Section16_Spontaneity_and_Free_Energy

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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 ʼ 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 ( Δ G, Δ 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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