Chemical equation

• Chemical equation is the representation of a chemical reaction in the form of symbols for substances that are involved in the reaction.
• It is a method used to present a chemical reaction with the help of symbol and formulae of the substance involved in it.
• The substances which combine or react are known as reactant.
• The reactant molecules on the left side or behind the arrow interact with each other such that new bonds are formed resulting in the formation of products.
• The new substance produced in a reaction are known as products.
• The products are on the right side or ahead of the arrowhead.
• A reacts with B to form X and Y as shown below:

TO MAKE EQUATIONS MORE INFORMATIVE:

• The chemical equation can be made more informative
• By indicating the physical states of the reactants and products.
• g. ‘s’ is used for solids, ‘l’ for liquids and ‘g’ for gas. For an aqueous solution, ‘(aq)’ is used.
• By indicating heat changes taking place in the reaction.
• g. An exothermic reaction is indicated by writing +Heat or +Heat energy or just +Energy on the product side of the equation.
• An endothermic reaction is indicated by writing +Heat or +Heat energy or just +Energy on the reactant side of the equation.
• By indicating the conditions under which the reaction takes place.
• E.g. If the reaction takes place in the presence of a catalyst, then the symbol or formula of the catalyst is written above or below the arrow sign.

BALANCED CHEMICAL EQUATION

• A chemical equation in which the number of atoms of all the molecules is equal on both sides of the equation is known as balanced chemical equation.
• A balanced chemical equation has equal masses of various elements in reactant and product.

UNBALANCED CHEMICAL EQUATION

• A chemical equation in which the number of atoms of all the molecules is equal on both sides of the equation is known as balanced chemical equation.
• An unbalanced chemical equation has unequal masses of various elements in reactant and product.

LAW OF CONSERVATION OF MASS:

• According to the law of conservation of mass, matter can neither be created nor destroyed.
• This means that the total mass of the reactants and products of a chemical reaction must remain the same.
• Therefore, for a complete chemical equation, the number of atoms of various elements on both sides are made equal, to make the equation balanced.
• The law of conservation of mass fails in the following equation because the number of atoms of the elements/molecules on the reactant side is not equal to the number of atoms of the elements/molecules on the product side.

BALANCING A CHEMICAL EQUATION

• Consider the above chemical reaction. The following steps are used for balancing the chemical equation:
• Step 1: The maximum number of atoms present on either side of the reaction is on product side for Oxygen which is 4.
• To make the number of oxygen atoms on both sides of the reaction equal, the number of oxygen atoms on reactant side is multiplied by 4.
• The equation becomes
  $\mathrm{Fe} + 4{\mathrm{H}}_2\mathrm{O} \to {\mathrm{Fe}}_3{\mathrm{O}}_4 + {\mathrm{H}}_2$
• Step 2: The number of hydrogen atoms is 8 on the reactant side which is more than that of the product side, hence we need to balance hydrogen atoms, and hence we multiply H₂on the product side by 4.
• The equation becomes
  $\mathrm{Fe} + 4{\mathrm{H}}_2\mathrm{O} \to {\mathrm{Fe}}_3{\mathrm{O}}_4 + 4{\mathrm{H}}_2$
• Step 3: Thus the number of hydrogen and oxygen atoms on both sides of the equation are same.
• Finally, to balance the number of iron atoms on both side of the equation the number of iron atoms on the reactant side is multiplied by 3.

• The number of atoms of each element on both sides of the equation is shown in the table given below:

Thus, the above reaction is a balanced chemical equation.

LAW OF DEFINITE PROPORTION:

• In a given chemical compound, the proportion by mass of the elements that compose it are fixed, independent of the origin of the compound or its mode of preparation