RECALL “ENERGETICS OF SOLUTIONS”Usually, exothermic reactions are spontaneous. However, some endothermic reactions are spontaneous. For example,MgSO4dissolving in water is endothermic and spontaneous.NH4Cl dissolving in water is endothermic and spontaneous.Some exothermic reactions are notspontaneous. For example,liquid water becoming solid water (ice) at room temperature is exothermic, but not spontaneous.Therefore, something in addition to ΔH determines spontaneity.Two factors involved in determining if a reaction will go to the right:ΔH and ΔSΔG = ΔH – TΔSΔG must be negative for a reaction to go to the right.Chem 162-2015 Chapter 19 Thermodynamics20
Concepts: Reaction: A + 2B 3C + D ΔG = ΔH - TΔSSpontaneous process: a reaction that goes forward without any outside intervention; a reaction that goes to the right.When ΔG is negative, the reaction is spontaneous (goes from left to right).When ΔG is positive, the reaction is not spontaneous (goes from right to left).When ΔG is zero, the reaction is at equilibrium.ΔH – TΔS = ΔG CaseΔH ΔS-(TΔS)ΔG Result1-+--Spontaneous at all T2--+-? +?Spontaneous at low T; nonspontaneous at high T3++-+? -?Nonspontaneous at low T; spontaneous at high T4+-++Nonspontaneous at all TWhat if ΔH is negative and –(TΔS) is negative (case 1), what will ΔG be? Answer: NegativeWhat if ΔH is positive and –(TΔS) is positive (case 4), what will ΔG be? Answer: PositiveWhat if ΔH is negative and –(TΔS) is positive (case 2), what will ΔG be? Answer: We don’t know. It depends on the relative magnitudes of ΔH and –T(ΔS). If ΔH is more strongly negative than –(TΔS) is positive, then ΔG will be negative. If –(TΔS) is more strongly postive than ΔH is negative, then ΔG will be positive. The –(TΔS) magnitude can be controlled by T. The larger the T the more important the –(TΔS) is. 16.23(modified): ΔH is -25kJ and ΔS is -100 J/K. Which will make the reaction spontaneous, large or small temperature? At what temperature will the reaction A + 2B 3C + D be neither spontaneous nor nonspontaneous (i.e., be at equilibrium, i.e., ΔG = 0)? Atwhat temperature will it be spontaneous to the right (i.e., ΔG < 0)? spontaneous to the left (i.e., ΔG > 0)? Problem solution: Case 2.ΔG = ΔH – TΔS. Since ΔH is negative, and –TΔS is positive, then a small T would make the –TΔS term small, making ΔG negative, and allow the reaction to go to the right.ΔG = ΔH - TΔSAt equilibrium ∆G = 00 = -25000J – (X x -100J/K)X = 250KSince a small T would make the reaction spontaneous, then less than 250K would make the –TΔS less positive, resulting in a negative ΔG, and a corresponding spontaneous reaction. If the temperature was >250K then –TΔS would be more positive, resulting in a positive ΔG, and a corresponding non-spontaneous reaction.
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