Spontaneity, Entropy, and Free Energy

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Slide 1

Spontaneity, Entropy and Free Energy

Slide 2

Spontaneous Processes and Entropy

First Law

“Energy can neither be created nor destroyed"

The energy of the universe is constant

Spontaneous Processes

Processes that occur without outside intervention

Spontaneous processes may be fast or slow

Many forms of combustion are fast

Conversion of diamond to graphite is slow

Slide 3

Entropy (S)

A measure of the randomness or disorder

The driving force for a spontaneous process is an increase in the entropy of the universe

Entropy is a thermodynamic function describing the number of arrangements that are available to a system

Nature proceeds toward the states that have the highest probabilities of existing

Slide 4

Positional Entropy

The probability of occurrence of a particular state depends on the number of ways (microstates) in which that arrangement can be achieved

Ssolid < Sliquid << Sgas

Slide 5

Second Law of Thermodynamics

"In any spontaneous process there is always an increase in the entropy of the universe"

"The entropy of the universe is increasing"

For a given change to be spontaneous, Suniverse must be positive

Suniv = Ssys + Ssurr

Slide 6

H, S, G and Spontaneity

G = H - TS

H is enthalpy, T is Kelvin temperature

Slide 7

Calculating Entropy Change in a Reaction

Entropy is an extensive property (a function of the number of moles)

Generally, the more complex the molecule, the higher the standard entropy value

Slide 8

Standard Free Energy Change

G0 is the change in free energy that will occur if the reactants in their standard states are converted to the products in their standard states

G0 cannot be measured directly

The more negative the value for G0, the farther to the right the reaction will proceed in order to achieve equilibrium

Equilibrium is the lowest possible free energy position for a reaction

Slide 9

For reactions at constant temperature: