NOTES
Cynesure Institute
Class 10
Chemical Reactions
and Equations
Cynesure institute
Dinesh
Maurya
Chemical Reaction:
A chemical reaction involves a chemical change in which substances react to form new
substances with entirely new properties.
Eg.
– Food gets digested in our body
– Rusting of iron
–Cooking of food
–Respiration in cells
Reactants and Products:
 Substances that react or take part in the chemical reaction are known as reactants.
 The substances formed after the chemical reactions are known as products.
 The arrow sign between the reactant and product shows the direction of reaction.
Observation of a Chemical Reaction:
A chemical reaction can be observed with the help of any of the following results:
a. Evolution of a gas
b. Change in temperature
c. Formation of a precipitate
d. Change in colour
e. Change of state
Physical And Chemical Change:
 Physical change: If a change involves change in colour or state but no new substance
is formed, then it is a physical change.
Eg. Growing of tree, turning ice into water, etc
 Chemical change: If a change involves formation of new substances, it is a chemical
change.
Eg. Rusting of iron, forming curd from milk, digestion of food, etc.
Chemical Reactions in everyday life:
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





Milk which left at room temperature during summers turns sour
An iron object is left exposed to humid atmosphere rusts
Grapes get fermented to form alcohol
Food is cooked
Food digested in our body
Respiration
Examples of Chemical reaction:
`
1. When magnesium ribbon is heated, it burns with a dazzling white flame and changes
into a white powder.
 This powder is magnesium oxide.
 Magnesium ribbon needs to be cleaned before burning to remove the layer of
magnesium oxide on it.
 It is formed due to the reaction between magnesium and oxygen present in the
air.
2. When sulfuric acid reacts with zinc, it releases hydrogen gas.
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Dinesh
Maurya
Chemical Equation:
The symbolic representation of a chemical reaction is called a chemical equation.
2H2
+
O2
2H20
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Reactant
Product
Features of a chemical equation:
a. The reactants are written on the left hand side with a plus sign between them.
b. The products are written on the right hand side with a plus sign between them.
c. An arrow separates the reactants from the products. The arrow head points
towards the products and indicates the direction of the reaction.
Balanced Chemical Equation:
A balanced equation is a chemical equation in which number of atoms of each
element is equal on both sides of the equation.
i.e. Number of atoms of an element on reactant side = Number of atoms of that
element on the product side.
Law Of Conservation Of Mass:
The total mass of the elements present in the products of a chemical reaction is
equal to the total mass of the elements present in the reactants.
Balancing of a chemical equation:
The process of equating the number of atoms on both the sides of a chemical
equation is known as balancing of a chemical equation.
 The method of balancing chemical equations is called also hit-and-trial method.
Rule of sign used in a chemical reaction:
a)
b)
c)
d)
The evolution of gas is indicated by an upward arrow.
The formation of precipitate is indicated by a downward arrow.
Heat evolved during the reaction is written as + Heat on the product side.
Heat absorbed during the reaction is written as + Heat on the reactant side.
Symbols of Physical States of Reactants and Products and Energy Required:
 The physical states of the reactants and products are mentioned along with their
chemical formulae.
 gaseous, liquid, aqueous and solid states of reactants and products are represented by
the notations (g), (l), (aq) and (s), respectively.
 The word aqueous (aq) is used when there is mixture of gas and liquid.
3Fe(s)
+
4H2O(g)
Fe3O4(s)
+
4H2(g)
 Sometimes the reaction conditions such as temperature, pressure, catalyst, etc., for the
reaction are indicated below the arrow in the equation.
***
TYPES OF CHEMICAL REACTIONS:
There are six types of chemical reactions as follows:
1. Combination reaction
2. Decomposition reaction
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Dinesh
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3.
4.
5.
6.
Displacement reaction
Oxidation
Reduction
Redox reaction
1. Combination reaction is a reaction in which two or more substances combine to give a
single product.
Combination reaction can be between two elements, between an element and a
compound or between two compounds.
Example:
CaO +
H2O
Quick Lime
Ca(OH)2
Slaked Lime
2H2 +
Hydrogen
2H2O
Water
O2
Oxygen
 Exothermic reactions: If heat is evolved during a reaction, then such a reaction is
known as Exothermic reaction. All combination reactions are exothermic reactions.
