1 Unit 10 Lesson 4 In Chemical Reactions

© 2009, Prentice-Hall, Inc. Spontaneous Processes Spontaneous processes (physical or chemical) are those that can proceed without any outside intervention. The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously return to vessel B.

Spontaneous Reactions A Spontaneous Reaction proceeds without needing a constant input of energy (they usually have an output of energy) –Think about iron rusting, it happens “automatically” and releases heat A non spontaneous reaction will not proceed without a constant input of energy –Think about water decomposing, it needs a constant supply of electricity 3

© 2009, Prentice-Hall, Inc. Spontaneous Processes Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.

5 Entropy (  S) A measure of the disorder or randomness in a system. The law of disorder states that the natural tendency of systems is to move towards a state of maximum disorder or randomness. It is related to the various modes of motion in molecules. Increasing disorder = +  S Decreasing disorder = -  S

6 Examples Increasing entropy examples (positive  S) Solid  Liquid  Gas –Gas particles are less “orderly” than solids Dissolving of a solid –Orderly crystals dissolve into ions in solutions When temperature increases –Particles begin to move faster, have more collisions, and more disorder

7 Spontaneous Reactions If a reaction is spontaneous it will occur without the addition of energy. The spontaneity of a reaction is measured by free energy. Negative free energy (  G) is spontaneous. –It means there is a “surplus” of energy so the reaction has more than enough to proceed

Gibb’s Law of Free Energy  G =  H – T  S +  G = Non-spontaneous  G = Spontaneous 8 Free Energy Enthalpy Entropy Temp (in K)

9 Example Problem Calculate the free energy for a reaction at 25°C if  H rxn = kJ/mol &  S rxn = 31.8 J/molK. Is the reaction spontaneous?  G =  H - T  S  G = ( kJ/mol) – (298 K .0318 kJ/molK)  G = kJ/mol Yes, it is spontaneous

10  G =  H - T  S EnthalpyEntropy Spontaneous Rxn? (neg.) Negative (exo)PositiveYes Positive (endo)Positive Only at high temperatures Negative (exo)Negative Only at low temperatures Positive (endo)NegativeNo