Chapter 19 Entropy and Free Energy-1

# Chapter 19 Entropy and Free Energy-1 - Chapter 19 Entropy...

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Chapter 19 Entropy and Free Energy Have you ever seen a cold object transfer heat (by itself) to a hot object? Have you ever seen a gas re-fill the glass vessel from which it escaped? The answer is, probably, no.(there is always a class-clown who will say,yes!) Do any of these processes violate the energy conservation law (First Law)? Nope! Then, why don’t they occur? The answer is Entropy (S) or disorder. The universe is getting disorganized every second, and there nothing we can do about it. Except, by using energy to organize some part of the universe while other part gets disorganized.

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Entropy and Spontaneity In general, a process will occur without input of energy if the process releases or disperses energy (exothermic). Other processes can occur spontaneously, even if they are endothermic, as long as the process increases the amount of disorganization of the system and its surroundings (the universe). Entropy is the amount of disorder of a system. A process that increases the disorder of the universe is spontaneous or allowed.
Second Law of Thermodynamics A spontaneous process is one that results in the increase of ENTROPY OF THE UNIVERSE. Entropy is a state function and, unlike enthalpy or energy, it can be calculated absolutely. Mathematically is defined as Heat transfer/temperature of transfer. And its change as Q/T = S And T S = Q (remember that Q can be E or H)

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Absolute Standard S o Using math calculations and the Third Law: “at zero K the entropy of any system is ZERO”. 2200 dS= dQ/T S 0 - 0 = Q/T from T=0K to T=298K These absolute standard molar entropies are found in tables. S S o = = Σ Σ S S o (products) - (products) - Σ Σ S S o (reactants) (reactants) Their S 0 can be used as per Hess law once calculated.
Standard Molar Entropies Standard Molar Entropies

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Calculating  S for a Reaction Calculating  S for a Reaction S S o = = Σ Σ S S o (products) - (products) - Σ Σ S S o (reactants) (reactants) This S 0 can be used as per Hess law for other reactions involving liquid water
Spontaneous Reactions Spontaneous Reactions In general, spontaneous In general, spontaneous reactions are reactions are exothermic exothermic . . Fe Fe 2 O 3 (s) + 2 Al(s) -> Fe(s) + Al (s) + 2 Al(s) -> Fe(s) + Al 2 O 3 (s) (s) H = - 848 kJ H = - 848 kJ

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Directionality of Reactions Directionality of Reactions Energy Dispersal Energy Dispersal Exothermic reactions involve a release of Exothermic reactions involve a release of stored chemical potential energy to the
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