Chemical Reactions and Equations
Introduction to Chemical Reactions and Equations
Physical and chemical changes
Chemical change - one or more new substances with new physical and chemical properties
Example: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
are formed.
(Blue) (Green)
Here, when copper sulphate reacts with iron, two new substances, i.e., ferrous sulphate and
copper are formed.
Physical change - change in colour or state occurs but no new substance is formed.
Example: Water changes to steam on boiling but no new substance is formed(Even though
steam and water look different when they are made to react with a piece of Na, they react
the same way and give the exact same products). This involves only change in state (liquid
to vapour).
Observations that help determine a chemical reaction
A chemical reaction can be determined with the help of any of the following observations:
a) Evolution of a gas
b) Change in temperature
c) Formation of a precipitate
d) Change in colour
e) Change of state
Chemical reaction
Chemical reactions are chemical changes in which reactants transform into products by
making or breaking of bonds(or both) between different atoms.
Types of chemical reactions
Taking into consideration different factors, chemical reactions are grouped into multiple
categories.
Chemical Reactions and Equations Page 1
,Chemical Reactions and Equations Page 2
, categories.
Few examples are:
● Combination
● Decomposition
● Single Displacement
● Double displacement
● Redox
● Endothermic
● Exothermic
● Precipitation
● Neutralisation
Chemical Reactions and Equations I
Word equation
A word equation is a chemical reaction expressed in words rather than chemical
formulas. It helps identify the reactants and products in a chemical reaction.
For example,
Sodium + Chlorine → Sodium chloride
The above equation means: "Sodium reacts with chlorine to form sodium chloride."
Symbols of elements and their valencies
A symbol is the chemical code for an element. Each element has one or two letter atomic
symbol, which is the abbreviated form of its name.
Valency is the combining capacity of an element. It can be considered as the number of
electrons lost, gain or shared by an atom when it combines with another atom to form a
molecule.
Writing chemical equations
Representation of a chemical reaction in terms of symbols and chemical formulae of the
reactants and products is known as a chemical equation.
Zn(s) + dil.H2SO4(aq) → ZnSO4(aq) + H2(↑)
(Reactants) (Products)
• For solids, the symbol is "(s)".
• For liquids, it is "(l)".
• For gases, it is "(g)".
• For aqueous solutions, it is "(aq)".
• For gas produced in the reaction, it is represented by "(↑)".
• For precipitate formed in the reaction, it is represented by "(↓)".
Balancing of a Chemical Reaction
Conservation of mass
According to the law of conservation of mass, no atoms can be created or destroyed in a
chemical reaction, so the number of atoms for each element in the reactants side has to
balance the number of atoms that are present in the products side.
In other words, the total mass of the products formed in a chemical reaction is equal to the
total mass of the reactants participated in a chemical reaction.
Balanced chemical equation
The chemical equation in which the number of atoms of each element in the reactants side
is equal to that of the products side is called a balanced chemical equation.
Steps for balancing chemical equations
Hit and trial method: While balancing the equation, change the coefficients (the numbers in
front of the compound or molecule) so that the number of atoms of each element is same
on each side of the chemical equation.
Short-cut technique for balancing a chemical
equation
Chemical Reactions and Equations Page 3
Introduction to Chemical Reactions and Equations
Physical and chemical changes
Chemical change - one or more new substances with new physical and chemical properties
Example: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
are formed.
(Blue) (Green)
Here, when copper sulphate reacts with iron, two new substances, i.e., ferrous sulphate and
copper are formed.
Physical change - change in colour or state occurs but no new substance is formed.
Example: Water changes to steam on boiling but no new substance is formed(Even though
steam and water look different when they are made to react with a piece of Na, they react
the same way and give the exact same products). This involves only change in state (liquid
to vapour).
Observations that help determine a chemical reaction
A chemical reaction can be determined with the help of any of the following observations:
a) Evolution of a gas
b) Change in temperature
c) Formation of a precipitate
d) Change in colour
e) Change of state
Chemical reaction
Chemical reactions are chemical changes in which reactants transform into products by
making or breaking of bonds(or both) between different atoms.
Types of chemical reactions
Taking into consideration different factors, chemical reactions are grouped into multiple
categories.
Chemical Reactions and Equations Page 1
,Chemical Reactions and Equations Page 2
, categories.
Few examples are:
● Combination
● Decomposition
● Single Displacement
● Double displacement
● Redox
● Endothermic
● Exothermic
● Precipitation
● Neutralisation
Chemical Reactions and Equations I
Word equation
A word equation is a chemical reaction expressed in words rather than chemical
formulas. It helps identify the reactants and products in a chemical reaction.
For example,
Sodium + Chlorine → Sodium chloride
The above equation means: "Sodium reacts with chlorine to form sodium chloride."
Symbols of elements and their valencies
A symbol is the chemical code for an element. Each element has one or two letter atomic
symbol, which is the abbreviated form of its name.
Valency is the combining capacity of an element. It can be considered as the number of
electrons lost, gain or shared by an atom when it combines with another atom to form a
molecule.
Writing chemical equations
Representation of a chemical reaction in terms of symbols and chemical formulae of the
reactants and products is known as a chemical equation.
Zn(s) + dil.H2SO4(aq) → ZnSO4(aq) + H2(↑)
(Reactants) (Products)
• For solids, the symbol is "(s)".
• For liquids, it is "(l)".
• For gases, it is "(g)".
• For aqueous solutions, it is "(aq)".
• For gas produced in the reaction, it is represented by "(↑)".
• For precipitate formed in the reaction, it is represented by "(↓)".
Balancing of a Chemical Reaction
Conservation of mass
According to the law of conservation of mass, no atoms can be created or destroyed in a
chemical reaction, so the number of atoms for each element in the reactants side has to
balance the number of atoms that are present in the products side.
In other words, the total mass of the products formed in a chemical reaction is equal to the
total mass of the reactants participated in a chemical reaction.
Balanced chemical equation
The chemical equation in which the number of atoms of each element in the reactants side
is equal to that of the products side is called a balanced chemical equation.
Steps for balancing chemical equations
Hit and trial method: While balancing the equation, change the coefficients (the numbers in
front of the compound or molecule) so that the number of atoms of each element is same
on each side of the chemical equation.
Short-cut technique for balancing a chemical
equation
Chemical Reactions and Equations Page 3
