5.6.5. Absolute Entropy and Third Law of Thermodynamics

Molecules of a substance may move in a straight line in any direction, they may spin like a top and the bonds in the molecules may stretch and compress. These motions of the molecule are called translational, rotational and vibrational motion respectively. When temperature of the system rises, these motions become more vigorous and entropy increases. On the other hand when temperature is lowered, the entropy decreases.

Molecules within a substance can exhibit different types of motion: translational (movement from one place to another), rotational (spinning around an axis), and vibrational (changes in the distance between atoms in a molecule). As the temperature of a substance increases, these movements become more energetic, leading to an increase in disorder, which is measured as entropy. Conversely, reducing the temperature slows down these motions, resulting in a decrease in entropy. In essence, more movement corresponds to higher entropy, while less movement relates to lower entropy.

Think about a room full of people. If people are dancing and moving around actively (high temperature), the room feels lively and chaotic (high entropy). However, if everyone is sitting still and quietly (low temperature), the room feels much more orderly and less chaotic (low entropy).

The entropy of any pure crystalline substance approaches zero as the temperature approaches absolute zero. This is called the third law of thermodynamics. This is so because there is perfect order in a crystal at absolute zero.

The Third Law of Thermodynamics states that as a system reaches absolute zero temperature (0 Kelvin), the entropy, or disorder, of a perfect crystal approaches zero. This implies that at this temperature, the molecules are in a perfectly ordered state, exhibiting no randomness or movement. Thus, perfect order at absolute zero results in zero entropy, but this only applies to pure crystalline substances and not to amorphous materials or mixtures, where some entropy remains even at absolute zero.

Imagine a perfectly organized set of building blocks arranged in a neat stack. At absolute zero, all movements stop, and the blocks align in perfect order. This orderly structure reflects zero entropy, exemplifying the idea that at the lowest temperature, disorder disappears completely.

The importance of the third law lies in the fact that it permits the calculation of absolute values of entropy of pure substance from thermal data alone.

The Third Law of Thermodynamics is crucial because it allows us to determine absolute entropy values for pure substances based only on their thermal properties. This means that scientists can calculate how much disorder a pure substance has at various temperatures, particularly as it approaches absolute zero. Understanding this helps in various scientific fields including chemistry, physics, and material science as it provides a fundamental baseline for measuring entropy.

Consider a set of scales. The Third Law establishes a fixed point, like zero weight, from which you can measure everything else. When scientists know that the entropy of a perfect crystal is zero at absolute zero, they can compare the entropy of other substances at higher temperatures against this baseline, much like weighing different objects against an empty scale.

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