the field of thermodynamics studies the behavior of energy (heat) flow. from this study, a number of...

The field of thermodynamics studies the behavior of energy (heat) flow. From this study, a number of physical laws have been established.

The laws of thermodynamics describe some of the fundamental truths observed in our Universe.

Thermodynamics isn’t just a good idea, it’s the law!

A way to think about thermodynamics, is to liken it to gambling. The Universe is the Great Casino.

You have to understand, that no matter what, in the long run, the house is going to win.

1. You can’t win. You can’t get something for nothing.

2. You can’t break even. The house always wins!

3. You can’t get out of the game.

These are the rules:

In the end, you’re going to lose….no matter what.

Every time you place a bet…the house takes it’s share.

You can’t make any new money,….the amount of money is fixed, and finite. Kind of like “table stakes”.

…and so first there was the second law, then there came

the first law, which was followed 80 years later by the zeroth law, and then

finally the third law, which may not actually be a law at all. So, what confuses you?

ThermodynamicsFirst Law: The energy of the universe is

constant. Energy can be neither created nor destroyed, so while energy can be converted to another form, the total energy remains constant.

This is merely a statement of conservation of energy.

Energy In Energy Out

Any Isolated System

Electric Motor

Electric Generator

Mechanical Work Out

For a long time, people tried to design and build Perpetual Motion Machines

Cool. I’m going to be rich!

The enthalpy of a substance is a measure of the energy that is released or absorbed by the substance when bonds are broken and formed during a reaction.

When bonds are formed, energy is released. In order to break bonds, energy must be absorbed.

H = Hproducts - Hreactants

Enthalpy – Heat of Reaction

The Universe

System

Surroundings

Consider a chemical reaction (system).

The Universe

System

Surroundings

Heat

When heat leaves the system, the reaction is said to be exothermic. (H = -)

Energy of the surroundings goes

up. H = (+)

The Universe

System

Surroundings

Heat

When heat enters the system, the reaction is said to be endothermic. (H = +)

Energy of the surroundings

goes down. H = (-)

The Second Law

You can’t break even….the house always takes it’s cut.

This all has to do with something called entropy.

The time has come the Walrus said, to speak of many things. Of shoes, and ships, and sealing

wax. Of Entropy, Enthalpy, and Free

Energy.

The answer is entropy.

So, what’s the question?

Why do things happen the way they do, and not in reverse?

Glassware breaks spontaneously when it hits the floor. Yet you can’t drop broken glass and have it form a graduated cylinder.

Why is it that….

A parked new car left to itself will become junk over time…..

The junk car will never become like new over time

A sugar cube dissolves spontaneously in hot coffee, but dissolved sugar never precipitates out to form a sugar cube?

x

Entropy

A measure of the disorder of a system.

Systems tend to change from a state of low entropy to a state of higher entropy. That is to say, if left to themselves, systems tend to increase their entropy.

Solids Liquids Solutions Gases

Increasing Entropy

Fewer Particles More Particles

The second law can be stated in many forms…

Heat can never pass spontaneously from a cold to a hot body. As a result of this fact, natural processes that involve energy transfer must have one direction, and all natural processes are irreversible.

If you do work, if you use energy, if you convert it from one form to another, you will lose some of it. No machine can be 100% efficient. This lost energy goes to increasing the disorder of the Universe.

The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process.

S universe = S system + S surroundings

For a spontaneous process:

S universe > 0

For an equilibrium process:

S universe = 0

The Second Law of Thermodynamics

What does it mean that for some process we find that S universe

is negative?

This means that the process is not spontaneous in the direction described. Rather, the process is

spontaneous in the opposite direction.

Heat Flo

w

Energy (heat) flows from the warm swimmer to the cold ocean, never from the cold ocean to the warm swimmer.

Dave’s Hand

John’s Hand

2.6 million to one

2.6 million to one

What are the odds?

Johns hand is one of a very select group of hands called a straight flush. Out of the 2.6 million possible hands, there are only 40 straight flushes.

Dave has junk. There are over a million hands that are junk.

In other words, there are very few combinations of five cards that form a straight flush, and very many combinations that result in junk.

John’s Hand 7 8 9 10 J Straight Flush

Dave’s Hand 4 8 7 3 K Junk

Microstate Macrostate

Entropy is defined in terms of microstates and macrostates.

The more microstates a macrostate can

have, the higher its entropy.

Hey, wake up loser. The law of entropy says

you’re probably going to get junk.

I

Microstates

Consider a tank with 4 air molecules.

I

II

Microstates

I

II

III

Microstates

This is the most likely

And so it is with air molecules. Considering the extremely large number of molecules in a house

for example, the odds are overwhelmingly high that the

molecules will spread out evenly.

Probably Not!

So, next time you’re thirsty, go ahead,

wander into the kitchen and get yourself a drink

of water. There’ll be plenty of air to breath when you get there.

Probably!

In this house we obey the laws of

thermodynamics.

“If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations – then so much the worse for Maxwell’s equations.

If it is found to be contradicted by observations – well, these experimentalists do bungle things sometimes.

But if your theory is found to be against the second law of thermodynamics I can give you no hope. There is nothing for you do do but collapse in deepest humiliation”

Sir Arthur Eddington - 1928

Entropy is given the symbol SS = npSproducts - nrSreactants

Two factors determine the spontaneity of a reaction…

1. Heat of Reaction - Enthalpy

2. Disorder - Entropy

Two factors determine the spontaneity of reactions…

1. Heat content - - - Enthalpy

Reactions with a (-) H tend to be spontaneous.

