Third Law of Thermodynamics Third Law of Thermodynamics The third law of thermodynamics deals with the entropies of the perfect crystalline substances at absolute zero of temperature. According to third law of thermodynamics, at absolute zero, the entropy of a perfectly crystalline substance is taken as zero. This law was first formulated by Nernst in 1906. Since entropy is related to disorder, according to third law, at absolute zero, there is least disorder or there is perfect order. Therefore, the entropy at perfect order is taken as zero. It may be noted that this law is true only for those substances which exist in the perfectly crystalline form at 0 K. However, if there are imperfections of any type in the perfect crystalline arrangement at 0 K, then the entropy will be larger than zero. Explanation :In simple terms, the third law states that the entropy of a perfect crystal approaches zero as the absolute temperature approaches zero. This law provides an absolute reference point for the determination of entropy.

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The entropy determined relative to this point is the absolute entropy. Mathematically, the absolute entropy of any system at zero temperature is the natural log of the number of ground states times Boltzmann's constant kB. The entropy of a perfect crystal lattice as defined by Nernst's theorem is zero provided that its ground state is unique, because ln(1) = 0. An example of a system which does not have a unique ground state is one containing halfinteger spins, for which time-reversal symmetry gives two degenerate ground states. For such systems, the entropy at zero temperature is at least ln(2)kB (which is negligible on a macroscopic scale). Some crystalline systems exhibit geometrical frustration, where the structure of the crystal lattice prevents the emergence of a unique ground state. Ground-state helium (unless under pressure) remains liquid. In addition, glasses and solid solutions retain large entropy at 0K, because they are large collections of nearly degenerate states, in which they become trapped out of equilibrium. Another example of a solid with many nearly-degenerate ground states, trapped out of equilibrium, is ice Ih, which has "proton disorder".. For the entropy at absolute zero to be zero, the magnetic moments of a perfectly ordered crystal must themselves be perfectly ordered; indeed, from an entropic perspective, this can be considered to be part of the definition of "perfect crystal". Only ferromagnetic, antiferromagnetic, and diamagnetic materials can satisfy this condition. Materials that remain paramagnetic at 0K, by contrast, may have many nearly-degenerate ground states (for example, in a spin glass), or may retain dynamic disorder (a spin liquid). Uses of Thermodynamics Third Law :- The thermodynamics third law helps us to calculate the absolute entropies of pure substances at different temperatures. The entropy (S) of the substance at different temperature T may be calculated by the measurement of heat capacity changes.

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The value of integral can be calculated from the graph of Cp/T against temperature (T). The area under the curve between 0 K and T K gives the value of integral and thus, the value of S at temperature T. When the entropy of one mole of the substance is expressed at 298 K and 760 mm of Hg pressure, it is called standard entropy of the substance and is denoted as So. The standard entropy change (DSo) for a chemical reaction can be calculated from the standard entropies of various substances in products and reactants as.

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