023

Chemical Reactions Essentials:

When the nature and the identity of the initial substance have somewhat changed it is said to be a chemical change and  whenever a chemical change occurs, a  Chemical Reaction takes place. These  Chemical Reactions can be represented as chemical equations.

Understanding  Chemical Reactions 

  • The description of a  Chemical Reaction in sentence form is quite long. It can be written in a shorter form. 
  • The simplest way to do this is to write it in the form of a word-equation.
  • When a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide. 
  • It can be written in words as:

Magnesium + Oxygen → Magnesium oxide

        (Reactants)                         (Product)

  • Reactants: The substances that undergo chemical change in the  Chemical Reactions are the reactants.
  • Product: The new substance formed during the  Chemical Reactions is a product. 
    • A word-equation shows change of reactants to products through an arrow placed between them. 
    • The reactants are written on the left-hand side (LHS) with a plus sign (+) between them and products are written on the right-hand side (RHS) with a plus sign (+) between them. 
    • The arrowhead points towards the products, and shows the direction of the  Chemical Reactions.

 Chemical Reactions Representation: Magnesium Burning in Oxygen

  • Chemical equations can be made more concise and useful if we use chemical formulae instead of words.
  • A chemical equation represents a  Chemical Reactions. 
    • Burning of magnesium ribbon in oxygen can be written as –

Mg + O2 → MgO

Balanced Chemical Equations: Preserving Mass in  Chemical Reactions 

  • Law of conservation of mass states that mass can neither be created nor destroyed in a  Chemical Reactions i.e the total mass of the elements present in the products of a  Chemical Reactions has to be equal to the total mass of the elements present in the reactants. 
    • In other words, the number of atoms of each element remains the same, before and after a  Chemical Reactions.
    • Hence, we need to balance a skeletal chemical equation. 

Example: Zn + H2SO4 → ZnSO4 + H2

Number of atoms of different elements on both sides of the arrow

Element Number of atoms in reactants (LHS)  Number of atoms in products (RHS)
Zn 1 1
S 2 2
H 1 1
O 4 4
  • As the number of atoms of each element is the same on both sides of the arrow, the equation is a balanced chemical equation.

Fe + H2O → Fe3O4 + H2 

  • Step I: List the number of atoms of different elements present in the unbalanced equation 

Number of atoms of different elements present in the unbalanced equation

Element Number of atoms in reactants (LHS)  Number of atoms in products (RHS)
Fe 1 3
H 2 2
O 1 4
  • Step II: Start balancing with the compound that contains the maximum number of atoms. 
    • It may be a reactant or a product. 
    • In that compound, select the element which has the maximum number of atoms. i.e. the compound is Fe3O4 and the element oxygen in it. To balance the oxygen atoms.

Atoms of Oxygen

Atoms of Oxygen In reactants In products
Initial  1 (in  H2O  ) 4 (in Fe3O4 )
To balance 1 x 4 4
  • Now the partly balanced equation becomes:

Fe + 4 H2O → Fe3O4 + H2 

  • Step III: Now to equalise the number of H atoms, make the number of molecules of hydrogen as four on the RHS. 
Atoms of Hydrogen
Atoms of Hydrogen In reactants In products
Initial  8 (in 4 H2O) 2 (in  H2)
To balance 8 2 x 4
  • The equation would be: Fe + 4 H2O → Fe3O4 + 4 H2 
  • Step IV: To equalise Fe, we take three atoms of Fe on the LHS . 
Atoms of Iron
Atoms of Iron In reactants In products
Initial  1 (in Fe) 3 (in Fe3O4 )
To balance 1 x 3 3
  • The equation would be: 3 Fe + 4 H2O → Fe3O4 + 4 H2 
  • Step V: Finally, the numbers of atoms of elements on both sides of equations are equal. 
    • This equation is now balanced. 
    • This method of balancing chemical equations is called the hit-and-trial method.

3Fe + 4H2O → Fe3O4 + 4H2  

  • Step VI: To make a chemical equation more informative, the physical states of the reactants and products are mentioned. 
    • The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively.

Note: The word aqueous (aq) is written if the reactant or product is present as a solution in water.

  • Thus, the balanced Equation becomes: 3Fe (s) + 4H2O (g) → Fe3O4 (s) + 4H2 (g)
  • Sometimes the reaction conditions, such as temperature, pressure, catalyst, etc., for the  Chemical Reactions indicated above and/or below the arrow in the equation. For example,

Chemical Reactions indicated

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