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Chemical Equilibrium
Major topic:
1
...
(understanding)
...
(applying):
Define law of mass action
...
(applying):

State the necessary condition for the
equilibrium and the ways that equilibrium can
be recognized(understanding) and
Write the equilibrium constant expression of
a reaction
...

A complete reaction is one in which in all
reactants have been converted into products
...
As a result,
after sometime no further change take place
...
in facts, these reactions do not stop;
rather they take place on both directions at

equal rate and attain the equilibrium state
...

Many examples of physical and chemical
equilibrium are found in nature
...

We inhale oxygen and exhale carbon dioxide,
while plants consume carbon dioxide and
release oxygen
...

Many environmental systems depend for
their existence on delicate equilibrium
phenomenon
...
The lives of aquatic
plants and animals are indirectly related to
concentration of dissolved oxygen in water
...
For example, when H₂
and O₂ (reactants) combine they from H₂O
(product)
...
They are supposed to complete and are
represented by putting a single arrow (→) between
the reactants and products
...
These reactions never go to
completion
...


These reactions proceed in both ways i
...
, they
consists of two reactions; forward and reverse
...

Let us discuss a reaction between hydrogen and
iodine
...
On heating, hydrogen and iodine
Vapours in a closed flask, hydrogen iodide is
formed, As a result, purple colour iodine fedes as it
reacts to form colourless hydrogen iodide
...

On the other hand, when only hydrogen iodide is
heated in a close flask, purple colour appears
because of formation of iodine vapours
...
This
reaction is reverse of the above
...

When both of these reactions are written together
as reversible reactions, they are represented as:
H₂ (g) + I₂(g)

»»»»

2HI (g)

««««
(Above symbol »«show double arrow)

Let us have another example, when calcium oxide
and carbon dioxide react, they produce calcium
carbonate
...
CO₂ escape out and reaction
goes to completion
...
When calcium carbonate
is heated in a closed flask, so that CO₂ can’t escape
out as
...
But eventually, the
reverse reaction speeds up and both reactions go
on at the same rate
...
It is written as
...


Thus when the rate of the forward reaction
take place at the rate of reverse reaction, the
composition of the reaction mixture remains
constant, it is called a Chemical equilibrium
state
...

(i)

(ii)

When reaction cases to proceed, it is
called static equilibrium
...
For
example a building remains standing
rather than failing down because all the
forces acting on it are balanced
...

When reaction does not stop, only the
rates of forward and reverse reactions
become equal to each other but take
place in opposite directions, this is
called dynamic equilibrium state
...

Rate of forward reaction = Rate of
reverse reaction
It is reversible reaction; dynamic
equilibrium is established before the
completion of reaction
...
But gradually forward
reaction slows down and reverses
reaction speeds up
...


For example in case of reaction between hydrogen
and iodine vapours, some of the molecules react
with each other to give hydrogen iodide
...

2HI(g) →→→ H₂(g) + I₂(g)

in the beginning, as the concentration of the
reactants is higher than that of the products, the
rate of the forward reaction is fast than the reverse
reaction
...
Thus, the equilibrium will set
up and concentration of various species(H₂,I₂,HI)
becomes constant, it is represented as
H₂(g) +I₂ double arrow 2HI(g)
Macroscopic Characteristic of forward and reverse
reactions
...
It is a reaction in which
reactants react to form
products
...
It takes place from left

Reverse Reaction
1
...

2
...
At initial stage, the rate
of forward reaction is
very fast
...
It slows down gradually

to left
3
...

4
...


Macroscopic Characteristics of Dynamic equilibrium
A few important characteristic features of dynamic
equilibrium are given below,
An equilibrium is achievable inly in a closed
system(in which substances can neither leave
nor enter
At equilibrium state a reaction does not stop
...
Even physical properties like
colour, density, etc remain the same

An equilibrium state is attainable from either
way, i
...




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