Question

what is Third law of thermodynamics​

Answer 1

$\huge{\underline{\sf{\red{Answer}}}}$

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The entropy of perfect crystals of all pure elements and compounds is zero at the absolute zero temperature.

Thus, absolute value of entropy unlike the absolute value of enthalpy for any pure substance can be calculated at any given temperature in standard state (298K, 1atm), it is standard absolute entropy.

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Answer 2

$\large{\underline{\underline{\sf{Third\:law\:of\:thermodynamics-}}}}$

According to this law,

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀" At absolute zero ( 0 k = -273°C ), entropy of a perfectly crystalline solid is zero. "

Let's take an example -

Case 1).

$\begin{array}{| c | c | c |}\cline{1-3} CO & CO & CO \\\cline{1-3} CO & CO & CO \\\cline{1-3} CO & CO & CO \\\cline{1-3}\end{array}$

Here, it is perfectly crystalline, so entropy will be zero (at zero Kelvin).

Case 2).

$\begin{array}{| c | c | c |}\cline{1-3} CO & CO & OC \\\cline{1-3} CO & CO & OC \\\cline{1-3} OC & OC & CO \\\cline{1-3}\end{array}$

Here, system isn't perfectly crystalline, so entropy will not be zero at zero Kelvin. {That kinda entropy is called residual entropy.}

★ With the help of third law of thermodynamics, we can find the absolute value of entropy..

$\large{\boxed{\sf{\traingle S = 2.303 \displaystyle \int\limits_{0}^{T} C_{p} log T dt}}}$

Here -

• T denotes temperature.
• Cp denotes molar heat capacity at constant pressure.