Second Law of thermodynamics
The second law of thermodynamics can be
understood through considering these processes:
 A rock will fall if you lift it up and then let go
 Hot pans cool down when taken out from the
stove.
 Ice cubes melt in a warm room.
What’s happening in every one of
those?
Energy of some kind is changing from being
localized (concentrated) somehow to
becoming more spreed out.
i.e in example 1:
The potential energy localized in the rock is
now totally spread out and dispersed in:
 A little air movement.
 Little heating of air and ground.
In the previous example
 System: rock above ground then rock on
ground.
 Surroundings: air + ground
 The second law of thermodynamics states that
energy (and matter) tends to become more
evenly spread out across the universe.
 i.e to concentrate energy (or matter) in one
specific place, it is necessary to spread out a
greater amount of energy (as heat) across the
remainder of the universe ("the surroundings").
What is entropy?
Entropy just measures the spontaneous
dispersal of energy: or how much energy is
spread out in a process as a function of
temperature.
Follow the Entropy
 Entropy a measure of disorder in the
physical system.
 the second law of thermodynamics – the
universe, or in any isolated system, the
degree of disorder (entropy) can only
increase.
 the movement towards a disordered state is
a spontaneous process.
So in a simple equation:
Entropy = “ energy dispersed”/ T
Entropy couldn't be expressed without the
inclusion of absolute temperature.
Entropy change ΔS shows us exactly how
important to a system is a dispersion of a
given amount of energy.
 i.e you can pump heat out of a refrigerator (to
make ice cubes), but the heat is placed in the
house and the entropy of the house increases,
even though the local entropy of the ice cube
tray decreases.
Entropy change Δ S
 In chemical terms entropy is related to the random
movements of molecules and is measured by T
ΔS.
 When a system is at equilibrium, no net reaction
occurs and the system has no capacity to do work.
Q=TΔS
entropy.
This is a condition of maximum
 Work can be done by system proceeding to
equilibrium and measure of the maximum
useful work is given by the following equation
W = - ΔH + T ΔS
Is the second law of thermodynamics violated in the
living cells?
NO!
 Cell is not an isolated system: it takes energy from its
environment to generate order within itself.
 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
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 Life
 For example, living things are highly
ordered, low entropy, structures, but they
grow and are sustained because their
metabolism generates excess entropy in
their surroundings.
 For living systems, approaching chemical
equilibrium means decay and death.
Entropy and Life
For living systems, approaching
equilibrium means decay and death.
The apparent paradox:
Life
S
Building blocks
Equilibrium
Gibbs Free energy
 Gibbs introduced the concept of free energy as an
another measure of the capacity to do useful work.
Free energy G is defined as
Δ G = ΔH- T ΔS
& W = - ΔH + T ΔS
Note that ΔG= -W
So that when the measure of W is positive (i.e the
system is doing useful work), the measure of ΔG
is negative and vice versa.
Energy supplied
Energy released
– Gibbs’ free energy (G)
 change in free energy
Product
Energy
must be
supplied.
Reactant
 endergonic - any reaction that
requires an input of energy.
 exergonic - any reaction that
releases free energy
Reactant
Energy is
released.
Product
 Glucose-1-p
Glucose-6-p
Since changes in free energy and enthalpy
are related only to the difference between
the free energies and enthalpies of
reactants and products, so we can
characterize the above reaction as:
Δ G = G g-6-p - G g-1-p
or
Δ H= H g-6-p - H g-1-p
 If the algebraic sign is:
1- negative, the reaction is exergonic (i.e it will
proceeds spontaneously from left to right as
written).
2-Positive, the reaction is endergonic, (i.e it
not proceeds spontenously.
3- Zero, the reaction is at equilibrium.
will
 When Δ H is:
1- negative, the reaction is exothermic (i.e it
gives off heat to its surroundings).
2-Positive, the reaction is endothermic (i.e it
take heat from its surroundings).
3- Zero, the reaction is isothermic ( no net
exchange of heat occurs with the
surroundings).
Standard free energy “Δ G°”
 “Δ G°” of a chemical reaction are calculated
at 25 C° and at 1 atmospheric pressure.
The biological standard free energy Δ G°−” is
more useful in biochemistry, here the
standard conditions are:
pH
=7
Temp = 37 C°
1 M concentrations of reactants and products.