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Thermodynamics

Thermodynamics. Brown, LeMay Ch 19 AP Chemistry Monta Vista High School. Review. 1 st Law of Thermodynamics In any process energy is neither created nor destroyed. When a system changes from one state to another ( D E = q + w), it Gains heat (+ q) or loses heat (- q) and/or

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Thermodynamics

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  1. Thermodynamics Brown, LeMay Ch 19 AP Chemistry Monta Vista High School

  2. Review 1st Law of Thermodynamics • In any process energy is neither created nor destroyed. • When a system changes from one state to another (DE = q + w), it • Gains heat (+ q) or loses heat (- q) and/or • Does work (- w) or has work done on it (+ w) • T and internal energy, E, are state functions (depend only on initial and final states of system and not path taken between them). • q and w are not state functions. But why does a reaction occur in a particular direction?

  3. 19.1: Spontaneous Processes • Reversible reaction: can proceed forward and backward along same path (equilibrium is possible) Ex: H2O freezing & melting at 0ºC • Irreversible reaction: cannot proceed forward and backward along same path Ex: ice melting at room temperature Spontaneous reaction: an irreversible reaction that occurs without outside intervention Ex: Gases expand to fill a container, ice melts at room temperature (even though endothermic), salts dissolve in water

  4. 19.2: Molecules & Probability • Spontaneity of a reaction is related to the number of possible states a system can have. Ex: 2 gas molecules are placed in a two-chambered container, yielding 4 possible states: • There is a ½ probability that one molecule will be in each chamber, and a ¼, or (1/2)2, probability that both will be in the right-side chamber.

  5. With 3 molecules: There is a ¾ probability that one molecule will be in one chamber and two in the other, and only a 1/8, or (1/2)3, probability that all 3 molecules will be in the right-side chamber. Frequency All on left Evenly distributed All on right

  6. As the number of molecules increases to 100, a bell-shaped distribution of probable states, called a Gaussian distribution, is observed. # molecules = 100 Carl Gauss(1777-1855) Frequency All on left Evenly distributed All on right

  7. Expanding this to 1 mole of molecules, there is only a (1/2)10^23 probability that every molecule will be in the right-side chamber. # molecules = 1023 Frequency All on left Evenly distributed All on right • The Gaussian distribution is so narrow that we often forget that it is a distribution at all, thinking of the most probable state as a necessity.

  8. This demonstrates that: • The most probable arrangements are those in which the molecules are evenly distributed. • Processes in which the disorder of the system increases tend to occur spontaneously. spontaneous non-spontaneous

  9. high K.E. low K.E. • These probability distributions apply to the motion and energy of molecules, and thus can predict the most probable flow of heat. • We call a process spontaneous if it produces a more probable outcome, and non-spontaneous if it produces a less likely one. spontaneous non-spontaneous evenly distributed K.E.

  10. Entropy Entropy (S): a measure of molecular randomness or disorder • S is a state function: DS = Sfinal - Sinitial + DS = more randomness - DS = less randomness • For a reversible process that occurs at constant T: • Units: J/mol.K

  11. 2nd Law of Thermodynamics • The entropy of the universe increases in a spontaneous process and remains unchanged in a reversible (equilibrium) process. • S is not conserved; it is either increasing or constant • Reversible reaction: DSUNIVERSE = SSYS + SSURR = 0 or SSYS = - SSURR • Irreversible reaction: DSUNIV = SSYS + SSURR > 0

  12. Examples of spontaneous reactions: Particles are more evenly distributed Particles are no longer in an ordered crystal lattice Ions are not locked in crystal lattice • Gases expand to fill a container: • Ice melts at room temperature: • Salts dissolve in water:

  13. 19.3: 3rd Law of Thermodynamics • The entropy of a crystalline solid at 0 K is 0. How to predict DS: • Sgas > Sliquid > Ssolid • Smore gas molecules > Sfewer gas molecules • Shigh T> Slow T Ex: Predict the sign of DS for the following: • CaCO3 (s) → CaO (s) + CO2 (g) • N2 (g) + 3 H2 (g) → 2 NH3 (g) • N2 (g) + O2 (g) → 2 NO (g) +, solid to gas -, fewer moles produced ?

  14. 19.4: Standard Molar Entropy, Sº • Standard state (º): 298 K and 1 atm • Units = J/mol·K • DHºf of all elements = 0 J/mol However, S° of all elements ≠ 0 J/mol·K • See Appendix C for list of values. • Where n and m are coefficients in the balanced chemical equation.

  15. 19.5: Gibbs free energy, G • Represents combination of twoforces that drive a reaction:DH (enthalpy) and DS (disorder) • Units: kJ/mol • DG = DH - TDS DG° = DH° - TDS° • (absolute T) Josiah Willard Gibbs(1839-1903)

  16. Determining Spontaneity of a Reaction If DG is :ion) • Positive Forward reaction is non-spontaneous; the reverse reaction is spontaneous • Zero The system is at equilibrium

  17. 19.6: Free Energy & Temperature • DG depends on enthalpy, entropy, and temperature: DG = DH - TDS DHDSDG and reaction outcome - + Always (- 2 O3 (g) → 3 O2 (g) + - Always +; non-spontaneous at all T 3 O2 (g) → 2 O3 (g) - - Spontaneous at low T; non-spontaneous at high T H2O (l) → H2O (s) + + Spontaneous only at high T ; non-spontaneous at low T H2O (s) → H2O (l)

  18. 19.7: Free Energy & Equilibria • Nernst Equation The value of DG determines where the system stands with respect to equilibrium. DG = DG° + RT ln Q (Nernst Equation) where R = 8.314 J/K•mol • Used for calculating DG under experimental conditions from standard conditions DG°. • How do you calculate DG° ? • Nernst Equation when the system is at equilibrium: Note that DG becomes zero at equilibrium and not DG°

  19. 19.7: Free Energy & Equilibria DGReaction outcome Negative Spontaneous forward rxn, K > 1 Positive Non-spontaneous forward rxn, K < 1 Zero System is at equilibrium, K = 1

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