balancing chemical equation

• Word equation

• Symbolic representation

• Balanced chemical equations

• Symbolic equation with physical states

Chemical Reactions and Equations

Sub-topic Contents

Representing a Chemical Reaction

Subtopic ID: 01.02.00

CBSE - 10

Chemistry


Word Equation



This is an example of a word equation to represent the reaction between barium carbonate and phosphoric acid.

Barium carbonate + Phosphoric acid

Barium phosphate + Water + Carbon dioxide

CBSE - 10

Chemistry



In order to handle the number of moles of reactants and products numerically it would be convenient to represent them in a symbolic equation.

Symbolic Representation


CBSE - 10

Chemistry

You would have watched the combustion of methane in air in any oil refinery unit. Let us write the chemical equation for this reaction. There are a few steps involved in balancing a chemical equation.

Combustion of Methane



Balanced Chemical Equations


CBSE - 10

Chemistry

In step-1, we need to write the word equation. Methane gives carbon dioxide and water on combustion in air.Step-1

Methane + Oxygen Carbon dioxide + Water





CBSE - 10

Chemistry

Let us represent the word equation by the respective molecular formula.

Step-2

CH4 + O2 CO2 + H2O

Methane + Oxygen Carbon dioxide + Water





CBSE - 10

Chemistry

Write down the number of atoms present on each side of the arrow. On the right hand side we have one carbon, four hydrogen and 2 oxygen atoms. On the right hand side we have 1 carbon, 2 hydrogen and 3 oxygen atoms.

Step-3

ElementNo. of atoms in

LHSNo. of atoms in

RHS

C 1 1

H 4 2

O 2 3





CH4 + O2 CO2 + H2O

CBSE - 10

Chemistry

The atoms are not balanced in this equation. Start balancing from the element with the greater number of atoms. Do not give priority to the atom found in more than one compound on either side of the arrow. For example, since oxygen is present in two compounds on the right hand side we should not give priority to the oxygen atom when balancing.

Step-4


LHSNo. of atoms

in RHS

C 1 1

H 4 2

O 2 3





CH4 + O2 CO2 + H2O

Balance the hydrogen atoms by multiplying by 2 on the right hand side.Step-5


LHSNo. of atoms in

RHS

C 1 1

H 4 2 x 2

O 2 3





CH4 + O2 CO2 + H2O

CBSE - 10

Chemistry

Write out the partially balanced equation.

Step-6

CH4 + O2 CO2 + 2H2O






LHSNo. of atoms in

RHS

C 1 1

H 4 2 x 2

O 2 3

CBSE - 10

Chemistry

Write down the number of atoms present on both sides of the arrow of the revised equation. Again check whether all the atoms are balanced.

Step-7

ElementNo. of atoms

in LHSNo. of atoms in

RHS

C 1 1

H 4 4

O 2 4

Step-7





CH4 + O2 CO2 + 2H2O

CBSE - 10

Chemistry

Balance the atoms of the remaining elements, here oxygen.

Step-7

ElementNo. of atoms


RHS

C 1 1

H 4 4

O 2 x 2 4

Step-8





CH4 + O2 CO2 + 2H2O

Write down the balanced equation.

Step-9

CH4 + 2O2 CO2 + 2H2O





ElementNo. of atoms


RHS

C 1 1

H 4 4

O 2 x 2 4

We can also mention the states of reactants and products within brackets. The subscripts g, l, s represent gas, liquid and solid respectively.

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)




Symbolic Equation with Physical States

g – gaseous state

l – liquid state

CBSE - 10

Chemistry

We can write the reaction condition above the arrow mark. Here, the reaction needs heating. This is indicated by a small triangle.

CH4(g) + 2O2(g) Δ

CO2(g) + 2H2O(l)




Symbolic Equation with Physical States

= Heat

CBSE - 10

Chemistry

balancing chemical equation

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