Chapter-17-Tro

Chapter-17-Tro - Chapter 17:Free energy and Thermodynamics...

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Spontaneous change and equilibrium : A spontaneous change is one that occurs by itself, without outside intervention . How can we predict if a given process (such as a chemical reaction) occurs spontaneously or not ? For example, if we mix N 2 (g) and H 2 (g) would they react to give NH 3 (g)? How can we predict the spontaneity of a given reactions? In the later part of 19 th century, Berthelet (France) and Thomsen (Denmark) thought that such predictions can be done using enthalpy. They 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 exothermic/endothermic alone is not a good criterion for spontaneity. To determine the criterion for spontaneity of chemical reactions at any given temperature let us consider some day to day observations first. (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? Chapter 17:Free energy and Thermodynamics
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The above mentioned two processes do not violate the first law of thermodynamics, but still are not practically observable phenomena ...they are unnatural or non-spontaneous. (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 natural processes . . Spontaneous processes Another thermodynamic quantity that has been suggested to predict the spontaneity of a given process is change in Entropy . What is Entropy? To understand entropy, let us consider the probability of getting head/tail when a coin is tossed. he probability of a coin, when tossed, showing head/tail = 1/2 When two coins are tossed, probability of getting both heads = (1/2)(1/2) = When three coins are tossed, probability of getting three heads = (1/2)(1/2)(1/2) =(1/2) 3 = 1/8. This indicates one possible arrangement out of eight possible arrangements. What are these arrangements? Spontaneity of a reaction should not be confused with the speed of that reaction. A reaction can be spontaneous, but may take million years for that reaction to occur. A catalyst can speed up a reaction (by lowering the energy barrier), but a catalyst cannot convert a non-spontaneous reaction into a spontaneous reaction.
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Arrangement 1 2 3 possibilities 3 heads H H H 1 2 heads,1 tail H H T H T H T H H 1 head, 2 tails H T T T H T T T H 3 tails T T T 1 3 3 Arrangements of, possibilities for, tossing three coins:
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Chapter-17-Tro - Chapter 17:Free energy and Thermodynamics...

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