CHAPTER: 3
Laws
of Thermodynamics
Main Points to focus:
•
Thermodynamics
•
First Law of thermodynamics
•
Second Law of thermodynamics
•
Terminologies
•
Application of first law of
thermodynamics
•
Application of 2nd law of thermodynamics
•
Energy transfer in Tropical Level or
Ecosystem
Energy:
All
life processes are driven by energy, and consequently, a cell or an organism
depressed of an energy source will soon die. Energy is defined by physicists
as the ability to do work and is governed by certain physical principles
such as the Laws of Thermodynamics.
The
forms of energy are:
Radiant
(light), thermal (heat), chemical, mechanical (motion), and electrical. One
focus of this plant physiology chapter is photosynthesis, the process that
converts radiant energy from the sun into the chemical energy of a sugar
molecule.
Transformations
from potential to kinetic and vice versa occur constantly in biological systems
and are part of the underlying principles of both photosynthesis and respiration.
Transfers
of work, heat, or matter and energy between the system and the surroundings may
take place across this boundary.
Boundaries
may be real or imaginary. For closed systems, boundaries are real while
for open system boundaries are often imaginary.
Types
of Thermodynamic Systems: There are
three types of thermodynamic systems, depending on the nature of the
boundary which are as follows:
Isolated
system: When the boundary is both sealed and
insulated, no interaction is possible with the surroundings. This is idealized
system.
Closed
system: Here the boundary is sealed but not insulated.
Therefore, A closed system is one which cannot transfer matter but can transfer
energy in the form of heat, work and radiation to and from its surroundings.
Open
system: In such system the boundary is open and unisolated therefore, An
open system is one which can transfer both energy and matter to and from its
surroundings.
Homogenous
System:
• A chemical system
which cannot be mechanically separated and
which has uniform physical properties throughout its mass or volume.
Heterogenous
System:
• A chemical system that contains various distinct and mechanically separable
parts or phases, such as a suspension and does
not have uniform physical properties.
Exothermic:
Heat
liberating reactions are known as exothermic
Endothermic:
Heat
absorbing reactions are known as endothermic
Exergonic
Reaction:
• An exergonic
reaction is a chemical reaction where the change in
the free energy is negative (there is a net release of free energy),
indicating a spontaneous reaction.
Endergonic
Reaction:
• In chemical
thermodynamics, an endergonic reaction (also called a nonspontaneous
reaction or an unfavorable reaction) is a chemical
reaction in which the standard change in free energy is
positive, and energy is absorbed.
Isothermal
Process: An isothermal
process is a change of a system, in which
the temperature remains constant: ΔT = 0
Enthalpy
(H):
• Enthalpy is
a measure of the total energy of a thermodynamic system. It
includes the internal energy, which is the energy required to create a
system, and the amount of energy required to make room for it by displacing
its environment and establishing its volume and pressure.
• Enthalpy is
a defined thermodynamic potential, designated by the letter "H",
that consists of the internal energy of the system (U) plus the
product of pressure (p) and volume (V) of the system:
H = U +
pV
• Enthalpy
is a thermodynamic potential. It is a state function and
an extensive quantity.
The unit of measurement in the International System of Units (SI) for
enthalpy is the joule, but other historical, conventional units are still
in use, such as the small and the large calorie.
Entropy(S):
• It
is a measure of the energy that is not available for work during a
thermodynamic process.
• It
is a measure of the randomness of the
thermodynamic system.
• Entropy, the
measure of a system’s thermal energy per unit temperature that
is unavailable for doing useful work. Because work is obtained
from ordered molecular motion, the amount of entropy is also a measure of
the molecular disorder, or randomness, of a system.
Entropy(S) = Unavailable work
Temperature
Significance
of entropy:
• Entropy
is a degree of disorder or randomness of the molecules of a system. It is
directly proportional to the disorder of the system.
• Entropy
is a function of thermodynamics probability.
Gibb’s
Free Energy:
• The
Gibbs free energy of a system at any moment in time is defined as the enthalpy
of the system minus the product of the temperature times the entropy of the
system.
G = H - TS
• The
Gibbs free energy of the system is a state function because it is defined in
terms of thermodynamic properties that are state functions. The change in the
Gibbs free energy of the system that occurs during a reaction is therefore equal
to the change in the enthalpy of the system minus the change in the product of
the temperature times the entropy of the system.
∆G = ∆H - ∆(TS)
• If
the reaction is run at constant temperature, this equation can be written as
follows.
∆ G = ∆ H - T
∆ S
• The
change in the free energy of a system that occurs during a reaction can be
measured under any set of conditions. If the data are collected under
standard-state conditions, the result is the standard-state free energy
of reaction (∆Go).
∆Go = ∆Ho - T
∆So
Thermodynamic Equilibrium:
A system in which the macroscopic
properties do not undergo any change with time is said to be in thermodynamic
equilibrium.
Thermal
equilibrium :-
A system is said to be
in thermal equilibrium, if there is no flow of heat from one position of
the system to another.
A
system is said to be in chemical equilibrium if
the composition of the various phases in the system remains the same
throughout.
