Question

Which law of thermodynamics help in calculating the absolute value of entropy at different temperatures ?

• A. First law
• B. Second law
• C. Third law
• D. Zeroth law

Answer 1

The correct option is C Third law

According to third law of thermodynamics absolute value of entropy can be caculated at any temperatures.

Theory:

Third law of thermodynamics : Entropy of pure and perfectly crystalline substance at $0K$ is zero. A perfect crystal is a crystal that contains no point, line, or planar defects. There are a wide variety of crystallographic defects. Kinetic energy is directly proportional to Temperature, when temperature becomes zero then kinetic energy will also be zero. Pure corresponds to no mixing of foriegn substances. For perfectly crystalline substance entropy $S=0$ at $0K$


Absolute entropy of a pure substance : Absolute value of entropy (unlike absolute value of enthalpy) can be calculated at any temperature. By the formula of entropy change.
$\Delta S=S_{\left(TK\right)}-S_{\left(0K\right)}={\int }_0^T\frac{n{C}_{m}dT}{T}$
After applying the lower limit $S_{\left(0K\right)}=0$ (perfectly crystalline substance)
$\Delta S=\int \frac{d{q}_{rev}}{T}$
$S_{\left(TK\right)}=S_{\left(0K\right)}+{\int }_0^T\frac{n{C}_{m}dT}{T}$
$=0+{\int }_0^T\frac{n{C}_{m}dT}{T}$

Standard entropy of pure substance $\left(S^o\right)$ : Entropy of a pure substance in its standard state that is pressure 1 bar and the temperature 298 K, is called the standard entropy of pure substance, It is denoted by $S^o$.