Section16_Spontaneity_and_Free_Energy

Section16_Spontaneity_and_Free_Energy - Page 1 of 22...

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Unformatted text preview: Page 1 of 22 Spontaneity and Free Energy By the end of this lecture, you will be able to: (1) State the 3 rd Law of Thermodynamics and use it to understand absolute entropies. (2) Predict a relative scale of entropy for substances under different conditions. (3) Understand changes in entropy and Gibbs free energy during chemical reactions. Page 2 of 22 The Third Law of Thermodynamics Ludwig Boltzmann identified an equation relating the number of possible ways ( W ) of arranging a system to entropy (S): S = k ln W where k is the Boltzmann constant. k = R / N A = 1.38 10-23 J K-1 mol-1 R is the universal gas constant (8.3145 J K mol-1 ) and N A is Avogadro s number (6.02 10 23 ). For a perfect crystal at 0 K, there is only one way of arranging the atoms, molecules or ions ( W = 1). Page 3 of 22 Thus, S(crystal, 0 K) = k ln 1 = 0 This is the third law of thermodynamics. Using the third law of thermodynamics, it is possible to determine absolute standard molar entropies (S) for many substances. Notes : This is unlike the case for Gibbs free energy (G, see below) and enthalpy (H) for which we can only determine differences ( G, H), not absolute values. S has units J mol-1 K-1 . Elements have nonzero standard entropies. Standard molar entropies for pure substances are always > 0. Page 4 of 22 Standard molar entropies for solutions may be < 0 and are relative to S of H + ( aq ), which is defined to be 0 J mol-1 K-1 . Prediction of relative S values (1) The effect of temperature For a given substance, S increases with increasing temperature. (2) Physical states and phase changes When a more ordered state changes into a less ordered one ( i.e. , from solid to liquid, or from liquid to gas), S increases. Page 5 of 22 (3) Dissolution of a solid or liquid The S of a dissolved solid or liquid is usually greater than that of the pure solute, but the type of solute and solvent play a role....
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Section16_Spontaneity_and_Free_Energy - Page 1 of 22...

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