Reversible Processes
A reversible process describes an ideal thermodynamic system where steps in a reaction, or changes in a physical process, can reverse direction such that the resulting entropy change for the system and surroundings is zero. If it is possible to restore a thermodynamic system and its surroundings to their original state (including the original entropic state) following a process then such a process is termed a reversible process. All other reactions and processes are irreversible.
Considering both the system and surroundings, all real processes are irreversible. Internally reversible processes demand no irreversibilities in the system and externally reversible process demand no irreversibilities outside the system.
A ideal reversible process describes a reaction or process in which the system its surroundings are in equilibrium throughout the process. In considering ideal gas expansions and contractions, for example, the system and surroundings are considered to be in equilibrium if they have the same temperature and the same pressure throughout the process. If at any time the system or the surrounds diverge from equilibrium then the process becomes irreversible thermodynamically. The definitional (and easily constructed) test of reversibility dictates that if a reaction or process is completely reversible then it can be driven to change direction by an infinitesimal change in some external variable.
To expand an ideal gas by a reversible process demands that outside and inside pressures be kept balanced as the expansion proceeds. The temperatures of system and surroundings must remain the same so that the system and surroundings are in equilibrium at all times. The only way to accomplish this is through a series of thermostatic steps in which the system does a maximum amount work on the surroundings (expand) while the surroundings does minimum work on the system.
Although the second law of thermodynamics states that all isolated systems move spontaneously toward a state of maximum entropy, as a practical matter, nearly reversible process can be designed if reaction or process conditions are not allowed to stray far from equilibrium. Because entropy increases create irreversible process and set a limiting boundary on the approach to reversibility, reversible processes must be designed to reduce entropy increase to a minimum. Physicists studying the thermodynamics of reversible processes or equilibrium dynamics use thermostatic techniques to create near reversible processes through a series of slow and gradual sequence steps conducted at, or very near, the equilibrium point of each step. In some cases, thermostatic techniques substitute a series of very slow and gradual changes into the transition from initial to final states.
In many idealized approximations (especially at the high school and undergraduate university level) students encounter idealized problems that are reversible. Very few, if any, real problems are so idealized and entropic and chaotic effects make natural reactions or processes irreversible.
Most reactions are irreversible because energy is usually transformed and work is done to cause a change in a system or its surroundings. In additon, in most reactions some energy is lost because of undesirable transformations of energy (e.g., heat lost during the use of electrical energy). No matter how well a system is insulated, or designed to reduce such factors as friction, there is always energy dissipation and entropy increase. By definition, reversible process demand 100% efficiency in the use and conversion of energy (any loss would preclude a return to the initial state). Because heat (generated by friction, etc.) can never be 100% recovered and reconverted into work, no machine can operate at 100% efficiency. As a consequence, machines can not perform reversible functions.
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