Laws of Thermodynamics
Thermal Equilibrium
It is observed that a higher temperature object which is in contact with a lower temperature object will transfer heat to the lower temperature object. The objects will approach the same temperature, and in the absence of loss to other objects, they will then maintain a constant temperature. They are then said to be in thermal equilibrium. Thermal equilibrium is the subject of the Zeroth Law of Thermodynamics.
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Zeroth Law of Thermodynamics
The "zeroth law" states that if two systems are at the same time in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
If A and C are in thermal equilibrium with B, then A is in thermal equilibrium with C. Practically this means that all three are at the same temperature, and it forms the basis for comparison of temperatures. It is so named because it logically precedes the First and SecondLaws of Thermodynamics.
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There are underlying ideas about heat associated with the zeroth law of thermodynamics, and one of those ideas was expressed by Maxwell as "All heat is of the same kind." If A is in thermal equilibrium with B, then every unit of internal energy that passes from A to B is balanced by the same amount of energy passing from B to A. This is true even if the atomic masses in A are different from those in B, and even if the amount of energy per unit mass in A is different because the material has a different specific heat. This implies that there is a measurable property that can be considered to be the same for A and B, a property upon which heat transfer depends. That property is called temperature.
First Law of Thermodynamics
The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes:
The first law makes use of the key concepts of internal energy, heat, andsystem work. It is used extensively in the discussion of heat engines. The standard unit for all these quantities would be the joule, although they are sometimes expressed in calories or BTU.
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It is typical for chemistry texts to write the first law as ΔU=Q+W. It is the same law, of course - the thermodynamic expression of the conservation of energy principle. It is just that W is defined as the work done on the system instead of work done by the system. In the context of physics, the common scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work, as in the pushing down of a piston in an internal combustion engine. In the context of chemical reactions and process, it may be more common to deal with situations where work is done on the system rather than by it.
Enthalpy
Four quantities called "thermodynamic potentials" are useful in the chemical thermodynamics of reactions and non-cyclic processes. They are internal energy, the enthalpy, the Helmholtz free energy and the Gibbs free energy. Enthalpy is defined by
H = U + PV
where P and V are the pressure and volume, and U is internal energy. Enthalpy is then a precisely measurable state variable, since it is defined in terms of three other precisely definable state variables. It is somewhat parallel to the first law of thermodynamics for a constant pressure system
Q = ΔU + PΔV since in this case Q=ΔH
It is a useful quantity for tracking chemical reactions. If as a result of an exothermic reaction some energy is released to a system, it has to show up in some measurable form in terms of the state variables. An increase in the enthalpy H = U + PV might be associated with an increase in internal energy which could be measured by calorimetry, or with work done by the system, or a combination of the two.
The internal energy U might be thought of as the energy required to create a system in the absence of changes in temperature or volume. But if the process changes the volume, as in a chemical reaction which produces a gaseous product, then work must be done to produce the change in volume. For a constant pressure process the work you must do to produce a volume change ΔV is PΔV. Then the term PV can be interpreted as the work you must do to "create room" for the system if you presume it started at zero volume.
The internal energy U might be thought of as the energy required to create a system in the absence of changes in temperature or volume. But if the process changes the volume, as in a chemical reaction which produces a gaseous product, then work must be done to produce the change in volume. For a constant pressure process the work you must do to produce a volume change ΔV is PΔV. Then the term PV can be interpreted as the work you must do to "create room" for the system if you presume it started at zero volume.
System Work
When work is done by a thermodynamic system, it is ususlly a gas that is doing the work. The work done by a gas at constant pressure is:
For non-constant pressure, the work can be visualized as the area under the pressure-volume curve which represents the process taking place. The more general expression for work done is:
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Work done by a system decreases the internal energy of the system, as indicated in the First Law of Thermodynamics. System work is a major focus in the discussion of heat engines.
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Second Law of Thermodynamics
The second law of thermodynamics is a general principle which places constraints upon the direction of heat transfer and the attainable efficiencies of heat engines. In so doing, it goes beyond the limitations imposed by thefirst law of thermodynamics. It's implications may be visualized in terms of the waterfall analogy..
Second Law: Heat Engines
Second Law of Thermodynamics: It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W . Some amount of heat QC must be exhausted to a cold reservoir. This precludes a perfect heat engine.
This is sometimes called the "first form" of the second law, and is referred to as the Kelvin-Planck statement of the second law. |
Second Law: Refrigerator
Second Law of Thermodynamics: It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator. The statements about refrigerators apply to air conditioners and heat pumps, which embody the same principles.
This is the "second form" or Clausius statement of the second law. |
Second Law: Entropy
Second Law of Thermodynamics: In any cyclic process the entropy will either increase or remain the same.
Since entropy gives information about the evolution of an isolated system with time, it is said to give us the direction of "time's arrow" . If snapshots of a system at two different times shows one state which is more disordered, then it could be implied that this state came later in time. For an isolated system, the natural course of events takes the system to a more disordered (higher entropy) state.
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References:
- Thermal Equilibrium. (n.d.). Retrieved December 29, 2015, from http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thereq.html
- First Law of Thermodynamics. (n.d.). Retrieved December 29, 2015, from http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/firlaw.html
- Second Law of Thermodynamics. (n.d.). Retrieved December 29, 2015, from http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html