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Chemical Thermodynamics Second Law of Thermodynamics Unlike energy, entropy is NOT conserved: • Spontaneous Processes • Entropy and the Second Law of Thermodynamics • The Molecular Interpretation of Entropy • Entropy Changes in Chemical Reactions • Gibbs Free Energy • Free Energy and Temperature • Free Energy and the Equilibrium Constant The Universe wants to become more and more disordered, it wants its total entropy to increase continually! Reversible process ΔSuniv = ΔSsys + ΔSsurr = 0 Irreversible process ΔSuniv = ΔSsys + ΔSsurr > 0 Spontaneous Processes First Law of Thermodynamics stated: Energy can not be created nor destroyed, it can be converted from one form to another or transferred from a system to surroundings (or vice versa) Entropy ΔS = Sfinal – Sinitial units J /K S = k ln W So, if increasing the temperature increases the number of microstates could we decrease the temperature to a point where all motion stops and we have a single microstate? The Third Law Of Thermodynamics The Entropy of a pure, crystalline substance at absolute zero (0 K) is zero Standard Free Energy The standard free energy change ΔGo is defined as the change in the free energy when reactants in their standard states are converted to products in their standard states: ΔGo = Σ n ΔG o (products) - Σ m ΔG o (reactants) Conventions used in establishing Standard free energies Solid Pure solid Liquid Pure liquid Gas 1 bar pressure solution 1M ∆Gof in standard state = 0 elements ΔGo At 0K, all the units in the lattice have no thermal energy, no motion, 1 Microstate Because is the difference in Gibbs free energy between products in their standard states and reactants in their standard states, it is a fixed quantity for a given reaction at a given temperature ΔG also depends on the composition of reaction mixture and varies and might even change sign as the reaction proceeds. S = k lnW W = 1 lnW = 0 Therefore: S = 0 What happens to Entropy as we continue to heat Melting the solid overcomes the forces holding the particles together, this gives them a greater degree of freedom Calculate the standard free energy change ΔG o for this reaction at 25 oC and 500 oC. What do the calculated values tell you? Given that ΔH o = -92.38 kJ and ΔSo = -198.4 J/K Equilibrium (isothermal) N2(g) + ΔGo = ΔHo - TΔSo 3H2 (g) 2NH3 (g) ΔS = qrev /T ΔSfus = ΔHfus /T Why is the Entropy of phase changes following the order Ssolid < Sliquid < Sgas Entropy Changes in Chemical Reactions Increasing the temperature from 25 oC to 500 oC changes the sign of ΔGo , indicating that a mixture of N2, H2 and NH3 at 1 bar pressure will spontaneously form more NH3 at 25 oC and spontaneously form more N2 and H2 at 500 oC Free Energy and Temperature