Chemistry
By: HIMANSHU SIR
Ch 1.
Chemical equations and reactions
1. Chemical
reactions- Chemical changes or chemical reactions are the changes in
which one or more new substances are formed. In simple words or rather in one
word a Chemical reaction simply represents "REARRANGEMENT ". The new product formed after a chemical
reaction is having entirely different set of properties .
2. Chemical
Equations – Representing a chemical reaction in terms of
symbols and formulae of the reactants and products is known as chemical
equation.eg:- C + O2
--> CO2
3. Balanced
Chemical equation – 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.
or in
simple words the number of atoms on reactant side should be equal to that on
the product side . It can be correlated to a beam balance in our daily life
having equal weight on both sides of it
.
CLASSIFICATION OF
CHEMICAL REACTIONS
(a) Combination
reactions- The reactions in which two or more substances combine to
form a new single product or substance
such reactions are called as combination reactions. For example,
A
+ B --> AB
(where A and B are two reactants and AB is the single new product formed)
2Mg(s) + O2 (g) → 2 MgO (s)
(b) Decomposition reactions – The reactions in
which a single reactant breaks/splits up into two or more simpler substances
are called decomposition reactions. For example,
ABC
--> A +
B + C
(where A , B ,
C are the new products formed )
2Pb (NO3)2
(s) → 2PbO(s) + 4NO2
(g) + O2 (g)
SUB DIVISIONS OF
DECOMPOSITION REACTION
THERMAL DECOMPOSITION : When decomposition of a substance
takes place on heating it is known as
thermal decomposition. eg :
ELECTROLYTIC DECOMPOSITION
--> The decomposition of a substance by passing electric current
through it is known as Electrolytic
Decomposition or Electrolysis. eg :
For
example, when electric current is passed through acidified water, it decomposes
into hydrogen and oxygen. Electric current
2H2O (l) → 2H2 (g) +
O2 (g)
NOTE : As Water molecule contains Hydrogen and Oxygen in the
ratio of 2:1 therefore on electolytic decomposition it gives double the volume
of Hydrogen in comparison to Oxygen.
PHOTOLYTIC
DECOMPOSITION :( Photo means light and lytic means breaking
) Certain
compounds when exposed to light undergo decomposition. For example, Silver
chloride when exposed to sunlight turns grey due to its decomposition into
silver and chlorine. Sunlight 2AgCl(s) → 2Ag(s) + Cl2 (g)
2AgBr(s)
→ 2Ag(s) + Br2 (g)
The decomposition of a substance by absorbing light energy
is called photolysis or photochemical decomposition.
NOTE : The reaction of photolytic decomposition of AgBr is
used in Black and White Photography .
(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) → ZnSO4 (aq) + Cu(s)
(ii) Cu(s) + 2AgNO3 (aq) → 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 double displacement reactions are precipitation
reactions and neutralization reactions.
(i) Precipitation reaction : In
precipitation reactions, aqueous solution
of two salts are mixedwhereby some salts precipitate due to mutual
exchange of ions between the two salts. For example
AgNO3 (aq)
+ NaCl (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.The nature of
salt formed depends upon the extent of neutralization that is both acids and
bases are atrong than they result neutral salt .eg:
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O
(e) Redox reaction :(Red-Reduction
+ Ox - Oxidation RedOx)
If a
substance gains oxygen or looses hydrogen during a reaction it is oxidized. On
the other hand if a substance loses oxygen or gains hydrogen during a reaction,
it is reduced. Both oxidation and reduction take place simultaneously and hence
these are called redox reaction.
Oxidation
ZnO + C → Zn + CO
Reduction Oxidation
Cl2 + H2S → 2HCl + S
Reduction
(f) Exothermic reaction and endothermic reaction : On
the basis of energy changes reaction can be classified as
(i) Exothermic reaction-( Hint : Exo means Exit to moive out ) A chemical
reaction in which heat energy is evolved or produced is called exothermic
reaction .
For e.g. (i) C (s) + O2 → CO2 (g) + Heat
(ii) Endothermic reaction- A chemical reaction in
which heat energy is absorbed is called endothermic reaction. CaCO3 +
Heat → CaO + CO2
5. Effects of oxidation reaction in everyday life:
(a) Corrosion – The process of damage of the upper layer of the
metal due to the action of air , water , moisture or acids is known as
corrosion .
OR
The
process of slow conversion of metals into their undesirable compounds due to
their reaction with moisture and other gases present in the atmosphere is
called corrosion of metals.
Some
common examples of corrosion are rusting of iron, formation of green layer on the surface of copper,
tarnishing of silver etc.
Formation
of brown layer on the surface of iron due to action of the above mentioned
factors on the surface of iron is called rusting of iron. Formula of Rust is Fe2O3.xH2O.
Rusting of iron is a serious problem.
METHODS TO PREVENT RUSTING / COROSION
1) GALAVANIZATION --THe coating of layer of Zn
on the surface of Iron which helps to prevent it from rusting .
2) By
Paint .
(b) Rancidity- The slow oxidation of food containg fat and oil by the
action of air is known as rancidity .The taste and odour of food materials
containing fat and oil changes when they are left exposed to air for a long
time. This is known as rancidity.
Rancidity can be prevented by using various methods such as
by adding antioxidants to the food materials Storing food in air tight container
By
Flushing out air with some inert gases like nitrogen.
