A science that includes the study of A science that includes the study of energy transformations and the energy transformations and the relationships among the physical relationships among the physical properties of substances which are properties of substances which are affected by these transformations.affected by these transformations.

Zeroth Law- If two thermodynamic systems are in thermal equilibrium with a third, they are also in thermal equilibrium with each other. A ⇔ B and B ⇔ C implies A ⇔ C.

- The increase in the internal energy of a system can be achieved in many ways, among which is by heating the system. “Energy can neither be created nor destroyed" or The energy of the universe is constant

First Law

Second Law- the fact that over time, ignoring the effects of self-gravity, differences in temperature, pressure, and density tend to even out in a physical system that is isolated from the outside world. Entropy is a measure of how far along this evening-out process has progressed.

- as a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value. This is based on the assumption that the entropy of a perfect crystal is zero at 0 K.

Third Law

1. Spontaneous - 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

2. Reversible a process or cycle such that the net change at each stage in the combined entropy of the system and its surroundings is zero.

3. Irrevesible there will be a certain amount of heat energy loss or dissipation due to intermolecular friction and collisions; energy that will not be recoverable if the process is reversed.

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 universe the 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 system that are available to a system

States that have the highest States that have the highest probabilities of existing are favorableprobabilities of existing are favorable

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 achieved arrangement 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

Things in the world tend toward lowest energy (-ΔH) and also tend toward greatest disorder (+ ΔS).More disorder can be recognized as:· greater # of moles of gas formed· gas > liquid > solid and (aq) > (s)· greater # of particles· greater volume formedexamples:

2NH3 (g) → N2(g) + 3H2(g) NaCl (s) → Na+ + Cl-

NaCl (s) → NaCl (l)Note: ΔS° can be calculated using Hess’s Law, but S° of elements is NOT 0. Also, S° is often reported in J/mol×K and cal/mol×K, not kJ and kcal… watch your units!

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

ΔG= ΔH-TΔS at constant temperature

• If ΔG is negative at constant T and P, the Process is spontaneous.

For reactions at constant temperature: G0 = H0 - TS0

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

Gibb’s-Helmholtz equation:

Let’s Check

•For the reaction H2O(s) → H2O(l)

Sº = 22.1 J/K mol Hº =6.02 kJ/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.

Positive Negative

ΔH

ΔS

ΔG

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

Value of H

Value of TS

Value of G

Spontaneity

Negative Positive

Positive Negative

Negative Negative

Positive Positive

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

Negative Spontaneous

Positive Nonspontaneous

???

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

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

Are diamonds forever?????

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 Energy The Dependence of Free Energy on Pressureon 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 of KTemperature Dependence of K

000 )ln( STHKRTG

So, ln(K) 1/T

0 ln( )G RT K

Nonstandard Free EnergyNonstandard Free Energy

0 ln( )G G RT Q