Thermodynamics

Thermodynamics

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Thermodynamics can be defined as the science of energy. Although every body has a feeling of what energy is, it is difficult to give a precise definition for it. Energy can be viewed as the ability to cause changes.

The name thermodynamics stems from the Greek words therme (heat) and dynamics (power), which is most descriptive of the early efforts to convert heat into power. Today the same name is broadly interpreted to include all aspects of energy and energy transformations, including power generation, refrigeration, and relationships among the properties of matter.

One of the most fundamental laws of nature is the conservation of energy principle. It simply states that during an interaction, energy can change from one form to another but the total amount of energy remains constant. Energy can not be created or destroyed. A rock falling off a cliff, for example, picks up speed as result of its potential energy being converted to kinetic energy.

Although the principles of thermodynamics have been in existence since the creation of the universe, thermodynamics did not emerge as a science until the construction of the first successful atmospheric steam engines in England by Thomas Savery in 1607 and Thomas Newcomen in 1712. These engines were very slow and inefficient, but they opened the way for the development of a new science: Thermodynamics.

All applications in nature involve some interaction between energy and matter; thus, it is hard to imagine an area that does not relate to thermodynamics in some matter. Therefore, developing a good understanding of basic principles of thermodynamics has long been an essential part of engineering education.
Thermodynamics is commonly encountered in many engineering systems and other aspects of life, and one does not need to go very far to see some application areas of it.

In fact, one does not need to go anywhere. The heart is constantly pumping blood to all parts of the human body, various energy conversions occur in trillions of body cells, and the body heat generated is constantly rejected to the environment. The human comfort is closely tied to the rate of this metabolic heat rejection. We try to control this heat transfer rate by adjusting our clothing to the environmental conditions.

Other applications of thermodynamics are right where one lives. An ordinary house is, in some respects, an exhibition hall filled with wonders of thermodynamics. Many ordinary household utensils and appliances are designed, in whole or in part, by using the principles of thermodynamics.

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Some examples include the electric or gas range, the heating and air conditioning systems, the refrigerator, the humidifier, the pressure cooker, the water heater, the shower, the iron, and even the computer, and the TV. On a large scale, thermodynamics plays a major part in the design and analysis of automotive engines, rockets, jet engines, and conventional or nuclear power plants, solar collectors, and the design of vehicles from ordinary cars to airplanes.

Stated simply: the total energy of the universe does not change. This does not mean that the form of the energy cannot change. Indeed, chemical energies of a molecule can be converted to thermal, electrical or mechanical energies.
The internal energy of a system can change only by work or heat exchanges. From this the change in the free energy of a system can be shown by the following equation:

DE= q - w Eqn. 1

When q is negative heat has flowed from the system and when q is positive heat has been absorbed by the system. Conversely when w is negative work has been done on the system by the surrounding and when positive, work has been done by the system on the surroundings.
In a reaction carried out at constant volume no work will be done on or by the system, only heat will be transferred from the system to the surroundings. The end result is that:

DE = q Eqn. 2

When the same reaction is performed at constant pressure the reaction vessel will do work on the surroundings. In this case:

DE = q - w Eqn. 3

where w = PDV Eqn. 4

When the initial and final temperatures are essentially equal (e.g. in the case of biological systems):

DV = Dn[RT/P] Eqn. 5

therefore, w = DnRT Eqn. 6

by rearrangement of equation 3 and incorporation of the statements in equations 4-6, one can calculate the amount of heat released under constant pressure:

q = DE + w = DE + PDV = DE + DnRT Eqn. 7

In equation 7 Dn is the change in moles of gas per mole of substance oxidized (or reacted), R is the gas constant and T is absolute temperature.

The second law of thermodynamics states that the universe (i.e. all systems) tend to the greatest degree of randomization. This concept is defined by the term entropy, S.

S = k lnW Eqn. 12

where k = Boltzmann constant (the gas constant, R, divided by Avagadros' number) and W = the number of substrates. For an isothermal reversible reaction the change in entropy can be reduced to the term:

DS = DH/T Eqn. 13

Whereas, enthalpy is a term whose value is largely dependent upon electronic internal energies, entropy values are dependent upon translational, vibrational and rotational internal energies. Entropy also differs from enthalpy in that the values of enthalpy that indicate favored reactions are negative and the values of entropy are positive. Together the terms enthalpy and entropy demonstrate that a system tends toward the highest entropy and the lowest enthalpy.
In order to effectively evaluate the course (spontaneity or lack there of) of a reaction and taking into account both the first and second laws of thermodynamics, Josiah Gibbs defined the term, free energy. Free energy:

DG = DH - TDS Eqn. 14

Free energy is a valuable concept because it allows one to determine whether a reaction will proceed and allows one to calculate the equilibrium constant of the reaction which defines the extent to which a reaction can proceed. The discussion above indicated that a decrease in energy, a negative DH, and an increase in entropy, a positive DS, are indicative of favorable reactions. These terms would, therefore, make DG a negative value. Reactions with negative DG values are termed exergonic and those with positive DG values endergonic. However, when a system is at equilibrium:

DG = 0 Eqn. 15

Gibb's free energy calculations allow one to determine whether a given reaction will be thermodynamically favorable. The sign of DG states that a reaction as written or its reverse process is the favorable step. If DG is negative then the forward reaction is favored and visa versa for DG values that are calculated to be positive.

Work can easily be converted to heat directly and completely, but converting heat to work requires the use of some special devices. These devices are called heat engines. Heat engines differ considerably from one to another, but all can be characterized by the following:

1. They receive heat from a high temperature source (solar energy, oil furnace, nuclear reactor, etc)

2. They convert part of this heat to work (usually in the form of a rotating shaft)

3. They reject the remaining waste heat to a low temperature sink (the atmosphere, rivers, etc)

4. They operate in a cycle.

Heat engines and other cyclic devices usually involve a fluid to and from which heat is transferred while undergoing a cycle. The term heat engine is often used in a broader sense to include work -producing devices that don't operate in thermodynamic cycles.

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