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[a] While processes in isolated systems are never reversible, [3] cyclical processes can be reversible or irreversible. [4] Reversible processes are hypothetical or idealized but central to the second law of thermodynamics. [3] Melting or freezing of ice in water is an example of a realistic process that is nearly reversible.
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Reversible process (thermodynamics), a process or cycle such that the net change at each stage in the combined entropy of the system and its surroundings is zero; Reversible reaction, a chemical reaction for which the position of the chemical equilibrium is very sensitive to the imposed physical conditions; so the reaction can be made to run ...
Isentropic process: a reversible adiabatic process, occurs at a constant entropy; Isobaric process: occurs at constant pressure; Isochoric process: occurs at constant volume (also called isometric/isovolumetric) Isothermal process: occurs at a constant temperature; Steady state process: occurs without a change in the internal energy
Cyclic processes were important conceptual devices in the early days of thermodynamical investigation, while the concept of the thermodynamic state variable was being developed. (3) Defined by flows through a system, a flow process is a steady state of flows into and out of a vessel with definite wall properties. The internal state of the ...
where a reversible path is chosen from absolute zero to the final state, so that for an isothermal reversible process Δ S = Q r e v T {\displaystyle \Delta S={Q_{rev} \over T}} . In general, for any cyclic process the state points can be connected by reversible paths, so that
For reversible processes, an isentropic transformation is carried out by thermally "insulating" the system from its surroundings. Temperature is the thermodynamic conjugate variable to entropy, thus the conjugate process would be an isothermal process , in which the system is thermally "connected" to a constant-temperature heat bath.
A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B. This is distinct from a reversible process in thermodynamics. Weak acids and bases undergo reversible reactions. For example, carbonic acid: H 2 CO 3 (l) + H 2 O (l) ⇌ HCO 3 − (aq) + H 3 O + (aq).