Third Law of Thermodynamics Equation (Simple & Easy)

The third law of thermodynamics equation is mentioned below. Here, I’ll also tell you why entropy of pure crystalline substances is zero.

So first of all, the statement of third law of thermodynamics is;

“The value of entropy of a completely pure crystalline substance is zero at absolute zero temperature”

Now let us see the equation of third law of thermodynamics.

You might be knowing that we can calculate the change in entropy using this formula.

S – S0 = kB  lnΩ

Here, 

  • S = entropy of the system
  • S0 = initial entropy
  • kB = Boltzmann constant
  • Ω = total number of microstates

If a substance is perfectly Crystalline, then its total number of microstates will be 1 (Ω = 1).

Now from the above equation, we get;

S – S0 = kB  lnΩ

            = kB  ln(1)

            = 0.   (Because ln 1 = 0)

Now, When the initial entropy of the system is taken as zero, the value of entropy S can be calculated easily.

Thus from above equation, we get;

S – 0 = 0

S = 0

Thus, the entropy of a pure crystalline substance is “zero” at absolute zero temperature.

I hope you have understood the third law of thermodynamics equation.

For more detailed information on Third law of thermodynamics, check the ultimate guide on Third law of thermodynamics.

(This guide has all the important knowledge about Third law of thermodynamics along with examples and lots more. You will love this guide for sure.)

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