Mirakle puhan sc.

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 Chemical Reactions are associated with chemical
change.
 Whenever a chemical change occurs chemical
reaction is said to take place.
 Chemical reactions are usually irreversible and a
new product is formed.

Signs of Chemical Reactions
There are five main signs that indicate a chemical reaction has taken place:
change in color Evolution of a gas Change in temperature Change in state

 A WORD EQUATION describes chemical change
using the names of the reactants and products.
Write the word equation for the reaction of methane gas
with oxygen gas to form carbon dioxide and water.
methane + oxygen
Reactant Product
CH4
O2 CO2 H2O+ + 22
carbon dioxide + water

 The simplest form of description of a
chemical reaction in a shorter form is by
writing it in the form of an equation. E.g.-
Mg + O2 MgO
 The reactants are written on the left side
and product is written on the right side.
 An arrow is placed between them to
show the direction of reaction.

 The equation must represent known
facts.
 The equation must contain the correct
formulas for the reactants and
products.
 The law of conservation of mass must
be satisfied.

Showing Phases inShowing Phases in
Chemical EquationsChemical Equations
Solid Phase – the substance is relatively rigid and has a definite
volume and shape. NaCl(s)
Liquid Phase – the substance has a definite volume, but is able
to change shape by flowing. H2O(l)
Gaseous Phase – the substance has no definite volume or shape,
and it shows little response to gravity. Cl2(g)
H2O(s) H2O(l) H2O(g)

• According to the law of conservation of mass, total
mass must be equal on the both sides of the
equation.
• This type of equation is known as a balanced
chemical reaction. E.g.-
2Mg + O2 2MgO
• Here both sides have two atoms of Magnesium and
two atoms of Oxygen.

Cl
Cl
Cl
H
H
H
ClClCl
Cl HH
H
H
H2 + Cl2  HCl H2 + Cl2  2 HCl
reactants products
H
Cl
reactants products
H
Cl
2
2
2 2
2 2
1
1
(unbalanced) (balanced)
Unbalanced and Balanced Equations

Reactants Products
1 C atom 1 C atom
4 H atoms 4 H atoms
4 O atoms 4 O atoms

?
Visualizing a Chemical Reaction
Na + Cl2 NaCl
___ mole Cl2 ___ mole NaCl___ mole Na
2
10 5 10
2
10 5 10

1. Combination reaction
2. Decomposition reaction
3. Single-displacement reaction
4. Double-displacement reaction
A + B  AB
AB  A + B
A + BC  AC + B
AB + CD  AD + CB
5. Redox reaction

 These type of reactions occur when two
reactants combine to form one or more
products. E.g. –
1. CaO + H2O Ca(OH)2
2. C + O2 CO2
 They are generally exothermic reactions which
involve evolution of heat during reaction.

Combination Reaction
General form: A + B  AB
element or element or compound
compound compound
Na
ClNa
Cl
2 Na + Cl2  2 NaCl

ClNa
Na
Cl

Decomposition Reaction
• The types of reaction in which a single reactant
breaks down to give simpler products are called
decomposition reaction. E.g.-
• When a decomposition reaction is carried out by
heating, it is known as thermal decomposition.
2 H2O 2 H2 + O2

H
H
H
H
H
H
H
H
O
Decomposition reaction
2 H2O 2 H2 O2
General form: AB A B
+
+
+
compound two or more elements
or compounds
OO
O

H2O H2 + O2
electricity
H2O2 H2O + O2
NI3 N2 + I2
2 2
Hydrogen Peroxide
Electrolysis of water
2 2
Nitrogen triiodide
AB A + B
General Form
2 3

• The reaction in which a more reactive
element displaces a less reactive element
from its solution is known as displacement
reaction.
• They are of two types:-
1.Single Displacement Reaction
2.Double Displacement Reaction

Double-replacement reaction
CaCO3 + 2 HCl  CaCl2 + H2CO3
General form:
AB + CD  AD + CB
Single-replacement reaction
Mg + CuSO4  MgSO4 + Cu
General form:
A + BC  AC + B

PbI2
K1+
Pb2+
lead (II) chloride + potassium iodide
Cl2
I
Pb2+
Pb
K1+
K
Cl1-
I1-
Cl1-
I1-
KCl
potassium chloride lead (II) iodide+
(aq) (ppt)
+ +

Double Displacement Reaction
K2CO3 (aq)
Potassium carbonate
BaCl2 (aq)
Barium chloride
2 KCl (aq)
Potassium chloride
BaCO3 (s)
Barium carbonate
+ +

Ca
Foiled again –Foiled again –
Aluminum loses to CalciumAluminum loses to Calcium
Element Reactivity

• If a substance gains oxygen during a reaction, it is
said to be oxidised.
• If a substance loses oxygen during a reaction, it is
said to be reduced.
• Reactions in which this type of change occurs is
known as Oxidation and Reduction reactions or
Redox reactions. E.g. –
CuO + H2 Cu +H2O

PRESENTED BY :- MIRAKLE PUHAN
10 `B’
Roll no. 25
SPECIAL THANKS TO SIR SURESH CHANDRA DAS

Mirakle puhan sc.

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