Chapter 20 Lecture 4

Chapter 20 Lecture 4 - Gibbs Free Energy Gibbs Free Energy...

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ibbs Free Energy ( Gibbs Free Energy ( G ) H sys 0 T S univ = T S sys H sys > 0 T S univ = S sys - > 0 –T S univ = H sys –T S sys < 0 G sys = H sys - T S sys For simplicity, we can write: G = H – T S G , the change in the free energy of a system, is a measure of the spontaneity of the process and of the useful energy available from it. 0 for a spontaneous process G < 0 for a spontaneous process G > 0 for a nonspontaneous process G = 0 for a process at equilibrium 20-39
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Predicting T Range for a Spontaneous Reaction uestion 1: Is the following reaction spontaneous at high or low Sample Problem Quest o s t e o o g eact o spo ta eous at g o o temperatures? 3H 2 (g) + N 2 (g) 2NH 3 (g) + heat olution Solution : Exothermic reaction ֜ H < 0 Less gaseous products ֜ S < 0 = H – S = (– – (– = (– +T(+)<0 T has a mall alue G H T S ( ) T( ) ( ) + T(+) < 0 if T has a small value Reaction is spontaneous at low T uestion 2: For the same reaction at certain conditions = 1 8 Question 2: For the same reaction at certain conditions, H = -91.8 kJ and S = -197 J/K. What is the T-range at which the reaction is spontaneous? olution: Solution: G = H – T S < 0 H < T S -91.8 kJ < T (-0.197 kJ/K) Multiply by (-1) 1 8 kJ > 197 kJ/K) ote: (<) flips to (>) 20-40 91.8 kJ > T (0.197 kJ/K) Note: (<) flips to (>) T < 91.8 kJ/0.197 kJ/K ֜ T < 466 K T -range
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Standard Free Energy Changes Standard free energy Change ( G o sys ) – or standard free energy of reaction ( G o sys ) – the free energy change for a reaction in which all reactants and products are present in their standard states at a specified temperature T G o sys = H o sys - T S o sys Standard free energy of formation ( G o f ) e tandard free energy for the reaction of formation of 1 mol the standard free energy for the reaction of formation of 1 mol of a substance from its elements at a specified temperature T (usually 298 K) o = º º G rxn Σ m G f (products) Σ n G f (reactants) f of an element in its standard state is zero f at
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Chapter 20 Lecture 4 - Gibbs Free Energy Gibbs Free Energy...

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