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Introduction
Chemical reaction is a process in which
one or more reactants react to produce one or
more products.
Concept Ladder
Type of Chemical reaction
(a) Irreversible Reactions Physical changes can
(b) Reversible reactions also be categorised into
Reversible and Irreversible
(a) Irreversible Reaction changes.
Unidirectional reactions are known as
irreversible reactions. i.e. reactants convert
to produce products and where products
cannot convert back to the reactants.
Ex.
(1) BaCl2(aq) + H2SO4(aq)
→ BaSO4(ppt)
+ 2HCl(aq)
(2) 2Mg(s) + O2(g) → 2MgO(s)
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(b) Reversible Reaction
Reversible reactions occur in both forward
Does a physical change
and backward directions and therefore
accompain a chemical change?
never go on to completion.
Ex.
(1) N2 (g) + O2 (g)
2 NO (g)
(2) 3 Fe (s) + 4 H2O (g)
Fe3O4 (s) + 4 H2 (g)
Note :
For any reversible reaction, container should
be closed.
Equilibrium
It is defined as the state when measurable
properties such as position speed, temperature,
concentration and pressure do not change with Concept Ladder
time.
In mechanical equilibrium,
Equilibirum can be of a no. of types.
the net forces on an object
For example: physical equilibrium, thermal
is zero.
Chemical Equilibrium
equilibrium, mechanical equilibrium, chemical
equilibrium etc.
1.
, Thermal Equilibrium
It is that type of equilibrium where the
thermal energy between two or more substances
are equal in nature.
The above figure shows two objects A and B
which are far from each other and when the come
in contact then their thermal energies become
equal. So, they are in thermal equilibrium with
each other.
Concept Ladder
Physical Equilibrium
The type of equilibrium which develops Solid-Liquid-Gaseous
between different phases and there is no change equilibrium can be seen in
in chemical composition. In physical equilibrium, it’s three states. The point
there is the existence of same substance in at which these three exist
different physical states. for H2O is known as triple
(1) Solid-liquid equilibrium point of water. Triple point
H2O (solid)
H2O (liquid) of water is 273.15K
Rate of transfer of ice is equal to the rate of
freezing of water
(2) Liquid-vapour equilibrium
Chemical Equilibrium
Rate of vaporization = Rate of condensation,
H2O (liquid)
H2O (vapour)
2.
,Conditions necessary for a Liquid-Vapour
Equilibrium
(i) The system must be a closed system. Concept Ladder
(ii) The system must be at a constant
temperature. For any pure liquid at 1 atm
(iii) The visible properties of the system should pressure, the temperature
not change with time. at which the liquid and
(3) Solid-vapour equilibrium vapor are at equilibrium is
Certain solid substances on heating get called normal boiling point
converted directly into vapour without of liquid.
passing through the liquid phase. This
process is called sublimation. The vapour
when cooled, gives back the solid, it is called
deposition or desublimation or sublimation.
Heat
NH4Cl (solid)
NH4Cl (vapour)
For example, Ammonium chloride when.
heated sublimes.
(4) Equilibrium between a solid and its solution
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If a solid solute is in contact with the
saturated solution, there exits a dynamic
equilibrium between the solid and the When a solution becomes
solution phase. supersaturated will you be able
Solid substance
Solution of the to observe equilibrium?
substance
Example : Sugar and sugar solution. In a
saturated solution, a dynamic is equilibrium
is established between dissolved sugar and
solid sugar.
Sugar (solid)
Sugar (aqueous)
Rate of dissolution on of solid sugar =Rate
of precipitation of sugar from the solution. Concept Ladder
At the equilibrium state, the no. of sugar
molecules going into the soln from the The number of precipitating
solid sugar is equal to the no. of molecules molecules at equilibrium
precipitating out from the soln, i.e., at state is equal to the number
equilibrium. of molecules dissolving in
Chemical Equilibrium
(5) Equilibrium between a gas & its solution in the solution.
a liquid.
Gases mixed in liquids. The dependent
factors for solubility of a gas in any liquid
are :
3.
, (a) Nature of the gas and liquid.
(b) Temp. of the liquid.
(c) Pressure (P) of the gas over the surface Concept Ladder
of the solution.
This equilibrium is defined by Henry’s law, When the condition is
which explains that the mass of a gas mixed changed at equilibrium
in a known mass of solvent at a temp. is state, the equilibrium
proportional to the solvent’s pressure of changes.
the given gas. It decreases with increases When temperature of a
of temperature. solution of gas is increased,
the rate of dissolution of
Chemical Equilibrium gas decreases.
In chemical equilibrium, concentration of
the reactants and products do not change with
time and the system does not show any further
change in properties.
In case of chemical equilibrium, rate of forward
reaction becomes equal to the rate of backward
reaction. i.e. Rate (forward reaction) = Rate
(backward reaction) Rack your Brain
Forward reaction (In an open glass container)
3 Fe (s) 4 H2O (g)
3 Fe (s) 4 H2O (g) Equilibrium state is dynamic in
If the container tube is closed, both the reactions nature. Why it is not static?
take place simultaneously and hence reaction
become reversible.
3Fe (s) + 4H2O (g)
Fe3O4 (s) + 4H2 (g)
Other example may include
CaCO3 (s)
CaO (s) + CO2 (g)
Characteristics of Equilibrium State
y A reaction (or a process) is said to be
in equilibrium when the rate of forward
reaction (process) becomes equal to the
rate of backward reaction (process).
y An equilibrium is dynamic in nature and not
static i.e., even after equilibrium is attained,
Chemical Equilibrium
the forward as well as the backward
reaction take place but at equal speeds.
y A chemical equilibrium can be established
only if none of the products is allowed to
escape out.
4.
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