CHEMICAL REACTIONS AND EQUATIONS.pdf

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CHEMICAL
REACTIONS AND EQUATION
S


DIFFERENCE BETWEEN PHYSICAL CHANGE AND CHEMICAL CHANGE

Physical Change

Chemical Chan
ge

(i)

Those changes in which no new
substances are formed are called
physical changes

(i)

Those changes in which the original
substances lose their chemical nature
and identity and form new chemical
substances with different properties
are called chemica
l changes

(ii)

It is a temporary change

(ii)

It is a permanent change

(iii)

It is easily reversible

(iii)

It is usually irreversible

(iv)

In a physical change the mass of
substance does not alter

(iv)

In a chemical change the mass of the
substance does
alter



CHEMICAL REACTION
:

The process by which two or more chemical substances
interact to form some new substances with different chemical properties.

Chemical reactions
are represented by balanced chemical equations with the help of chemical symbols of

the
substances involved.


Characteristics of chemical reactions
:

The easily observable changes that take
place in a chemical reaction are called characteristics of chemical reaction.


•

Formation of Precipitate
: Example




AgNO
3

(aq) + NaCl(aq)
→

AgCl (pp
t) + NaNO
3

(aq)


•

Evolution of gas : Example




Z
n(s) +
dil
HCl
→

Z
nCl
2

+

2
H


•

Change in Colour: Example




Pb(NO
3
)
2
(aq) + KI(aq)
→

(
)
(aq)
KNO
ppt
PbI
3
2
+









Yellow


•

Change in state: Example




Solid wax burns

to form water vapour and
c
arbon dioxide.


•

Change in temperature: Example




CaO + H
2
O
→

Ca(OH)
2

+ Heat
(Exothermic)




(
)
OH
4
NH
2
BaCl
Heat


Cl
4
NH
2
OH
Ba
+
→
+
+
(Endothermic)


Types of Chemical Reactions

Combination
reactions

Decomposition
reactions

Displacement
reactions

Double displacement
reactions

Oxidation
–
Reduction
(Redox) Reactions




Type
s

of Chemical Reactions:


(i)

Addition or combination reaction:

Two or more substances combine to form a
single substance.






CaO + CO
2

⎯
→

CaCO
3


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(ii)

Decomposition reaction:

One chemical substance splits to give two or more
substances either by heat energy (Thermolysis) or light (Photolysis) or by electricity
(E
lectrolysis).

Thermolysis

3
2
2
3
NO
NO
PbO
)
NO
(
Pb
+
+
⎯
→
⎯


Electrolysis

−
+
+
⎯
⎯
⎯
→
⎯
OH
H
O
H
current
electric
2

Photolysis

2AgCl
⎯
→

2Ag + Cl
2


(iii)

Displacement Reaction:

More reactive element displaces less reactive element
from its compound or salt.

Fe + CuSO
4

⎯
→

FeSO
4

+ Cu


(i
v)

Double Displacement Reaction:

Two elements interchange their respective
salts or ions to form new compounds.

FeCl
3

+ CuSO
4

⎯
→

FeSO
4

+ CuCl
2



(v)


(a)

Oxidation

(b)

Reduction


→

addition of oxygen or


→

removal of hydrogen or


→

loss of electrons

Natur
al oxidation in everyday life:


→

Corrosion

& Rancidity



→

loss or removal of oxygen


→

gain or addition of hydrogen


→

gain of electrons

Natural reduction in everyday life
:


→

Hydrogenation




Redox Reactions

:
Both oxidation and
reduction taking place
simultaneously in the
same chemical reaction.




Oxidizing agent :
A

substance which brings about the oxidation of other substance
but itself gets reduced in a reaction.




Reducing agent :

A substance which brings about reduction of the other substance
bu
t itself gets oxidized in a reaction



Some examples of redox reactions :

(a)


Zn + Cu
2+
Zn
2+

+ Cu

Oxiding

Gain of electrons : Reduction

agent

Loss of electrons : oxidation

Zn + CuSO
4
ZnSO
4

+ Cu

Reducing

agent




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(b)


Cu + 2Ag
+
Cu
2+

+ 2Ag

Reducing

Loss of electrons : Oxidation

agent

oxidising

agent

Gain of electrons : Reduction

Cu + 2AgNO
3
Cu(NO
3
)
2

+ 2Ag





(c)



Mn
4+
+ 2Cl
–
Mn
2+
+ Cl
2

oxidising

Gain of electrons : Reduction

agent

Reducing

agent

Loss of electrons : oxidation

MnO
2
+ 4HCl MnCl
2
+ Cl
2
+ 2H
2
O






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