CH4
+ O2
CO2 + H20 + Heat
 During digestion, food is broken down into simpler substances.
 For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are
broken down to form glucose.
 This glucose combines with oxygen in the cells of our body and provides energy.
 The special name of this reaction is respiration.
C6H12O6
Glucose
+
6O2
→
6CO2 +
6H2O +
Energy
 The decomposition of vegetable matter into compost is also an example of an exothermic
reaction.
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2. Decomposition reaction: In a decomposition reaction, a single reactant decomposes to
give two or more products.
Decomposition reactions require energy in the form of heat, light or electricity.
Types of decomposition reactions:
a.
Decomposition reactions which require heat are known as thermolytic
decomposition reactions
2Fe SO4
Ferrous Sulphate
b.
Fe2O3 +
Ferric Oxide
Decomposition reactions which
decomposition reactions.
AgCl2
Silver Chloride
c.
Heat
Sunlight
require
light
Ag
Cl2
+
SO2
are
+
known
SO3
as
photolytic
Decomposition reactions which require electricity are known as electrolytic
decomposition reactions
Electric Current
H20
H2 + O2
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 Endothermic reaction. If energy is absorbed from the surroundings, then such a
reaction is known as endothermic reaction. All decomposition reactions are
endothermic.
CaCO3
∆
CaO
+ CO2
Here, ∆ shows heat given.
3. Displacement reaction: A reaction in which a more active element displaces less active
element from its salt solution.
Fe + CuSO4
FeSO4
Copper Sulphate
Iron Sulphate
Here, Fe is a more reactive metal than copper.
Example :
+
Cu
Double displacement reaction: A chemical reaction in which there is an
exchange of ions between the reactants to give new substances is called double
displacement reaction.
Example:
Na2SO4
+ BaCl2
BaSO4
+ 2NaCl
Sodium Sulphate
Barium Chloride Barium Sulphate
Sodium
Chloride
Reactivity series is a list of metals arranged in the order of decreasing reactivity.
The most reactive metal is placed at the top and the least reactive metal is placed at the
bottom.
Precipitation reaction: An insoluble solid known as precipitate is formed during
a double displacement reaction. Such reactions are also known as precipitation
reactions.
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4. Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen.
2Cu + O2
2CuO
Copper is getting oxidized
 If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidised.
5. Reduction is a chemical process in which a substance gains hydrogen or loses oxygen.
CuO + H2
Cu + H2O
Copper Oxide is reduced
If a substance gains hydrogen or loses oxygen during a reaction, it is said to be
reduced.
Note: In terms of electronic concept, Oxidation is defined as a loss of electrons while
reduction is defined as a gain of electrons.
6. Redox reaction: A reaction in which oxidation and reduction take place simultaneously in
a reaction, is known as a redox reaction.
 A substance that loses oxygen or gains hydrogen is known as an oxidizing agent.
 A substance that loses hydrogen or gains oxygen is known as a reducing agent.
 An oxidising agent gets reduced whereas a reducing agent gets oxidised.
CuO
+
Fe
Fe2O3
+ Cu
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Here in this chemical reaction, Cu is the oxidizing agent and Fe is reducing agent. Also,
Fe is getting oxidized and Cu is getting reduced.
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CORROSION:
Corrosion is the slow eating up of metals by the action of air and moisture on their
surfaces.
Corrosion in case of Iron is known as Rusting.
Chemically, rust is hydrated ferric oxide (Fe2O3.xH2O)
Advantages of corrosion: Though corrosion is undesirable, it can be advantageous in
case of aluminium which on exposure to air, gets coated with a protective layer of
aluminium oxide. This protects the metal underneath from further corrosion and
damage.
***
Rancidity: When oils and fats or foods containing oils and fats are exposed to air, they
get oxidised due to which the food becomes stale and gives a bad taste or smell. This is
called Rancidity.
Rancidity can be prevented by:
a) Adding antioxidants i.e. the substances which prevent oxidation
b) Refrigeration
c) Storing the food in air-tight containers
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