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Atomic Structure | Chapter 12 | 8th Science - Chemical Equation | 8th Science : Chapter 12 : Atomic Structure

Chapter: 8th Science : Chapter 12 : Atomic Structure

Chemical Equation

1. Steps in writing the skeleton equation 2. Balancing chemical equation 3. Information conveyed by a balanced chemical equation

Chemical Equation

A chemical equation is a short hand representation of a chemical reaction with the help of chemical symbols and formulae. Every chemical equation has two components: reactants and products. Reactants are the substances that take part in a chemical reaction and the products are the substances that are formed in a chemical reaction.

 

1. Steps in writing the skeleton equation

Before writing the balanced equation of a chemical reaction, skeletal equation is written. The following are the steps involved in writing the skeletal equation.

1. Write the symbols and formulae of each of the reactants on the left hand side (LHS) and join them by plus (+) sign.

2. Follow them by an arrow (→) which is interpreted as gives or forms.

3. Write on the right hand side (RHS) of arrow the symbols and formulae for each of the products.

4. If the product is a gas it should be represented by upward arrow (↑) and if it is a precipitate it should be represented by downward arrow(↓).

Example: Mg + H2SO4 → MgSO4 + H2

5. The equation thus written is called as skeleton equation (unbalanced equation).

 

2. Balancing chemical equation

According to law of conservation of mass, the total mass of all the atoms forming the reactants should be equal to that of all the atoms forming the products. This law will hold good only when the number of atoms of all types of elements on both sides is equal. A balanced chemical equation is one in which the total number of atoms of any element on the reactant side is equal to the total number of atoms of that element on the product side.

There are many methods of balancing a chemical equation. Trial and error method (direct inspection), fractional method and odd number-even number method are some of them. While balancing a chemical equation following points are to be borne in mind.

1. Initially the number of times an element occurs on both sides of the skeleton equation should be counted.

2. An element which occurs least number of times in reactant and product side must be balanced first. Then, elements occuring two times, elements occuring three times and so on in an increasing order must be balanced.

3. When two or more elements occur same number of times, the metallic element is balanced first in preference to non-metallic element. If more than one metal or non-metal is present then a metal or non-metal with higher atomic mass (refer periodic table to find the atomic mass) is balanced first.

4. The number of molecules of reactants and products are written as coefficient.

5. The formula should not be changed to make the elements equal.

6. Fractional method of balancing must be employed only for molecule of an element (O2,H2,O3,P4,…) not for compound (H2O, NH3,…)

Now let us balance the equation for the reaction of hydrogen and oxygen which gives water. Write the word equation and balance it.

Step 1: Write the word equation.

Hydrogen  + Oxygen →   Water

Step 2: Write the skeleton equation. H2 + O2 → H2O

Step 3: Select the element which is to be balanced first based on the number of times an element occurs on both sides of the skeleton equation.


Step 4: In the above case, both elements occur one time each. Here, preference must be given to oxygen because it has higher atomic mass (refer periodic table).

Step 5: To balance oxygen, put 2 before H2O on the right hand side (RHS).

H2 + O2 → 2H2O

Step 6: To balance hydrogen, put 2 near hydrogen (H2) on the left hand side (LHS).

2H2+O2    →   2H2O

(H = 4 0 = 2) (H = 4 0 = 2)

Now, on both sides number of hydrogen atoms is four and oxygen atoms is two. Thus, the chemical equation is balanced.

 

3. Information conveyed by a balanced chemical equation

A balanced chemical equation gives us both qualitative and quantitative information. It gives us qualitative informations such as the names, symbols and formulae of the reactant molecules taking part in the reaction and those of the product molecules formed in the reaction. We also can get quantitative information like the number of molecules/ atoms of the reactants and products that are taking part in the reaction. However, a chemical equation does not convey the following.

i. Physical state of the reactants and the products.

ii. Heat changes (heat liberated or heat absorbed) accompanying the chemical reaction.

iii. Conditions such as temperature, pressure, catalyst etc. , under which the reaction takes place.

iv. Concentration (dilute or concentrated) of the reactants and products.

v. Speed of the reaction.


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