We now that shows the relationship between the

enthalpy

change of a system and the

entropy

change of the surroundings

deltaS(univ)= deltaS(sys) + deltaS(surr)

Calculating entropy changes for a reaction

Standard entropy change for a reaction- the change in S for a process in which all reactants are in their standard state.

for a gas - 1 atm

for liquid or solid-the most stable form at 1 atm and temperature of interest

for solution - 1 M concenctration

Calculating Free Energy changes in Reactions

Energy & Entropy

What is the

Chemical Potential

?

Enthalpy? No! Not Enthalpy alone.

Spontaneous processes occur because they release energy from the system.

Most spontaneous processes proceed from higher potential energy to lower potential energy. - Exothermic, -deltaH

There are some spontaneous processes that proceed from lower potential energy to higher potential energy. - Endothermic, +deltaH

How can something absorb potential energy, yet have a net release of energy?

Heat Transfer and Entropy

delta

S

universe = delta

S

sys. + delta

S

surround.

What If the entropy of the system decreases? Then the entropy of the surroundings must increase by a larger amount.

Processes like heat flow from hot to cold, water vapor condensing or water freezing are spontaneous, even though entropy decrease. How?

The entropy increase of the surroundings must come from heat released by the system; the process must be exothermic!

Energy

The first law of thermodynamics is that energy cannot be created or destroyed.

two ways energy is “lost” from a system

deltaE = q + w

deltaE = deltaH + P(deltaV)

deltaE is a state function.

Every energy transition results in a “loss” of energy. (Heat Tax)

No Perpetual Motion!

**Free Energy & Thermodynamics**

Entropy

Entropy (

S

) is a thermodynamic function that increases as the number of energetically equivalent ways of arranging the components increases.

Units - J/K*mol

S =

k

ln W

k = Boltzmann constant = 1.38 × 10^-23 J/K

W

is the number of energetically equivalent ways a system can exist. Unitless

Practice

C3H8(g) + 5 O2(g) --> 3 CO2(g) + 4 H2O(g)

1. deltaG(rxn) = deltaH(rxn) - TdeltaS(rxn)

estimate at any temperature

2. Using free energy of formation values

deltaG(rxn) = deltaG(prod) - deltaG(react)

only at 25 C ( 298K)

3. For a stepwise reaction (Hess's Law)

Spontaneity

A fundamental goal of

Thermodynamics

is to predict spontaneity or processes.

Spontaneous

- Processes that will occur without outside intervention.

Nonspontaneous processes require energy input to go.

determined by looking at a

chemical potential

that predicts direction of change.

Spontaneous processes are

irreversible

.

Any

reversible

process is at equilibrium.

If this

potential

energy after reaction < before the reaction, the reaction is thermodynamically favorable.

Spontaneity

does not equal

fast or slow

More freedom of motion increases the randomness of the system. When systems become more random, energy is released. We call this energy,

entropy

A - 1 microstate

B - 1 microstate

C - 6 microstates

Therefore, state C has higher entropy than either state A or B.

energy more dispersed over 6 possibilities.

Macrostates

There are two factors that determine whether a reaction is spontaneous.

enthalpy change

and

entropy change

of the system.

The enthalpy change is favorable for exothermic reactions and unfavorable for endothermic reactions

deltaS = Sfinal - Sinitial (a state function)

Entropy change is favorable when the result is a more random system. deltaS is positive.

Some changes that increase the entropy are:

Reactions whose products are in a more random state

Solid more ordered than liquid; liquid more ordered than gas

Reactions that have larger numbers of product molecules than reactant molecules

Increase in temperature

Solids dissociating into ions upon dissolving

The second law of thermodynamics

- for a spontaneous process the total entropy change of the

universe

must be positive

For reversible process deltaSuniv = 0

A chemical system proceeds in the direction that increases the entropy of the universe.

Change is temperature dependent (units J/K)

delta

S

surr = deltaHsys/T

Kinetics - Speed of a reaction-How Fast (Chapter 13)

Looks at how long it takes for a reaction to reach equilibrium. The kinetics of a process is effected by:

Concentration of reactants

Temperature

Use of a catalyst

Mechanism of the reaction

Equilibrium – the extent of a reaction

Looks at how far a reaction proceeds toward products before the rate of the forward process equals the rate of the reverse process. The equilibrium expression and equilibrium constant (Keq) describes mathematically the extent of reaction.

