![]() ![]() Here we further explore the nature of this state function and define it mathematically. In Chapter 13, we introduced the concept of entropy in relation to solution formation. The crystal must be perfect, or else there will be some inherent disorder. Because the entropy of a substance depends on the amount of substance, the pressure, and the temperature, it is convenient to describe the entropy of a substance in terms of S m °, its standard molar entropy, i.e., as the entropy of 1 mol of substance at the standard pressure of 1 atm (101.3 kPa) and given temperature. Third law: The entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K). To help explain why these phenomena proceed spontaneously in only one direction requires an additional state function called entropy (S), a thermodynamic property of all substances that is proportional to their degree of "disorder". The third law of thermodynamics defines absolute zero on the entropy scale. Moreover, the molecules of a gas remain evenly distributed throughout the entire volume of a glass bulb and never spontaneously assemble in only one portion of the available volume. Since there is no disorder in this state, the entropy can be defined as zero. 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). At any temperature over 0K, the conventional entropies. The absolute entropy of any substance can be calculated using equation (1) in the following way. The standard entropy of elements and compounds are already determined, and therefore we usually refer the actual entropy of a substant to its standard value Substance S0 (J/K mol) H 2 O(l) 69. The standard entropy (S) is the absolute entropy of a pure material at 25C (298 K) and 1 atm pressure. The term standard state is used to describe a reference state for substances, and is a help in thermodynamical calculations (as enthalpy, entropy and Gibbs. For example, after a cube of sugar has dissolved in a glass of water so that the sucrose molecules are uniformly dispersed in a dilute solution, they never spontaneously come back together in solution to form a sugar cube. The absolute entropy is difficult to determine, because it is difficult to determine a number of microstates corresponding to a particular macrostate. For a full video: see Thus enthalpy is not the only factor that determines whether a process is spontaneous. When water is placed on a block of wood under the flask, the highly endothermic reaction that takes place in the flask freezes water that has been placed under the beaker, so the flask becomes frozen to the wood. The absolute entropy of a pure substance at a given temperature is the sum of all the entropy it would acquire on warming from absolute zero (where \(S0\)) to the particular temperature. The reaction of barium hydroxide with ammonium thiocyanate is spontaneous but highly endothermic, so water, one product of the reaction, quickly freezes into slush. ![]()
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