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Understanding Gibbs Free Energy in Chemical Reactions

Learn about spontaneous and nonspontaneous reactions, entropy, and free energy in chemical systems. Discover how to calculate Gibbs free energy and determine reaction spontaneity.

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Understanding Gibbs Free Energy in Chemical Reactions

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  1. Gibbs Free Energy

  2. 18.4 Free Energy and SpontaneousReactions • A spontaneous reaction occurs naturally and favors the formation of products at the specified conditions.

  3. 18.4 Free Energy and SpontaneousReactions • A nonspontaneous reaction is a reaction that does not favor the formation of products at the specified conditions. Photosynthesis is a nonspontaneous reaction that requires an input of energy.

  4. 18.4 Free Energy and SpontaneousReactions • Spontaneous reactions produce substantial amounts of products at equilibrium and release free energy. • Free energy is energy that is available to do work.

  5. 18.4 Entropy • Entropy is a measure of the disorder of a system. • A positive ΔS is means that a system is getting more disordered or more chaotic.

  6. 18.4 Entropy

  7. 18.4 Entropy • An increase in entropy favors the spontaneous chemical reaction; a decrease favors the nonspontaneous reaction.

  8. 18.4 Entropy • For a given substance, the entropy of the gas is greater than the entropy of the liquid or the solid. Similarly, the entropy of the liquid is greater than that of the solid.

  9. 18.4 Entropy • Entropy increases when a substance is divided into parts.

  10. 18.4 Entropy

  11. 18.4 Entropy • Entropy tends to increase when temperature increases. As the temperature increases, the molecules move faster and faster, which increases the disorder.

  12. Enthalpy, Entropy, and Free Energy • Ent alpy h H ΔH is the difference between the chemical potential energy of the products and the chemical potential energy of the reactants

  13. Gibbs Free-Energy Is the Gibbs free-energy change positive or negative in a spontaneous process? 18.4 Gibbs Free-Energy

  14. AGAH -TAS

  15. 18.4 Gibbs Free-Energy • The numerical value of ΔG is negative in spontaneous processes because the system loses free energy.

  16. Gibbs Free energy questions are all about find out if ΔG is negative or positive. If ΔG is negative then the reaction is spontaneous The secret is the negative (subtraction) symbol between the ΔH and the T Δs

  17. 18.4 Gibbs Free-Energy - + - +

  18. 18.4 Gibbs Free-Energy - - + -

  19. 18.4 Gibbs Free-Energy + - - +

  20. 18.4 Gibbs Free-Energy - - - +

  21. 18.4 Gibbs Free-Energy - - - -

  22. 18.4 Section Quiz. • 1. Free energy from a reaction is the amount of energy that is • absorbed by an entropy decrease. • equal to the enthalpy change. • wasted as heat. • available to do work.

  23. 18.4 Section Quiz. • 2. Free energy is always available from reactions that are • endothermic. • nonspontaneous. • at equilibrium. • spontaneous.

  24. 18.4 Section Quiz. • 3. Choose the correct words for the spaces: Spontaneous reactions produce ________ and substantial amounts of _________ at equilibrium. • free energy, products • no free energy, reactants • free energy, reactants • no free energy, products

  25. 18.4 Section Quiz. • 4. Which of the following involves a decrease in entropy? • Natural gas burns. • A liquid freezes. • Dry ice sublimes. • Water evaporates.

  26. 18.4 Section Quiz. • 5. A reaction is spontaneous if • enthalpy decreases and entropy increases. • enthalpy increases and entropy increases. • enthalpy decreases and entropy decreases. • enthalpy increases and entropy decreases.

  27. 18.4 Section Quiz. • 6. Which is the correct expression of the Gibbs free energy equation: • ΔG= ΔH - ΔTS • ΔG= ΔS -TΔH. • ΔG= ΔH -TΔS. • ΔG= ΔH.T.ΔS

  28. I Do • Determine if the following reaction would be spontaneous at 25oC in the forward reaction • N2(g) + 3 H2 (g) 2 NH3 (g) • ΔHo = -92.22 kJ ΔSo = -198.75 J/K • Step 1 convert units: • ΔHo = -92220 J • ΔSo = -198.75 J/K • T = 298K • Step 2 • Write out the Gibbs free energy equation • ΔG = ΔH – TΔS • Step 3 apply parentheses • ΔG = ΔH – (TΔS) • Step 4 substitute in values • ΔG = -92220 - (298 x -198.75) • ΔG = -92220 - (-59227.5) • ΔG = -32992.5J • Step 5 • If ΔG is negative then the reaction is spontaneous in the forward direction • 298K • -92220 J

  29. I Do • Determine if the following reaction would be spontaneous at 25oC in the forward reaction • N2(g) + 3 H2 (g) 2 NH3 (g) • Step 2 Write out the Gibbs free energy equation • ΔG = ΔH – TΔS

  30. I Do. • Determine if the following reaction would be spontaneous at 25oC in the forward reaction • N2(g) + 3 H2 (g) 2 NH3 (g) • Step 3 apply parentheses • ΔG = ΔH – (TΔS)

  31. I Do. • Determine if the following reaction would be spontaneous at 25oC in the forward reaction • N2(g) + 3 H2 (g) 2 NH3 (g) • Step 4 substitute in values • ΔG = -92220 - (298 x -198.75) • ΔG = -92220 - (-59227.5) • ΔG = -32992.5J • Step 5 • If ΔG is negative then the reaction is spontaneous in the forward direction

  32. I Do. • Determine if the following reaction would be spontaneous at 25oC in the forward reaction • N2(g) + 3 H2 (g) 2 NH3 (g) • Step 5 • If ΔG is negative then the reaction is spontaneous in the forward direction

  33. END OF SHOW

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