Chemical Chemical ThermodynamicsThermodynamics

Spontaneous Processes and Spontaneous Processes and EntropyEntropy

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

Entropy (S)Entropy (S) A measure of the randomness or A measure of the randomness or

disorder disorder The driving force for a spontaneous The driving force for a spontaneous

process is an increase in the entropy of process is an increase in the entropy of the universethe universe

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

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

Positional EntropyPositional Entropy The probability of occurrence of The probability of occurrence of

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

SSsolidsolid < S < Sliquidliquid << S << Sgasgas

Second Law of Second Law of ThermodynamicsThermodynamics

"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

H, H, S, S, G and SpontaneityG and Spontaneity

Value of H

Value of TS

Value of G

Spontaneity

Negative Positive Negative

Spontaneous

Positive Negative Positive Nonspontaneous

Negative Negative ??? Spontaneous if the absolute value of H is greater than the absolute value of TS (low temperature)

Positive Positive ??? Spontaneous if the absolute value of TS is greater than the absolute value of H (high temperature)

G = G = H - TH - TSSH is enthalpy, T is Kelvin temperatureH is enthalpy, T is Kelvin temperature

Calculating Entropy Change in Calculating Entropy Change in a Reactiona Reaction

0 0 oreaction products reactantsp rS n S n S

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

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

Standard Free Energy ChangeStandard 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

For reactions at constant temperature:G0 = H0 - TS0

Calculating Free Energy Calculating Free Energy Method #1Method #1

Calculating Free Energy: Method Calculating Free Energy: Method #2#2

An adaptation of Hess's Law:Cdiamond(s) + O2(g) CO2(g) G0 = -397 kJCgraphite(s) + O2(g) CO2(g) G0 = -394 kJ

CO2(g) Cgraphite(s) + O2(g) G0 = +394 kJ

Cdiamond(s) Cgraphite(s)

G0 =

Cdiamond(s) + O2(g) CO2(g) G0 = -397 kJ

-3 kJ

Calculating Free Energy Calculating Free Energy Method #3Method #3

Using standard free energy of formation (Gf0):

0 0 0(products) (reactants)p f r fG n G n G

Gf0 of an element in its standard state is zero

The Dependence of Free The Dependence of Free Energy on PressureEnergy on Pressure

Enthalpy, H, is not pressure dependent

Entropy, S entropy depends on volume, so it

also depends on pressure

Slarge volume > Ssmall volume

Slow pressure > Shigh pressure

Free Energy and EquilibriumFree Energy and Equilibrium

Equilibrium point occurs at the lowest value of free energy available to the reaction system

At equilibrium, G = 0 and Q = K

G0 K

G0 = 0 K = 1

G0 < 0 K > 1

G0 > 0 K < 1

Temperature Dependence Temperature Dependence of Kof K

000 )ln( STHKRTG

R

S

TR

H

R

S

RT

HK

0000 1)ln(

So, ln(K) 1/T

Free Energy and WorkFree Energy and Work The maximum possible useful work

obtainable from a process at constant temperature and pressure is equal to the change in free energy

The amount of work obtained is always less than the maximum

Henry Bent's First Two Laws of Thermodynamics

First law: You can't win, you can only break even

Second law: You can't break even