Thermodynamics

– used to predict the

spontaneity

of a reaction. (

Why They Occur

)

Involves the study of changes in the system and surroundings of:

Enthalpy - H (Chapter 6)

Entropy - S

Free energy - G (a combination of H and S).

Looking Back

Catalyst?

Using q(sys) to quantify deltaS(surr)

-q(sys) --> +deltaS(surr)

entropy change of surrounding inversely related to temperature -

deltaS(surr) --> -q(sys)/T

at constant P

q(sys) = deltaH(sys)

so at

constant P & T

deltaS(surr) = - deltaH(sys)/T

So an exothermic system will lead to an increase in entropy of the surrounding that is inversely related to temperature

deltaH(rxn) = -2044KJ

a. calculate the entropy change in the surroundings associated with this reaction at 25 degrees C.

b. Determine the sign of the entropy change for the system.

c. Determine the sign of the entropy change for the universe. Will the reaction be spontaneous?

deltaS(univ)= deltaS(sys) - deltaH(sys)/T

(-T)deltaS(univ)= (-T)deltaS(sys) + (T)deltaH(sys)/T

(-T)deltaS(univ)= deltaH(sys) - (T)deltaS(sys)

the right side of this equation is the thermodynamic function

Gibbs Free Energy(G)

G = H -TS

the change in free energy:

deltaG = deltaH - T(deltaS)

deltaG = -TdeltaS(univ)

Since deltaS(univ) is a basis for spontaneity, So deltaG must also be an indicator of spontaneity.

Gibbs free energy is the

chemical potential

we have been looking for.

It represents the maximum Useful work available from a reaction

Chemical systems tend toward

lower free energy

- deltaG - indicates a spontaneous process

+ deltaG indicates a nonspontaneous process

CCl4(g) --> C (s, graphite) + 2 Cl2(g) deltaH = +95.7KJ, deltaS = +142.2 J/K

Calculate deltaG at 25 C. Is it spontaneous?

Practice:

C2H4(g) + H2(g) --> C2H6(g) deltaH = -137.5 KJ, deltaS = -120.5 J/K

Calculate deltaG at 25 C. Is it spontaneous?

At constant P and T

endothermic processes

like Melting favored at high T

decreasing entropy processes

like freezing favored at low T

deltaS(rxn)

deltaS(rxn) = S(prod) - S(react)

Third law of thermodynamics

- The entropy of a perfect crystal at absolute zero (0 K) is zero.

Standard molar entropy

values are measured against this

Calculate deltaS(rxn) for the following equation.

4 NH3(g) + 5 O2(g) --> 4 NO(g) + 6 H2O(g)

The change if Free energy of a chemical reaction represents the maximum amount of energy available, or

Free

, to do useful work (if deltaG is negative).

this is often less then the change in enthalpy

deltaG represents the

theoretical

limit of how much work can be done by the reaction.

In

Rea

l

reactions the available energy is even

less

then deltaG because of additional energy loss (heat tax)

deltaS = -80.5 KJ

"Free" Energy

If deltaG is positive it represents the theoretical minimum amount of energy required to make the reaction occur

slower current will reduce loss

recharging will also take more energy

Free Energy for Nonstandard States

Why?

H20(l) <-> H2O(g)

At standard state:

deltaG = +8.59 kJ/mol

Standard state - P(H2O) = 1 atm

Normal state - P(H2O) << 1 atm

normal conditions not standard conditions!

not spontaneous

deltaG @ nonstandard conditions-

deltaG = deltaG(stand) +

RT

ln

Q

Q

- reaction quotient

@ standard conditions

Q

= 1

@equilibrium at 298K

Q

=

K

=0.0313 and deltaG = 0

other nonstandard - plug in

Q

lower P(H2O) = evaporation

higher P(H2O) = condense

Standard Free Energy and Equilibrium

Spontaneity [delta G(stand)] and the equilibrium constant (K) and connected.

if a forward process occurs with a large negative deltaG then the K value will be large

if a forward process occurs with a large positive deltaG then the K value will be small

@ equilibrium DeltaG =0

deltaG(rxn) = -

RT

lnK

Temperature dependence and K

deltaG(rxn) = deltaH -TdeltaS

ln(K2/K1) = (-deltaH(rxn)/R)(1/T2 - 1/T1)

Can use to find deltaH from K at 2 temperatures or find K at a temperature if we know K at another temp and deltaH