A process that proceeds of its own accord, without any outside assistance, is termed a spontaneous process. The reverse process which doesn’t proceed on its own is referred to as non spontaneous or unnatural process.
In general the tendency to proceed naturally is called the spontaneity.
A spontaneous change is unidirectional and for such processes to occur time is no factor.
Normally it seems like exothermic processes occur spontaneously. But take a cube of ice as example! It would undergo melting when it is left to stand outside, which is an endothermic process. Thus arose the need of inventing another driving force that affects spontaneity. This was known as the entropy change.
What happens in spontaneous processes is that the molecules go into a state of higher randomness than the current state. A change that brings about randomness is more likely to occur than that one brings about order.
Take the example of a ball bouncing ;
When a change occurs the total energy remains constant, according to the first law. The direction of the change is determined by the distribution of energy. In spontaneous processes things tend to go into a state where energy is more chaotically dispersed.
The ordered directional motion of the ball gets converted into the random thermal motion of the floor molecules. As a result the bouncing height decreases as shown above.
On the other way around the ball placed on a hot floor should start bouncing as the reversible process. In order to do so the random thermal energy which is chaotically dispersed should accumulate into the ball performing directional movement.
Reversible processes occur without degrading the quality of the energy
Entropy
Entropy is the thermodynamic state quantity that is a measurement of the randomness or disorder. It’s a state function.
The second law of thermodynamics says that in a spontaneous process the entropy of the universe increases.
∆Suniverse ˃ 0
Entropy is related to the heat energy or the degraded energy.
∆S ∞ q
S depends on the temperature as well. If it’s already heated a bit of extra heat won’t help to create much disorder as dramatic as it does when the temperature is very law. Imagine two iron balls that are heated up to 100 ͦ C are placed in two rooms where the room temperatures are 10 ͦ C and 90 ͦ C. Assume that from both balls a q amount of heat is lost to the surrounding. Considering the impact the two energy losses can have on the entropy of the surrounding it appears that the second surrounding’s entropy change will not create any huge impact as the temperature is high and is in state of high entropy. Therefore we can say that;
∆S= q/T or dS= dq/T
∆S is a state function. { Sfinal-Sinitial} and q is a path function. In order to obtain a consistent value ∆S is specified by the reversible pathway. In a reversible path the work done is maximum. As ∆U is constant the qrev is maximum as well.
∆S= dqrev /T →(1)
As work done in a reversible path is larger than that of an irreversible path q must be larger as well for the reversible if ∆U is fixed.
dq sys rev ˃ dqsysirrev
[dq sys rev]/T ˃ [dqsysirrev]/T
∆Ssys ˃ [dqsysirrev]/T →Clacius inequality
The entropy change of the universe is that of the system plus that of the surrounding.
dSuni = dSsys + dSsurr
dSuni = dqrevsys / T + dqrevsurr / T
For irreversible processes
Since the surrounding are so large as to be unaffected by the transfer of heat such a transfer can be considered reversible from the point of view of the surrounding.
dqsurr = dqrevsurr
dSuni = dqrevsys / T + dqsurr / T
In any processe the surroundings gains the heat that is lost from the system. As this is irreversible;
[-dqsys] = dqsurr
dSuni = dqrevsys / T – dqsys / T
according to the Clausius inequality;
[dq sys rev]/T ˃ [dqsysirrev]/T
Therefore in irreversible/ spontaneous processes dSuni ˃ 0 (+)
For reversible processes
dSuni = dqrevsys / T + dqrevsurr / T
[-dqrevsys] = dqrevsurr
dSuni = dqrevsys / T - dqrevsys / T= 0
Therefore in reversible processes dSuni = 0
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