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Understanding Spontaneous Processes in Chemistry

Explore the concepts of enthalpy, entropy, and Gibbs free energy to predict the spontaneity of chemical reactions. Learn how nature favors lower energy states and increased chaos. Discover how temperature influences entropy and spontaneity.

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Understanding Spontaneous Processes in Chemistry

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  1. Spontaneity

  2. Recap of Enthalpy • Describes chemical potential energy stored in matter. • Can only measure changes in enthalpy. • Enthalpy is arithmetical. • Reverse equation, reverse sign of H • Multiply equation by some #, multiply H by that number. • Add two equations, add the H’s.

  3. Spontaneous Processes • A physical or chemical change that occurs with no outside intervention. • Some energy may be supplied to get the process started – activation energy

  4. Examples of Exothermic Spontaneous Processes • Burning methane gas, CH4, in the bunsen burner. CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) H = -891 kJ • Iron rusting 4Fe(s) + 3O2(g)  2Fe2O3(s) H = -1625 kJ

  5. Some Spontaneous Processes are Endothermic • Dissolving NH4NO3(s) in water is an example of a spontaneous endothermic process. • NH4NO3(s)  NH4+(aq) + NO3-(aq) • Ice melting is another spontaneous, endothermic process. • H2O(s)  H2O(l)

  6. Nature is lazy & disorganized • 2 driving forces in nature • Get to the lowest energy state • Get to the most chaotic state • For a system to get lower in energy, it must release energy. • So nature favors exothermic processes with -H. • But spontaneous endothermic processes occur, so something else is important, too.

  7. Entropy, S • A measure of the disorder or randomness of the particles of a system. • Law of Disorder: spontaneous processes always occur in the direction that increases the chaos of the universe. Unmelt.mov Unmix.mov Unshatr.mov

  8. Chaos • The more degrees of freedom a system has, the more chaotic it can be. • Degrees of freedom = ways you can move & places you can be. • Gases have much more entropy than liquids & liquids have more than solids.

  9. Entropy = 0? • A pure crystal with no imperfections at 0K. • Every atom is where it’s supposed to be. • Nothing is moving.

  10. Changes in Entropy, S • S = Sfinal – Sinitial or Sproducts – Sreactants • Nature wants to increase S, so Sfinal > Sinitial • Nature wants S to be positive.

  11. Predicting Changes in Entropy • Sgas > Sliquid > Ssolid • SMixture > SPureSubstance • SDissolved Solid > SSolidbut • Sgas > SDissolved Gas

  12. Predicting Entropy • Which has more entropy, 1 mole of dry ice or 1 mole of CO2 gas? • 1 mole of CO2 gas

  13. Predicting Changes in Entropy • 2SO3(g)  2SO2(g) + O2(g) S > 0 • 2 moles of gas on the reactant side. • 3 moles of gas on the product side. • Plus, the product side is a mixture.

  14. Temperature & Entropy • An increase in temperature increases the random motion of the particles, so entropy increases with temperature.

  15. Predicting Spontaneity • Use Gibbs free energy expression • G = H - TS • If G is negative, rxn is spontaneous. • If G is positive, rxn is not spontaneous.

  16. G = H + (-TS)

  17. G = H - TS • H is negative; S is positive. • H is negative; S is negative. • H is positive; S is positive. • H is positive; S is negative. Always spontaneous. Spontaneity depends on temperature. Spontaneity depends on temperature. Never spontaneous.

  18. G = H - TS G is negative for spontaneous processes.

  19. Summary • Nature has 2 driving forces • Tends to minimize enthalpy (potential energy). Wants H to be negative. • Tends to maximize entropy (chaos). Wants S to be positive.

  20. Summary • Spontaneity is determined by the combination of the enthalpy change and the entropy change.

  21. Predict the sign of G • LiBr(s)  Li+(aq) + Br-(aq) + 48.83 kJ • The reaction is exothermic so H is negative. The chaos increases so S is positive. Both enthalpy and entropy are going in the direction preferred by nature. This reaction is always SPONTANEOUS.

  22. Predict the sign of G • N2(g) + 2 O2(g) + 66.4 kJ  2 NO2(g) • The reaction is endothermic so H is positive. The chaos decreases (3 moles of a gas to 2 moles of a gas) so S is negative. Neither the enthalpy nor the entropy goes in the direction that nature prefers, so this reaction is NEVER spontaneous.

  23. Predict the sign of G • 2 H2(g) + O2(g)  2 H2O(l) + 571.6 kJ • The reaction is exothermic: H is negative. • Chaos decreases: S is negative. • One driving force, enthalpy, is with nature. • Entropy is going against nature. • CANNOT predict the spontaneity of this rxn.

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