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1、Spontaneous Processes A spontaneous process is one that does occur under the given set of conditions. If a reaction does not occur under a given set of conditions, it is said to be non-spontaneous.Spontaneous ProcessesSome types of spontaneous processes:Phase transitions (melting, freezing)Mixing an
2、d dissolving of solutes into solutionExpansion of gases into an evacuated bulbHeat transfer from hot to cold objectsMovement towards chemical equilibriumEveryday Examples: Water freezes spontaneously below 0oC and melts spontaneously above 0oC 1 atm A lump of sugar spontaneously dissolves in a cup o
3、f coffee, but dissolved sugar does not spontaneously reappear in its original form Expansion of a gas in an evacuated bulb is a spontaneous process. The gathering of all gas molecules into one location is not spontaneous Heat flow from a hotter object to a colder one is spontaneous, but the reverse
4、never happens spontaneously Iron exposed to water and oxygen forms rust, but rust does not spontaneously change to ironSpontaneous ProcessWhat makes a process spontaneous? Often, but not always, a system that is spontaneous tries to minimize its energy. A spontaneous system always maximizes its ENTR
5、OPY.Entropy (S) Entropy (S) is a direct measure of the randomness or disorder of a system At disorder increases, Entropy increases Entropy can also be related to probabilityOrder of Entropy (S) For all substances, the particles in the solid state are more ordered than those in the liquid state, whic
6、h are both more ordered than those in the gaseous state. Ssolid Sliquid 0)Entropy as a state function Like energy and enthalpy, entropy is a state function. S = Sf Si If the change results in an increase in randomness, then Sf Si State functions are properties that are determined by the state of the
7、 system.Second Law of Thermodynamics The entropy of the universe increases in a spontaneous process and remains unchanged in and equilibrium process. Spontaneous: Suniv = Ssys+ Ssurr 0Equilibrium: Suniv = Ssys+ Ssurr =0Entropy Changes in the System Entropy change (S) in the system aA + bB cC + dD S
8、standard entropy of reaction Srxn = sum of S for products - sum of S for reactants Srxn = S(products) - S(reactants)Entropy Changes in Surroundings Ssurr = -Hsys / TThis assumes that both the system and the surroundings are at temperature TMr. Gibbs and His Free Energy “Free energy” does not mean “w
9、ithout cost”. Rather, it means “energy available to do work” The Sign and Value of the free energy will determine the spontaneity of a reaction.More Mr. Gibbs Free energy is used to express the spontaneity of a reaction more directly. G = H TS, where all quantities pertain to the system. The change
10、in free energy in a constant temp. process can be expressed by: G = H - TS If G is negative, then the reaction must be spontaneous.Standard Free-Energy Changes Standard free energy of reaction is the free energy change for a reaction when it occurs under standard state conditions, when reactants in
11、their standard states are converted to products in their standard states. G = Gf (products) - Gf (reactants) If both H and S are positive, G will be negative only when TS is greater than H If H is negative, and S is negative, G will always be positive, regardless of temperature IfH is neg. and S is
12、positive, then G will be negative always If H is negative and S is negative, G will be negative only when TS is smaller than HTemp and Chemical Reactions Ball park estimates of when a reaction becomes spontaneous Procedure: find H and S at 25 C set H - TS = 0 solve for T, estimated temperature at wh
13、ich reaction becomes spontaneousPhase Transitions G = 0 at the temperature of a phase transitionthus, H - TS = 0so S = H / T Example: Sice water = H / T H = 6010 J/mole molar heat of fusion of water T = 273 freezing point of water is 0 C Sice water = 6010 J/mole . 273 K = 22.0 J/mole .K (entropy inc
14、rease) Swater ice = - 22.0 J/mole .K (entropy decrease)Free Energy and Chemical Equilibrium Once a reaction starts, standard state conditions no longer exist for the products or the reactants. For any chemical system:G = G + (RT) ln Q If G is not zero, then the system is not at equilibrium and it will spontaneously shift toward the equilibrium state At equilibrium: G = 0 and Q = KG = - RT ln K When K is very small o
