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ENTHALPY, ENTROPY AND GIBBS FREE ENERGY

ENTHALPY, ENTROPY AND GIBBS FREE ENERGY. ENTHALPY . The heat change associated with a chemical process ∆ H o rxn = Σ n ∆ H f o prod - Σ n ∆H f o react ∆ H rxn = negative Exothermic ∆ H rxn = positive Endothermic. Calculate the enthalpy change ( ∆H)for the following reaction

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ENTHALPY, ENTROPY AND GIBBS FREE ENERGY

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  1. ENTHALPY, ENTROPY AND GIBBS FREE ENERGY

  2. ENTHALPY • The heat change associated with a chemical process • ∆Horxn = Σn∆Hfoprod - Σn∆Hforeact • ∆Hrxn = negative • Exothermic • ∆Hrxn = positive • Endothermic

  3. Calculate the enthalpy change (∆H)for the following reaction CH4(g)+2O2(g) CO2(g) +2H2O(l) ΔHorxn= Σn ΔHfoproducts - Σ nΔHforeactants

  4. ENTROPY • Entropy is the degree of disorder • Represented with the symbol S • Matter changes from a more ordered to less ordered state Examples: 2H2O  2H2 + O2 H2(l)  H2(g)

  5. States of matter • Ssolid < Sliquid << Sgas • Ssolid< Saquesous ions

  6. Calculate the entropy change(∆S) for the following reaction • CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔSorxn= ΣnΔSfoproducts - Σ nΔSforeactants

  7. GIBBS FREE ENERGY • Gibbs free energy (∆G) is a measure of the chemical reaction potential of a system • If ∆G is negative, the reaction is spontaneous • If ∆G is positive, the reaction is not spontaneous

  8. Calculate the change in free energy for the following reaction CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ∆Gorxn = Σ n∆Gfoprod - Σ n∆Gforeact

  9. Gibbs Free Energy • Enthalpy and Entropy can be combined to predict reaction spontaneity ∆G = ∆H - T∆S

  10. For a Gas • Slarge vol > Ssmall vol • Slow press > Shigh press • Because S depends on P, we can look at eqn:

  11. G = Go + RTln(Q) • Q = rxn quotient • R = gas law const 8.31 J/Kmol • T = temp (in K) • Go =free energy at 1atm

  12. Try it. Calculate G at 700 K C(s,graph)+H2O(g)->CO(g)+H2(g) • PH2O = .85 atm • PCO = 1.0 x 10-4atm • PH2 = 2.0 x 10-4atm

  13. So, G? • Tells us whether products or reactants are favored • does not mean that the rxn will go to completion • Tells us rxn will go to equilibrium

  14. A(g) --> B(g) • 1 mole A at 2 atms • As A goes to B, GA will decrease and GB will increase • GA = GB at equilibrium

  15. Equilibrium exists when: • Gprod = Greact • G = Gprod - Greact = 0

  16. G = Go + RTln(Q) • At equilibrium, G = 0 and Q = K • Go = -RTln(K)

  17. Case 1: Go = 0 • Free energy of reactants and products are equal • System is at equilibrium

  18. Case 2: Go < 0 • Goprod < Goreact • System will adjust right • K will be greater than 1

  19. Case 3: Go > 0 • Goreact < Goprod • System will adjust left (toward reactants) • K < 1

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