absolute entropy definition
absolute entropy: represents the entropy change of a substance taken from absolute zero to a given temperature. The Third Law of Thermodynamics. The third law of thermodynamics defines absolute zero on the entropy scale. Third law: The entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K). In statistical thermodynamics, entropy (usual symbol S) is a measure of the number of microscopic configurations ω that a thermodynamic system can have when in a state as specified by some macroscopic variables. The absolute entropy of any substance can be calculated using equation (1) in the following way. Imagine cooling the substance to absolute zero and forming a perfect crystal (no holes, all the atoms in their exact place in the crystal lattice). Since there is no disorder in this state, the entropy can be defined as zero. Now start introducing small amounts of heat and measuring the temperature change. Even though equation (1) only works when the temperature is constant, it is approximately correct when the temperature change is small. Measuring Entropy. One useful way of measuring entropy is by the following equation: where S represents entropy, DS represents the change in entropy, q represents heat transfer, and T is the temperature. Using this equation it is possible to measure entropy changes using a calorimeter. In this equation, S is the entropy of the system, k is a proportionality constant equal to the ideal gas constant divided by Avogadro's constant, ln represents a logarithm to the base e, and W is the number of equivalent ways of describing the state of the system. It is often said that entropy is an expression of the disorder, or randomness of a system, or of our lack of information about it. The second law is now often seen as an expression of the fundamental postulate of statistical mechanics through the modern definition of entropy. The absolute entropy (S rather than δS) was defined later, using either statistical mechanics or the third law of thermodynamics, an otherwise arbitrary additive constant is fixed such that the entropy of a pure substance at absolute zero is zero. By definition, neither heat nor work can be transferred between an isolated system and its surroundings. We can apply the second law of thermodynamics to chemical reactions by noting that the entropy of a system is a state function that is directly proportional to the disorder of the system.
It represents the entropy change of a substance taken from absolute zero to a given temperature.