Entropy changes, chemical reactions compounds

In a chemical reaction, the change in enthalpy and the change in entropy are determined by the nature of the compounds involved. 

If, at 0 K, entropies of compounds are the sum of nonzero values assigned to each atom, conservation of atoms would ensure that the entropy change for all chemical reactions would be zero. This way of satisfying the third law would not be in agreement with the microscopic interpretation of entropy given in Chapter 5. 

In other words, every chemical reaction takes place without change in entropy at the absolute zero. From this it follows that the entropy of a compound is equal to the sum of the atomic entropies. The assumption made by Planck in addition to Nernst s theorem, viz. that the entropy of all substances vanish like the specific heats at the absolute zero, is sufficient but not necessary for the derivation of the heat theorem. 

This observation is the Nernst heat theorem, named after its discoverer, the German physicist Walther Nernst. It immediately suggests a choice of reference state The entropy of any pure element in its equilibrium state is defined to approach zero as T approaches 0 K. From the Nernst theorem, the entropy change for any chemical reaction, including one in which elements react to give a pure compound, approaches zero at 0 K. The most general form of this statement is the third law of thermodynamics ... 

The total energy released in a chemical reaction has two components, enthalpy and entropy. Change in enthalpy (AH, measured in Jmol- ) is a direct measure of the energy emitted or absorbed by a reaction. Change in entropy (AS, measured in Jmol 1K ) is a measure of the degree of disorder. Most reactions proceed to increase disorder, for example by splitting a compound into constituent ions or atoms. Enthalpy and entropy are related ... 

Unlike enthalpies for which values at absolute zero are important, hard to measure, and must be tabulated carefully, entropies of most pure compounds can be taken to be zero at absolute zero. Tabulations of entropy are thus simpler than tables of enthalpies, and finding entropy changes for chemical reactions from tables is often more precise than finding the comparable enthalpy change because there is no comparable law for enthalpies. 

Thus the absolute entropies of elements and compounds can be. established. These can be used to determine the entropy changes accompanying chemical reactions. 

In this equation ArS refers to the entropy change of any balanced chemical reaction at temperature T. If the reaction is the formation of a compound from its elements, ArS becomes AfS. ... 

The third law of thermodynamics asserts that if the entropies of all samples of pure perfect crystalline elements are taken as zero, the entropies of all samples of pure perfect crystalline compounds can also consistently be taken as zero. Entropies relative to the entropy at zero temperature are called absolute entropies. The values of these absolute entropies can be used to calculate entropy changes of chemical reactions. 

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