Entropy units9/24/2023 ![]() The theory developed by Boltzmann and others, is known as statistical mechanics. The question of why entropy increases until equilibrium is reached was answered very successfully in 1877 by a famous scientist named Ludwig Boltzmann. While the second law, and thermodynamics in general, is accurate in its predictions of intimate interactions of complex physical systems behave, scientists are not content with simply knowing how a system behaves, but want to know also why it behaves the way it does. For example, the orbiting of the planets around the Sun may be thought of as practically reversible: A movie of the planets orbiting the Sun which is run in reverse would not appear to be impossible. Some processes in nature are almost reversible. Such processes are irreversible: An ice cube in a glass of warm water will not spontaneously form from a glass of cool water. For example, a glass of warm water with an ice cube in it will have a lower entropy than that same system some time later when the ice has melted leaving a glass of cool water. When bodies of matter or radiation, initially in their own states of internal thermodynamic equilibrium, are brought together so as to intimately interact and reach a new joint equilibrium, then their total entropy increases. Thermodynamic entropy has a definite value for such a body and is at its maximum value. A body of matter and radiation eventually will reach an unchanging state, with no detectable flows, and is then said to be in a state of thermodynamic equilibrium. Irreversibility is described by a law of nature known as the second law of thermodynamics, which states that in an isolated system (a system not connected to any other system) which is undergoing change, entropy increases over time. Mixing coffee and burning wood are "irreversible". If a movie that shows coffee being mixed or wood being burned is played in reverse, it would depict processes impossible in reality. A more physical interpretation of thermodynamic entropy refers to spread of energy or matter, or to extent and diversity of microscopic motion. The word 'entropy' has entered popular usage to refer a lack of order or predictability, or of a gradual decline into disorder. ![]() For example, cream and coffee can be mixed together, but cannot be "unmixed" a piece of wood can be burned, but cannot be "unburned". Thus, this concept of entropy (measure of randomness) has led to the conclusion that all substances in their normal crystalline state at absolute zero temperature would be in the condition of maximum orderly arrangement, because all motion has essentially ceased at ‘0 K.’ In other words, entropy of a substance at 0 K is minimum.In thermodynamics, entropy is a numerical quantity that shows that many physical processes can go in only one direction in time. When two gases are mixed, the molecules of the gases intermix to achieve more randomness. The process of vaporisation produces an increase in randomness in the distribution of molecules, hence an increase in entropy. ![]() For example, when a solid changes to a liquid, an increase in entropy takes place, because with the breaking of the orderly arrangement of the molecules in the crystal to the less orderly liquid state, the randomness increases. Conversely, if the change is one in which there is an increase in orderliness, there is a decrease in entropy. Physical significance: Entropy has been regarded as a measure of disorder or randomness of a system. Thus when a system goes from a more orderly to less orderly state, there is an increase in its randomness and hence entropy of the system increases. It is an exothermic reaction, therefore, favours the process, but randomness factor opposes the reaction.Īs the reaction takes place, the energy factor must be greater than the randomness factor. (b) The reaction between hydrogen and oxygen to form water. Since the process is known to take place, randomness factor must be greater than energy factor. Evaporation of water is endothermic, therefore, energy factor opposes the process. When the two tendencies act in the opposite direction, the tendency with the greater magnitude determines whether the process is feasible or not. (iii) the driving force is the resultant of the magnitude of the two tendencies. (ii) the two tendencies may work in the same direction or opposite direction in a process and (i) the two tendencies act independent of each other, The overall tendency of a process to take place by itself is called the driving force. (ii) the tendency to acquire a state of maximum randomness or disorder. ![]() (i) the tendency to acquire a state of minimum energy, and The overall tendency of a process to occur can be expressed on the resultant of two tendencies namely:
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