1)Chemical reaction— Chemical changes or
chemical reactions are the changes in
which one or more new substances are formed.
2)Chemical Equations –
Representation of a chemical reaction in terms of symbols and
formulae of the reactants and products is
known as chemical equation.
3)Balanced Chemical equations – The chemical equation in which the no. of atoms of different
elements is same on both sides of the arrow
is called balanced chemical equation.
4) The chemical
reactions can be classified into different types such as—
a)
Combination reaction – The reactions in
which two or more substances combine to form a new substance are called
combination reaction. For example,
2Mg(s) + O2 (g) à 2 MgO (s)
b) Decomposition reaction - The reaction in which a single compound breaks up into two or
more simpler substances are called decomposition reactions. For example,
2Pb (NO3)2
(s) à 2PbO (s) + 4NO2 (g) +O2
(g)
The
decomposition of a substance by passing electric current through it is known as
electrolysis (Electrical Decomposition).
The
decomposition of a substance on heating is known as thermal decomposition.
The
decomposition of a substance by absorbing light energy is called photochemical
decomposition.
c)
Displacement reactions -The chemical
reactions in which a more reactive element displaces a less reactive element
from a compound are known as displacement reactions. For example,
i)
Zn (s) + CuSO4 (aq) à ZnSO4
(aq) + Cu (s).
ii)
Cu (s) + 2AgNO3 (aq)
à Cu (NO3)2 (aq)
+2Ag (s).
d) Double Displacement
Reactions - The chemical reactions in which
compounds react to form two different compounds by mutual exchange of ions are
called double displacement reactions.
These reactions
take place in solution two common types of this reaction are precipitation
reactions and neutralization reactions
i)
Precipitation reaction : In this
reactions, aqueous solution of two salts are mixed whereby
Some salts precipitate
due to mutual exchange of ions between the two salts. For example
AgNO3 (aq)
+ NaCI (aq) -----> AgCl(s) + NaNO3.
ii)
Neutralization reaction: In this type of
reaction an acid reacts with a base to form salt and water by exchange of ions.
NaOH
(aq) + HCl (aq) à NaCl
(aq) + H2O.
e)
Redox reaction: Chemical reaction which
shows both oxidation and reduction reaction.
Oxidation: Reaction that involves the gain of oxygen or loss of hydrogen.
Reduction: Reaction that shows the loss of oxygen or gain of hydrogen.
Both oxidation and reduction take place simultaneously and hence called
redox reaction.
ZnO +
C à Zn +CO
ZnO reduce to Zn ---- reduction
C oxidize to CO ------oxidation
f)Exothermic
reaction and endothermic reaction: On the basis of energy changes during
chemical
reaction, they can be classified as
i)
Exothermic reaction: A chemical reaction
in which heat energy is produced.
C + O2à CO2 (g) + heat
ii)
Endothermic reaction: A chemical
reaction in which heat energy is absorbed.
CaCO3 + Heat à CaO + CO2
5 Corrosion
– The process of slow conversion of metals into
their undesirable compounds due to their reaction with oxygen, water, acids,
gases etc. present in the atmosphere is called corrosion.
Rusting – Iron when reacts with oxygen and
moisture forms red substance called rust. Chemical composition of rust is Fe2O3.xH2O.
Its chemical name is Hydrated Iron (III) oxide.
6
Rancidity – The taste and odour of food materials
containing fat and oil changes when they are left exposed to air for long time.
This is called rancidity. It is caused due to oxidation of fat and oil present
in food material.
It can be prevented by using various
methods such as by adding antioxidants to the food materials. Storing food in
air tight container and by flushing out air with nitrogen.
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