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Spontaneity

Spontaneity. 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.

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Spontaneity

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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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