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Writing and Balancing Chemical Equations

A chemical change can be represented by using symbols and formulae of various species involved in the change. Such a representation is known as chemical equation.

Representation of a chemical change in terms of symbols and formulae of the reactants and products is known as chemical equation of the reaction.

For example, when zinc(II) trioxocarbonate(IV) is heated, zinc oxide and carbon dioxide are formed. This chemical change may be represented by the chemical equation given below:

ZnCO3             ZnO  + CO2

 

STEPS FOR WRITING CHEMICAL EQUATION

Step 1: Writing Skeletal Equation

In order to write a chemical equation for a reaction, the first step is to write symbols and formulae of various reactants and products. For example, when zinc(II) trioxocarbonate(IV) reacts with dilute hydrochloric acid it gives zinc chloride, carbon dioxide and water. This is represented by the following ‘word equation’.

Zinc(II) trioxocarbonate(IV) +Hydrochloric ac

Zinc chloride + Carbon dioxide + water

Using symbols and formulae for various reactants and products we Get,

 

ZnCO3 + HCl                           ZnC12 + CO2 + H2O

Equations such as given above in which no attempt has been made to equalize the number of atoms of various elements on both the sides of the equation are known as skeletal equations. Thus:

A skeletal equation is an equation in which various reactants and products are represented by their respective formulae but no attempt is made to equalize the number of atoms of various elements on both ‘the sides of the equation.

Step II: Balancing of Chemical Equation

We know from Dalton’s atomic theory that atoms can neither be created nor destroyed. Therefore, number of atoms of various elements on both the sides of the chemical equation must be equal.

After writing skeletal equation, the next step is to equalize the number of atoms of various elements on both the sides of the equation by multiplying various formulae by appropriate coefficients. This process is known as balancing of chemical equations.

 

In order to equalize the number of atoms of various elements, the various species are multiplied by appropriate numbers. For example, formation of ammonia is represented by the balanced chemical equation given below:

N2 + 3H2          2NH3

An equation in which number of atoms of each element is equal on both the sides of the equation is known as balanced chemical equation.

Sometimes the skeletal equation and the balanced equation may be identical. Some Examples are given below:

Some balanced chemical equations are discussed below:

 

A. Synthesis (Composition)

Two or more elements or compounds may combine to form a simple or complex compound.

Examples of synthesis reactions

 

1. Metal + oxygen                               metal oxide

2Mg(s) + O 2(g)                      2MgO(s)

 

2. Nonmetal + oxygen                        nonmetallic oxide

C(s) + O2(g)                            CO2(g)

 

3. Metal oxide + water                        metallic hydroxide

Mg O(s) + H2O(l)                    Mg(OH)2(s)

 

4. Nonmetallic oxide + water                         acid

CO2(g) + H2O(l)                      H2CO3(aq)

 

5. Metal+ nonmetal                             salt

2Na(s) + Cl2(g)                                    2NaCl(s)

 

6. A few nonmetals combine with each other.

2P(s) + 3Cl2 (g)                                   2PCl3(g)

B. Decomposition

 

A single compound breaks down into its component parts or simpler compounds.

 

Examples of decomposition reactions:

 

1. Metallic carbonates, when heated, form metallic oxides and CO2(g)

 

CaCO3(s)         CaO(s) + CO2{g)

 

2. Most metallic hydroxides, when heated, decompose into metallic oxides and water.

Ca(OH)2(s)       CaO(s) + H2O(g)

 

3. Metallic chlorates, when heated, decompose into metallic chlorides and oxygen.

2KC1O3(s)      2KCl(s) + 3O2(g)

 

4. Some acids; when heated, decompose into nonmetallic oxides and water.

H2S04             H2O(l) +SO3(g)