Second Law of thermodynamicsSecond Law of thermodynamics

The second law of thermodynamics can be The second law of thermodynamics can be understood through considering these processes:understood through considering these processes:

A rock will fall if you lift it up and then let goA rock will fall if you lift it up and then let go Hot pans cool down when taken out from the Hot pans cool down when taken out from the

stove.stove. Ice cubes melt in a warm room.Ice cubes melt in a warm room.

What’s happening in every one of What’s happening in every one of thosethose??

Energy of some kind is changing from being Energy of some kind is changing from being localized (concentrated) somehow to localized (concentrated) somehow to

becoming more spreed out.becoming more spreed out.

i.e in example 1i.e in example 1::

The potential energy localized in the rock is The potential energy localized in the rock is now totally spread out and dispersed in:now totally spread out and dispersed in:

A little air movement.A little air movement. Little heating of air and ground.Little heating of air and ground.

In the previous exampleIn the previous example SystemSystem: rock : rock aboveabove ground then rock ground then rock onon

ground.ground.

Surroundings: air + groundSurroundings: air + ground

The The second law of thermodynamics states that states that energy (and matter) tends to become more energy (and matter) tends to become more evenly spread out across the universe.evenly spread out across the universe.

i.e to concentrate energy (or matter) in one i.e to concentrate energy (or matter) in one specific place, it is necessary to spread out a specific place, it is necessary to spread out a

greater amount of energy (as heat) across the greater amount of energy (as heat) across the remainder of the universe ("the surroundings"). remainder of the universe ("the surroundings").

What is entropyWhat is entropy??

Entropy just measures the Entropy just measures the spontaneousspontaneous dispersaldispersal of energy: or how much energy is of energy: or how much energy is

spread outspread out in a process as a function of in a process as a function of temperature. temperature.

Follow the EntropyFollow the Entropy

EntropyEntropy a measure of disorder in the a measure of disorder in the physical system.physical system.

the the second law of thermodynamics –second law of thermodynamics – the the universe, or in any isolated system, the universe, or in any isolated system, the

degree of disorder (entropy) can only degree of disorder (entropy) can only increase. increase.

the movement towards a disordered state is the movement towards a disordered state is a a spontaneous process.spontaneous process.

So in a simple equationSo in a simple equation::

Entropy = “ energy dispersed”/ TEntropy = “ energy dispersed”/ T

Entropy couldn't be expressed without the Entropy couldn't be expressed without the inclusion of absolute temperature.inclusion of absolute temperature.

Entropy change Entropy change ΔΔS shows us exactly how S shows us exactly how important to a system is a dispersion of a important to a system is a dispersion of a

given amount of energy. given amount of energy.

i.e you can pump heat out of a refrigerator (to i.e you can pump heat out of a refrigerator (to make ice cubes), but the heat is placed in the make ice cubes), but the heat is placed in the

house and the entropy of the house increases, house and the entropy of the house increases, even though the local entropy of the ice cube even though the local entropy of the ice cube

tray decreases. tray decreases.

Entropy change Entropy change ΔΔ S S

In chemical terms entropy is related to the random In chemical terms entropy is related to the random movements of molecules and is measured by T movements of molecules and is measured by T ΔΔS.S.

When a system is at equilibrium, no net reaction When a system is at equilibrium, no net reaction occurs and the system has no capacity to do work.occurs and the system has no capacity to do work.

Q = T Q = T ΔΔ S This is a condition of maximum S This is a condition of maximum entropy.entropy.

Work can be done by system proceeding to Work can be done by system proceeding to equilibrium and measure of the maximum equilibrium and measure of the maximum useful work is given by the following equation useful work is given by the following equation

W = - W = - ΔΔH + T H + T ΔΔSS

Is the second law of thermodynamics violated in the Is the second law of thermodynamics violated in the living cells?living cells?

Cell is not an isolated system: it takes energy from its Cell is not an isolated system: it takes energy from its environment to generate order within itself.environment to generate order within itself.

Part of the energy that the cell uses is converted into Part of the energy that the cell uses is converted into heat. heat.

