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13 Μαΐ 2023 · Changes in entropy (ΔS Δ S), together with changes in enthalpy (ΔH Δ H), enable us to predict in which direction a chemical or physical change will occur spontaneously. Before discussing how to do so, however, we must understand the difference between a reversible process and an irreversible one.
For reversible processes (the most efficient processes possible), the net change in entropy in the universe (system + surroundings) is zero. Phenomena that introduce irreversibility and inefficiency are: friction, heat transfer across finite temperature differences, free expansion, ...
28 Απρ 2023 · We introduce heuristic arguments to infer that \(\Delta S=0\) is not possible for a spontaneous process in an isolated system. From this, we show that \(\Delta S_{universe}>0\) for any spontaneous process and hence that \(\Delta S_{universe}=0\) is not possible for any spontaneous process.
Entropy Changes in Reversible Processes. Suppose that the heat absorbed by the system and heat lost by the surrounding are under completely reversible conditions. In other words, qrev is the heat absorbed and lost by the surrounding at temperature T, then we can say that the entropy change in the system will be given by the following relation. (26)
1 Αυγ 2016 · In spite of their not being real, reversible processes are most important in thermodynamics because definite equations can be obtained only by considering reversible changes; irreversible changes can only be described with the aid of inequalities when equilibrium thermodynamics is used.
The change in entropy for any process that leads to a transformation between an initial state ``a'' and a final state ``b'' is therefore where is the heat exchanged in the actual process. The equality only applies to a reversible process.
The infinitesimal change of entropy in a reversible process can thus be written as \[dS=\left(\dfrac{\delta Q}{T}\right)_{rev}\] where is the entropy and is the absolute temperature.