Process:
Whenever the condition of a system changes, it is said to have undergone a
process. Thus a process may be defined as the operation by which a
system changes from one state to another.
Adiabatic
process (Thermally insulated from the surroundings):
A
process in which no heat is exchanged between the system and surroundings is
called adiabatic process (Q= O). System in which
such processes occur are thermally insulated from the surroundings.
Reversible process
takes place by infinitesimal small steps, the process would take infinite time
to occur. Such a process is-idealized
and is true in principle only. On the other hand, an irreversible process takes
place in finite time. Thus all processes which actually occur are irreversible.
Thermodynamics:
• Energy exists
in many forms, such as heat, light, chemical energy, and electrical energy.
• Energy
is the ability to bring about change or to do work.
• Thermodynamics
is the study of energy.
• It is the science concerned with the relations
between heat and mechanical energy or work, and the conversion of one into
other:
• Modern
thermodynamics deals with the properties of system of the
description(explanation) of which temperature is necessary coordinate.
• Energy has no mass and no occupy
space but it can be seen in impact.
• Thermodynamics describes the macroscopic
physics of matter and energy, especially including heat transfer, using the
concept of the thermodynamic system.
First
Law of Thermodynamics:
Energy can be changed from one form to another, but
it cannot be created or destroyed. The total amount of energy and matter in the
Universe remains constant, merely changing from one form to another. The First
Law of Thermodynamics (Conservation) states that energy is always conserved, it
cannot be created or destroyed. In essence, energy can be converted from one
form into another.
Second
Law of Thermodynamics:
It states that "in all energy exchanges,
if no energy enters or leaves the system, the potential energy of the state
will always be less than that of the initial state."
• This
is also commonly referred to as entropy.
• In
the process of energy transfer, some energy will dissipate as heat.
• Entropy is
a measure of disorder: cells are NOT disordered and so have low entropy. The
flow of energy maintains order and life. Entropy wins when organisms cease to
take in energy and die.
• When
energy is changed from one form to another, a decrease in energy quality always
occurs.
• The
results of experiments have been summarized in what is called the Second Law of
Thermodynamics: When energy is changed from one form to another, some of the
useful energy is always degraded to lower quality, more dispersed, less useful
energy.
• The
degraded energy usually takes the form of heat given off at a low temperature
to the surrounding(Environment).
Example
of Second law of thermodynamics :
In
living system, solar energy is converted into chemical energy (food
molecule)and then into mechanical energy (moving, thinking, and living). During
each of these conservations, high-quality energy is degraded and flows into the
environment as lower quality heat.
Application
of fist law of thermodynamic:
• A
living organism is an open system, it exchange both matter (water, nutrient)
and energy(light) with its surrounding.
• Living
organism extract energy from their environment and convert some of it into
useful form of energy to produce work and return some energy to the environment
as heat together with product molecule.
• Example:-
• Leaves
absorb energy from their surrounding in two ways:
1) as direct incident irradiation from the sun
&
2)as infrared. (Photosynthesis and
leaf temperature change)
Application
of second law of thermodynamics:
• Energy
is transfer from one organism to another, (Plant-herbivore-carnivore) a large
part of most energy is degraded as heat,
no longer moveable and leftovers is stored as living tissue.
• In
Photosynthesis, Pigment System (PS I) and PS II need both endothermic &
exothermic reaction.
• In
living system there are anabolic (synthesis of more complex to simpler
one ) and catabolic process. In anabolic, the molecule are placed in
definite order showing low entropy value where as catabolic the ordered
molecule are displaced giving maximum disorder & entropy value to the
system is high.
Energy
transfer in Tropical Level or Ecosystem:
Applied
in Ecology
Only
a small fraction of the plant food on land is actually harvested by animals;
most products of primary production are consumed by decomposers.
Transfer
of energy from one trophic level to the next higher trophic level is defined as
ecological efficiency. Such efficiencies of transfer of energy from one trophic
level to the next are low, generally only about 5 to 10 percent.
The amount of energy at each trophic level
decreases as it moves through an ecosystem. As little as 10 percent of the
energy at any trophic level is transferred to the next level; the rest is lost
largely through metabolic processes as heat. If a grassland ecosystem has
10,000 kilocalories (kcal) of energy concentrated in vegetation, only about
1,000 kcal will be transferred to primary consumers, and very little (only 10
kcal) will make it to the tertiary level. Energy pyramids such as this help to
explain the trophic structure of an ecosystem: the number of consumer trophic
levels that can be supported is dependent on the size and energy richness of
the producer level.
Facts
about Thermodynamics:
• All
organisms require energy to continue and to replace themselves,
• The
ultimate source of practically all Earth's energy is the Sun.
• One
can think of our Sun as "feeding" the earth via its radiant energy.
But 99 percent or more of this incident solar radiation goes unused by
organisms and is lost as heat and heat of evaporation
Only
about 1 percent is actually captured by plants in photosynthesis and stored as
chemical energy. Moreover, energy available from sunlight varies widely over
the earth's surface both in space and in time.