The heat is discharged into the cell's environment and The heat is discharged into the cell's environment and disorders it. disorders it.

The total entropy increasesThe total entropy increases

NO!

Part of the energy that the cell uses is converted into heat.

The heat is discharged into the cell's environment and disorders it ►►

►► The total entropy increases

Entropy and LifeEntropy and Life For example, For example, living thingsliving things are highly are highly

ordered, low entropy, structures, but ordered, low entropy, structures, but they grow and are sustained because they grow and are sustained because

their metabolism generates their metabolism generates excess excess entropyentropy in their surroundings. in their surroundings.

For For living systemsliving systems, approaching , approaching chemical equilibrium means decay and chemical equilibrium means decay and death.death.

Entropy and LifeEntropy and Life

For For living systemsliving systems, approaching , approaching equilibrium means decay and death.equilibrium means decay and death.

Building blocks

The apparent paradox:

S

Life

Equilibrium

Gibbs Free energyGibbs Free energy Gibbs introduced the concept of free energy as an Gibbs introduced the concept of free energy as an

another measure of the capacity to do useful work.another measure of the capacity to do useful work.

Free energy G is defined as Free energy G is defined as ΔΔ G = G = ΔΔH- T H- T ΔΔSS & W = - & W = - ΔΔH + T H + T ΔΔSS Note that Note that ΔΔG= -WG= -WSo that when the measure of W is positive (i.e the So that when the measure of W is positive (i.e the system is doing useful work), the measure of system is doing useful work), the measure of ΔΔG G is negative and vice versais negative and vice versa..

– Gibbs’ free energy (G)Gibbs’ free energy (G) change in free energychange in free energy

endergonicendergonic - any reaction that - any reaction that requires an input of energy. requires an input of energy.

exergonicexergonic - any reaction that - any reaction that releases free energyreleases free energy

Reactant

Product

Energymust besupplied. E

ner

gy

sup

plie

dE

ner

gy

rele

ased

Reactant

Product

Energy isreleased.

Glucose-1-p Glucose-6-pGlucose-1-p Glucose-6-p

Since changes in free energy and enthalpy Since changes in free energy and enthalpy are related only to the difference between are related only to the difference between

the free energies and enthalpies of the free energies and enthalpies of reactants and products, so we can reactants and products, so we can

characterize the above reaction as:characterize the above reaction as:

ΔΔ G = G G = G g-6-pg-6-p - G - G g-1-pg-1-p

oror

ΔΔ H= H H= H g-6-pg-6-p - H - H g-1-pg-1-p

If the algebraic sign is:If the algebraic sign is:

1- negative, the reaction is exergonic (i.e it will 1- negative, the reaction is exergonic (i.e it will proceeds spontaneously from left to right as proceeds spontaneously from left to right as written).written).

2-Positive, the reaction is endergonic, (i.e 2-Positive, the reaction is endergonic, (i.e it will it will not proceeds spontenously.not proceeds spontenously.

3- Zero, the reaction is at equilibrium.3- Zero, the reaction is at equilibrium.

When When ΔΔ H is: H is:11 - -negative, the reaction is exothermic (i.e it negative, the reaction is exothermic (i.e it

gives off heat to its surroundings) gives off heat to its surroundings)..

22--Positive, the reaction is endothermic (i.e it Positive, the reaction is endothermic (i.e it take heat from its surroundings) take heat from its surroundings)..

33 - -Zero, the reaction is isothermic ( no net Zero, the reaction is isothermic ( no net exchange of heat occurs with the exchange of heat occurs with the surroundings)surroundings)..

Standard free energy “Standard free energy “ΔΔ G G”°”°

““ΔΔ G°” of a chemical reaction are calculated G°” of a chemical reaction are calculated at 25 C° and at 1 atmospheric pressure.at 25 C° and at 1 atmospheric pressure.

The biological standard free energy The biological standard free energy ΔΔ G°−” is G°−” is more useful in biochemistry, here the more useful in biochemistry, here the standard conditions are:standard conditions are:

pH = 7pH = 7

Temp = 37 C°Temp = 37 C°

1 M concentrations of reactants and products. 1 M concentrations of reactants and products.