entropy

Entropy:-
Clausius (1854) introduced another most important thermodynamic function called entropy to make more clear and thorough understanding of the second and law of thermodynamics. Entropy of a system is a measure of randomness or disorder of the system and is denoted by the symbol S. it is a state function (that is, the change in entropy is independent of path). It can also be regarded as the measure of unavailable energy. The change in entropy of a system (or a reaction) is given by
?S= S_final - S_initial
= S_2- S_1= dq/T
Entropy is expressed in joule degree-1 (J K-1 mol-1).
Second law of thermodynamics states a spontaneous process is always accompanied by an increase in the total entropy of the system and its surroundings.


Entropy of Chemical Changes
The entropy change of a chemical reaction is given by the difference between the sum of the entropies of all the products and the sum of the entropies of all the reactants.
Consider a chemical reaction of the type
aA + bB + ….. ? cC + dD + …
the entropy change of the reaction is given by
?S= ( c SC + d SD + ….) – ( aSA + bSB + …)
Where SA, SB, SC and SD are the entropies of pure substances A, B, C, and D … taking part in the reaction so, in general
?S= ??S_products - ??S_reactants