Chapter-12-Tro

Chapter-12-Tro - Spontaneous change: A spontaneous change...

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Spontaneous change: A spontaneous change is one that occurs spontaneously. How can we predict the spontaneity of a process? In the later part of 19 th century, Berthelet (France) and Thomsen (Denmark) thought that spontaneity is associated with liberation of heat in a reaction. That would make exothermic reactions spontaneous, but not endothermic reactions. But this is contradicted by spontaneous dissolution of NH 4 NO 3 in water which is endothermic. Therefore the suggestion made by Berthelet and Thomsen was not accepted. To find a criterion for spontaneity of chemical reactions at any given temperature, some day to day observations will be useful. (1). Can you imagine witnessing a metal, when left in the open environment, increase in its temperature to more than the temperature of surroundings, even though heat absorbed by metal is equal to heat lost by surroundings? (2). Can you imagine witnessing the molecules of an ideal gaseous substance in a container confined to a small portion of the container, even though the internal energy of an ideal gas does not depend on volume? (3). On the other hand, if the metal at a higher temperature is left in the open environment, it will equilibrate to a lower temperature (4). The gas molecules expand naturally from a higher pressure to a lower pressure The processes in (3) and (4) are Spontaneous processes occurring naturally
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A thermodynamic quantity that has been suggested to predict the spontaneity of a given process is the change in Entropy . Entropy has units of Joules/K. For spontaneous processes change in entropy is positive or entropy increases . Entropy is a measure of spontaneity Ar gas Ne gas In this uniform mixing of inert gases, potential energy is not lowered. Yet this is a spontaneous process which is attributed to entropy. Entropy is a measure of the molecular disorder of a state. Higher disorder… higher entropy. Entropy is a measure of the probability of a state Higher probability of occurring means higher entropy; Uniform mixing is more probable than segregated gases. Remove separating wall Separating wall Segregated gases (ordered state) Uniformly mixed gases (disordered state) To assess if a process is to be favored, two factors are to be considered: (a). Is potential energy is lowered or increased ?; (b). Is entropy decreased or increased The dominant one among these two will decide the outcome.
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Solutions : A solution is a homogenous mixture of different substances. The major component in the solution is called solvent and the others are solutes . We normally deal with solutions that contain two substances. For example if we add a small amount of salt to a flask containing larger amount of water, this solution contains water solvent and salt solute. Solubility of a substance is the maximum amount that can dissolve in a given amount of solvent at a given temperature . The process of dissolving a solute in a solvent is called
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This note was uploaded on 03/06/2010 for the course CHEM 102B taught by Professor A during the Spring '09 term at Vanderbilt.

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Chapter-12-Tro - Spontaneous change: A spontaneous change...

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