Laws of Thermodynamics

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 (∆G^o).

                                                 ∆G^o =  ∆H^o - T ∆S^o

 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.