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Chapter 16 Spontaneity, entropy and free energy

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Spontaneous l A reaction that will occur without outside intervention. l We cant determine how fast. l We need both thermodynamics and kinetics to describe a reaction completely. l Thermodynamics compares initial and final states. l Kinetics describes pathway between.

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Thermodynamics l 1st Law- the energy of the universe is constant. l Keeps track of thermodynamics doesnt correctly predict spontaneity. l Entropy (S) is disorder or randomness l 2nd Law the entropy of the universe increases.

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Entropy l Defined in terms of probability. l Substances take the arrangement that is most likely. l The most likely is the most random. l Calculate the number of arrangements for a system.

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l 2 possible arrangements l 50 % chance of finding the left empty

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l 4 possible arrangements l 25% chance of finding the left empty l 50 % chance of them being evenly dispersed

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l 4 possible arrangements l 8% chance of finding the left empty l 50 % chance of them being evenly dispersed

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Gases l Gases completely fill their chamber because there are many more ways to do that than to leave half empty. l S solid <S liquid <<S gas l there are many more ways for the molecules to be arranged as a liquid than a solid. l Gases have a huge number of positions possible.

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Entropy l Solutions form because there are many more possible arrangements of dissolved pieces than if they stay separate. l 2nd Law S univ = S sys + S surr If S univ is positive the process is spontaneous. If S univ is negative the process is spontaneous in the opposite direction.

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For exothermic processes S surr is positive. For endothermic processes S surr is negative. Consider this process H 2 O(l) H 2 O(g) S sys is positive S surr is negative S univ depends on temperature.

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Temperature and Spontaneity l Entropy changes in the surroundings are determined by the heat flow. l An exothermic process is favored because by giving up heat the entropy of the surroundings increases. The size of S surr depends on temperature S surr = - H/T

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S sys S surr S univ Spontaneous? +++ --- +-? +-? Yes No, Reverse At Low temp. At High temp.

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Gibb's Free Energy l G=H-TS l Never used this way. G= H-T S at constant temperature l Divide by -T - G/T = - H/T- S - G/T = S surr + S - G/T = S univ If G is negative at constant T and P, the Process is spontaneous.

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Lets Check For the reaction H 2 O(s) H 2 O(l) Sº = 22.1 J/K mol Hº =6030 J/mol Calculate G at 10ºC and -10ºC Look at the equation G= H-T S Spontaneity can be predicted from the sign of H and S.

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G= H-T S H S Spontaneous? +- At all Temperatures ++ At high temperatures, entropy driven -- At low temperatures, enthalpy driven +- Not at any temperature, Reverse is spontaneous

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Third Law of Thermo l The entropy of a pure crystal at 0 K is 0. l Gives us a starting point. l All others must be>0. l Standard Entropies Sº ( at 298 K and 1 atm) of substances are listed. Products - reactants to find Sº (a state function). l More complex molecules higher Sº.

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Free Energy in Reactions Gº = standard free energy change. l Free energy change that will occur if reactants in their standard state turn to products in their standard state. l Cant be measured directly, can be calculated from other measurements. Gº= Hº-T Sº l Use Hesss Law with known reactions.

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Free Energy in Reactions There are tables of Gº f. l Products-reactants because it is a state function. l The standard free energy of formation for any element in its standard state is 0. l Remember- Spontaneity tells us nothing about rate.

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G= H-T S H S Spontaneous? +- At all Temperatures ++ At high temperatures, entropy driven -- At low temperatures, enthalpy driven +- Not at any temperature, Reverse is spontaneous

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Third Law of Thermo l The entropy of a pure crystal at 0 K is 0. l Gives us a starting point. l All others must be>0. l Standard Entropies Sº ( at 298 K and 1 atm) of substances are listed. Products - reactants to find Sº (a state function) l More complex molecules higher Sº.

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Free Energy in Reactions Gº = standard free energy change. l Free energy change that will occur if reactants in their standard state turn to products in their standard state. l Cant be measured directly, can be calculated from other measurements. Gº= Hº-T Sº l Use Hesss Law with known reactions.

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Free Energy in Reactions There are tables of Gº f l Products-reactants because it is a state function. l The standard free energy of formation for any element in its standard state is 0. l Remember- Spontaneity tells us nothing about rate.

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Free energy and Pressure G = Gº +RTln(Q) where Q is the reaction quotients (P of the products /P of the reactants). CO(g) + 2H 2 (g) CH 3 OH(l) l Would the reaction be spontaneous at 25ºC with the H 2 pressure of 5.0 atm and the CO pressure of 3.0 atm? Gº f CH 3 OH(l) = -166 kJ Gº f CO(g) = -137 kJ Gº f H 2 (g) = 0 kJ

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How far? G tells us spontaneity at current conditions. When will it stop? l It will go to the lowest possible free energy which may be an equilibrium. At equilibrium G = 0, Q = K Gº = -RTlnK

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Gº K =0=1 0 >0<0

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Temperature dependence of K Gº= -RTlnK = Hº - T Sº ln(K) = Hº/R(1/T)+ Sº/R l A straight line of lnK vs 1/T

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Free energy And Work l Free energy is that energy free to do work. l The maximum amount of work possible at a given temperature and pressure. l Never really achieved because some of the free energy is changed to heat during a change, so it cant be used to do work.

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Spontaneity, Entropy and Free Energy. Spontaneous Processes and Entropy First Law “Energy can neither be created nor destroyed" The energy of the universe.

Spontaneity, Entropy and Free Energy. Spontaneous Processes and Entropy First Law “Energy can neither be created nor destroyed" The energy of the universe.

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