Spontaneous process A spontaneous
process is one which is feasible and takes place by itself and has an urge to
take place, i.e. takes place when it is property initiated. Spontaneous
reactions are irreversible.
Thus, two types of spontaneous processes are:
(I) which occur without initiation. Examples are: (I) Evaporation of water in an open vessel. (ii) Dissolution of salt in water etc. (ii)Which require initiation. Examples are (I) carbon is ignited in oxygen
Thus, two types of spontaneous processes are:
(I) which occur without initiation. Examples are: (I) Evaporation of water in an open vessel. (ii) Dissolution of salt in water etc. (ii)Which require initiation. Examples are (I) carbon is ignited in oxygen
(II) H2 and O2 combine
when electric discharge is passed through the mixture.
Non - Spontaneous process
It
is define as the one which has no tendency to occur and proceeds when energy is
continuously supplied from outside.
Examples.
Decomposition of water in to H2 and O2 is
non-spontaneous
Enthalpy change and spontaneity
In most of the spontaneous reactions ∆H is negative. It may be noted that a spontaneous reaction has an urge
to proceed in a particular direction under a given set of conditions. Thus,
there is some kind of force which drags the process in a particular direction.
To know this driving force, consider a few examples.
(I)
C(s) + O2(g) Ã CO2(g)
(II)
N2(g)+3H2(g)Ã 2NH3(g)
From
these reactions, we see that system moves towards lower energy. Lower the energy more is the stability. Hence,
only exothermic reactions (∆H= -ve) should be spontaneous.
But
it is not fully true. We know that evaporation of water, dissolutions of salt
etc. Are spontaneous but proceed with the absorption of heat energy.
Spontaneous and Randomness
Processes for which ∆H is +ve are
studied and are found to proceed towards more randomness or disorderly state.
It may note that every system has a tendency to attain a state of maximum
stability and disorder or more random state is more stable.
Hence, an endothermic reaction can
be spontaneously only if it proceeds with the net increase in randomness
Factors on which Randomness depends
(i)
Physical state: Randomness increases when a system changes
from solid to the gaseous state. The randomness increases as: Solids<
Liquids< Gases
(ii)
Temperature: The increase in
temperature increases the K.E and hence randomness increases.
(iii) Pressure: Gases at low pressure have more randomness.
Entropy (S)
It is a thermodynamic property of a
system which measures the degree of disorder or randomness of the constituent
particles of the system. It is denoted by the system S.
Characteristics
of Entropy
(i)
Entropy is an extensive property.
(ii)
Like internal energy and enthalpy, it
is a state function.
(iii) The
value of entropy change does not depend upon the path followed.
(iv) Entropy
has an exact differential.
(v)
The change in entropy in a process
only on the initial and the final state of the system ∆S= Sfinal – Sinitial
(vi)
Units: Entropy is measured in cal K-1 mol-1
Physical significance of Entropy
1.
We know
that in a cyclic process, the total heat cannot be completely converted
into-work. When heat is supplied to the gas, only the collision with the face
of the piston push the piston upward and brings about expansion of gas. The
collision of gas molecules with one another and with container are not
effective collisions and result in the loss of some energy. As some of the heat
is lost due to random motion of gas molecules, we can correlate the increase in
entropy with unavailable work.
2.
When a
liquid starts boiling, its temperature becomes constants till the whole liquid
changes into vapours. We see that continuous heating does not increase the
temperature at this stage. The reason is that some work is done in disturbing
the regular arrangement in the liquid state. Thus, work done on the system
increase the randomness but not the temperature at the boiling points.
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