1. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Sub-topic Contents
• Word equation
• Symbolic representation
• Balanced chemical equations
• Symbolic equation with physical states
Subtopic ID: 01.02.00

2. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Word Equation This is an example of a word
equation to represent the
reaction between barium
Barium carbonate + Phosphoric acid carbonate and phosphoric
acid.
 Barium phosphate + Water + Carbon dioxide
Subtopic ID: 01.02.01

3. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Symbolic Representation In order to handle the number
of moles of reactants and
products numerically it would
be convenient to represent
them in a symbolic equation.
Subtopic ID: 01.02.02

4. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Balanced Chemical Equations You would have watched the
combustion of methane in air
Combustion of Methane 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.
Subtopic ID: 01.02.03

5. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Balanced Chemical Equations In step-1, we need to write the
word equation. Methane
gives carbon dioxide and
Step-1 water on combustion in air.
Methane + Oxygen  Carbon dioxide + Water
Subtopic ID: 01.02.03

6. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Balanced Chemical Equations Let us represent the word
equation by the respective
molecular formula.
Step-2
CH4 + O2  CO2 + H2O
Methane + Oxygen  Carbon dioxide + Water
Subtopic ID: 01.02.03

7. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Balanced Chemical Equations Write down the number of
atoms present on each side of
Step-3 the arrow. On the right hand
side we have one carbon, four
hydrogen and 2 oxygen
CH4 + O2  CO2 + H2O atoms. On the right hand side
we have 1 carbon, 2 hydrogen
and 3 oxygen atoms.
No. of atoms in No. of atoms in
Element LHS RHS
C 1 1
H 4 2
O 2 3
Subtopic ID: 01.02.03

8. Chemical Reactions and Equations
Representing a Chemical Reaction
Balanced Chemical Equations The atoms are not balanced in
this equation. Start balancing
Step-4 from the element with the
greater number of atoms. Do
not give priority to the atom
CH4 + O2  CO2 + H2O found in more than one
compound on either side of
No. of atoms in No. of atoms the arrow. For example, since
oxygen is present in two
Element LHS in RHS compounds on the right hand
side we should not give
C 1 1 priority to the oxygen atom
H 4 2 when balancing.
O 2 3
Subtopic ID: 01.02.03

9. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Balanced Chemical Equations Balance the hydrogen atoms
by multiplying by 2 on the right
Step-5 hand side.
CH4 + O2  CO2 + H2O
No. of atoms in No. of atoms in
Element LHS RHS
C 1 1
H 4 2x2
O 2 3
Subtopic ID: 01.02.03

10. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Balanced Chemical Equations Write out the partially
balanced equation.
Step-6
CH4 + O2  CO2 + 2H2O
No. of atoms in No. of atoms in
Element LHS RHS
C 1 1
H 4 2x2
O 2 3
Subtopic ID: 01.02.03

11. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Balanced Chemical Equations Write down the number of
atoms present on both sides
Step-7 of the arrow of the revised
equation. Again check
whether all the atoms are
balanced.
CH4 + O2  CO2 + 2H2O
Step-7
No. of atoms No. of atoms in
Element in LHS RHS
C 1 1
H 4 4
O 2 4
Subtopic ID: 01.02.03

12. Chemical Reactions and Equations
Representing a Chemical Reaction
Balanced Chemical Equations Balance the atoms of the
remaining elements, here
Step-8 oxygen.
CH4 + O2  CO2 + 2H2O
Step-7
No. of atoms No. of atoms in
Element in LHS RHS
C 1 1
H 4 4
O 2x2 4
Subtopic ID: 01.02.03

13. Chemical Reactions and Equations
Representing a Chemical Reaction
Balanced Chemical Equations Write down the balanced
equation.
Step-9
CH4 + 2O2  CO2 + 2H2O
No. of atoms No. of atoms in
Element in LHS RHS
C 1 1
H 4 4
O 2x2 4
Subtopic ID: 01.02.03

14. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Symbolic Equation with Physical States 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)
g – gaseous state
l – liquid state
Subtopic ID: 01.02.04

15. CBSE - 10 Chemical Reactions and Equations
Chemistry Representing a Chemical Reaction
Symbolic Equation with Physical States 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)
= Heat
Subtopic ID: 01.02.04