In chemistry, the second law of thermodynamics is mainly focused on entropy.
The entropy statement of the second law of thermodynamics is given below.
Entropy statement of Second law of thermodynamics:
“In all the spontaneous processes, the entropy of the universe increases.”
Above statement can be mathematically written as;
The entropy statement of the second law gives us the clear idea whether the process will occur on its own or not.
Let us take the example of a hot coffee.
Now we have to find whether cooling of a hot coffee will take place on its own or not.
By finding the entropy change in the universe, we can easily check whether the process will occur on its own or not.
Let me show you this with a mathematical proof.
-The temperature of coffee is 50 °C. (i.e 323 K)
-The temperature of surrounding is 20 °C. (i.e 293 K)
Let say coffee (system) releases 10 joules of heat to the surrounding.
That means, the surroundings will absorb 10 joules of heat.
So we can write,
Qsystem = -10 J
Qsurrounding = +10 J
Now, change in entropy of universe is given by equation;
∆Suniverse = ∆Ssystem + ∆Ssurrounding
= ∆Qsystem/Tsystem + ∆Qsurrounding/Tsurrounding
= (-10/ 323) + (+10/293)
= +0.00316 J/K
This is a positive value,
Hence, the entropy of the universe is increasing here. Thus it satisfies the entropy statement of second law of thermodynamics (∆Suniverse should be greater than 0).
So, we can say that this process is spontaneous. This process of releasing heat to the surrounding will occur on its own.
Well, in the above example you were already knowing that coffee is going to lose heat to the surrounding.
But in some other examples of chemical reactions, you may not be able to predict whether the chemical reaction will occur on its own or not.
So, calculating the entropy change for that particular reaction becomes important.
I hope you have clearly understood the entropy statement of the Second law of thermodynamics in chemistry.
For more detailed information on Second law of thermodynamics, check the ultimate guide on Second law of thermodynamics.
(This guide has all the important knowledge about Second law of thermodynamics along with real life examples and lots more. You will love this guide for sure.)
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