Thermodynamics Brown, LeMay Ch 19 AP Chemistry Monta Vista High School To properly view this presentation on the web, use the navigation arrows below and left-click on each page to view any animations.

2 Review 1 st Law of Thermodynamics In any process energy is neither created nor destroyed. When a system changes from one state to another (  E = q + w), it 1.Gains heat (+ q) or loses heat (- q) and/or 2.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 Reversible reaction: can proceed forward and backward along same path (equilibrium is possible) Ex: H 2 O 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 19.1: Spontaneous Processes

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. All on left Evenly distributed All on right Frequency

6 All on left Evenly distributed All on right Frequency 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 ( )

7 All on left Evenly distributed All on right Frequency # molecules = 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. 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. spontaneousnon-spontaneous

9 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. spontaneousnon-spontaneous high K.E. low K.E. evenly distributed K.E.

10 Entropy Entropy (S): a measure of molecular randomness or disorder S is a state function:  S = S final - S initial +  S = more randomness -  S = less randomness For a reversible process that occurs at constant T: Units: J/K

11 2 nd 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:  S UNIVERSE = S SYS + S SURR = 0 or S SYS = - S SURR Irreversible reaction:  S UNIV = S SYS + S SURR > 0

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

: 3 rd Law of Thermodynamics The entropy of a crystalline solid at 0 K is 0. How to predict  S: S gas > S liquid > S solid S more gas molecules > S fewer gas molecules S high T > S low T Ex: Predict the sign of  S for the following: 1.CaCO 3 (s) → CaO (s) + CO 2 (g) 2.N 2 (g) + 3 H 2 (g) → 2 NH 3 (g) 3.N 2 (g) + O 2 (g) → 2 NO (g) +, solid to gas -, fewer moles produced ?

: Standard Molar Entropy, Sº Standard state (º): 298 K and 1 atm Units = J/mol·K  Hº f 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.

: Gibbs free energy, G Represents combination of two forces that drive a reaction:  H (enthalpy) and  S (disorder) Units: kJ/mol   G =  H - T  S  G° =  H° - T  S°  (absolute T) Called “free energy” because  G represents maximum useful work that can be done by the system on its surroundings in a spontaneous reaction. (See p. 708 for more details.) Josiah Willard Gibbs ( )

16 Determining Spontaneity of a Reaction If  G is: Negative (the free energy of system decreases) The reaction is spontaneous (proceeds in the forward direction) Positive Forward reaction is non-spontaneous; the reverse reaction is spontaneous ZeroThe system is at equilibrium

: Free Energy & Temperature  G depends on enthalpy, entropy, and temperature:  G =  H - T  S  H  S  G and reaction outcome -+Always (-); spontaneous at all T 2 O 3 (g) → 3 O 2 (g) +-Always +; non-spontaneous at all T 3 O 2 (g) → 2 O 3 (g) --Spontaneous at low T; non-spontaneous at high T H 2 O (l) → H 2 O (s) ++Spontaneous only at high T ; non-spontaneous at low T H 2 O (s) → H 2 O (l)

: Free Energy & Equilibria The value of  G determines where the system stands with respect to equilibrium.  G =  G° + RT ln Q where R = J/Kmol or  G° = - RT ln K eq or

: Free Energy & Equilibria  G ° Reaction outcome NegativeSpontaneous forward rxn, K > 1 PositiveNon-spontaneous forward rxn, K < 1 ZeroSystem is at equilibrium, K = 1