2. Disorder - - - Entropy

Reactions with a (+) S tend to be spontaneous

Typically, the entropy is expected to increase for processes in which…

Liquids or solutions are formed from solids

Gases are formed from either solids or liquids.

The number of molecules increases during a chemical reaction.

The temperature of a substance is increased.

So = np S products - nrS reactants

To Calculate the Change of Entropy (So)

Cool, we now have two separate means to predict

spontaneity.

Enthalpy, for which -H means the process tends to be

spontaneous, -and-

Entropy, for which +S means the process tends to be

spontaneous.

But what if they contradict each

other in predicting spontaneity?

Combines the concepts of enthalpy and entropy in predicting spontaneity of a reaction.

Defined as the maximum amount of energy that can be taken out of a reaction to do useful work.

G = H - TS

Gibb’s Free Energy

G = H - TS

If G is (+) the reaction is not spontaneous

If G is (-) the reaction is spontaneous

If G is (0) the reaction is at equilibrium

T is in Kelvin

Gibb’s Free Energy

Can be calculated two ways…

G = H - TS

G = np Gproducts - nr Greactants

OR

Consider Melting Ice…

H2O (solid) + Heat H2O (liquid)

H = (+) Endothermic

S = (+) entropy increases

G = H – T S= (+) – T(+)

Must be < 0 to occur

The reaction will occur spontaneously at some temperature T and above.

Example Consider Freezing Water…

H2O (liquid) H2O (solid) + Heat

H = (-) Reaction is exothermic

S = (-) Entropy decreases going from a liquid to a solid

G = H - T S

At some temperature T and below, the S becomes larger, making G (-)

(This favors spontaneity)

(This favors non-spontaneity)

= (-H) – T(-S)

= (-H) + T(S)

The reaction will occur spontaneously at some temperature T and below. This temperature is of course 0o C.

Another Example…

Will the following reaction occur spontaneously at room temp (25oC)?

CH4(g) + 2O2(g) CO2(g) + 2H2O (g)

1. Calculate S

S = npSproducts - nrSreactants

CH4 + 2O2 CO2 + 2H2O (gas)

S = [S(CO2) + 2S(H2O)] – [S(CH4) + 2S(O2)]S = [ (213.6) + 2 (188.7)] – [ (186.2) + 2 (205)]

S = -5.2 joules/K mole

Next, calculate H

H = [H(CO2) + 2H(H2O)] – [H(CH4) + 2H(O2)]H = [(-395.3) + 2(-241.8)] – [(-74.86) + 2(0)]

H = - 804.0 kJ/mole = -804,000 J/mole

Note: Be careful about the units.

H = npH products - nrH reactants

Now, Calculate G

G = H - T S

= -804,000 – (298k)(-5.2)

= -802,450 J/mole = -802 kJ/mole

Or, Calculate G another way

G = npGproducts - nrGreactants

G = [G(CO2) + 2G(H2O)] – [G(CH4) + 2G(O2)]

G = [(-394.4) +2(-228.6)] – [(-50.8) + 2(0)]

G = -800.8 kJ

The other laws…

The Third Law: It isn’t possible to reach absolute zero temperature. It is important to understand that all motion does not cease at this temperature, rather this is the lowest temperature that energy can be extracted from the substance.

The Zeroth Law: If A is in thermal equilibrium with C, and B is also in thermal equilibrium with C, then A is in thermal equilibrium with B. This establishes temperature as the fundamental variable in thermodynamics.

Gibbs Free Energy

ΔGo = ΔHo -TΔSo

The “o” superscript indicates all substances are in their standard states – that is, at standard conditions (25oC, 1 atm pressure)

ΔG = ΔH -TΔS

OK, what if it’s not at standard conditions?

Lets consider pressure….

ΔG = ΔGo + RT ln(P)

Where R is the gas constant…

R = 8.3145 J/K·mol

ΔG = ΔGo + RT ln(P)From this…

ΔG = ΔGo + RT ln(Q)

Where Q is the reaction quotient.

See sec. 16.7

Now, lets consider the reaction when it’s at equilibrium….

At equilibrium……

Q = k where k is the equilibrium constant

ΔG = 0

ΔG = ΔGo + RT ln(Q)

Now, lets consider the reaction when it’s at equilibrium….

At equilibrium……

Q = k where k is the equilibrium constant

ΔG = 0

ΔG = ΔGo + RT ln(Q)

0

Now, lets consider the reaction when it’s at equilibrium….

At equilibrium……

Q = k where k is the equilibrium constant

ΔG = 0

ΔG = ΔGo + RT ln(Q)

0

ΔGo = -RT ln(k)

k

ΔGo = -RT ln(k)ooo STHkRTG )ln(

ΔGo = ΔHo -TΔSo

RT

ST

RT

Hk

oo

)ln(

R

S

TR

Hk

oo

1)ln(

R

S

TR

Hk

oo

1)ln(

This is a linear equation with the form y = mx + b

y = m x + b

R

S

TR

Hk

oo

1)ln(

This is a linear equation with the form y = mx + b

y = m x + b

1/T

ln(k)

R

S

TR

Hk

oo

1)ln(

This is a linear equation with the form y = mx + b

y = m x + b

1/T

ln(k)

Slope

R

H o

R

S

TR

Hk

oo

1)ln(

This is a linear equation with the form y = mx + b

y = m x + b

1/T

ln(k)

Y intercept

R

SoSlope

R

H o