In Physics a reversible process is defined as one in which the system can be returned to its initial conditions via the same path (along the PV Diagram), and every point along the path is an equilibrium state. An irreversible physical process is one that does not meet those conditions.

In Chemistry, reactions involving aqueous and gaseous products or reactants are reversible, meaning that they flow in both directions. Each reversible chemical reaction has a K value that relates the ratio of the concentration of products to reactants (taken to the power of their coefficients) at equilibrium. Irreversible chemical reactions are ones that either consist of only solids or liquids, or are reversible chemical reactions that just have a extremely large/small K value where they practically move in only one direction.

From a physics point of view a process is irreversible if it increases the entropy of the universe, and a reversible process is where the entropy of the universe remains constant. Does this mean that all reactions involving aqueous and gaseous products or reactants don't increase or decrease the entropy of the universe? Is it possible that reversible physical (thermodynamic) systems are not related to reversible chemical reactions?

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    $\begingroup$ Welcome to Physics! If you don't get a good answer here after a few days, you might try asking the same question over on our sister site Chemistry. $\endgroup$ Commented Mar 15, 2023 at 13:28
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    $\begingroup$ I have to say I have always been bothered by the definition of "reversible" in physics. Imagine two bodies, connected by a very weak thermal link but otherwise insulated, initially at different temperatures. Heat will flow in a definite direction, from the hotter body to the colder, until temperatures are equalised. Assume this process is so slow that it is quasistatic or smooth. The entropy change is well-defined at all times, $dS=\delta Q/T$. Yet this process cannot be logically described as "reversible"– it will flow in one direction only. $\endgroup$
    – printf
    Commented Mar 16, 2023 at 1:47
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    $\begingroup$ @printf That's why quasistatic and reversible are distinct characteristics, as emphasized in Wikipedia, for example, with this exact example. $\endgroup$ Commented Mar 16, 2023 at 6:00

2 Answers 2


Is it possible that reversible physical (thermodynamic) systems are not related to reversible chemical reactions?

Correct. No real process is thermodynamically reversible; entropy is generated whenever energy moves down a gradient (e.g., in temperature or pressure or any intensive variable), and such gradients are needed to drive net processes.

Thermodynamic reversibility is an idealization we use when we don’t care to calculate the entropy production or when we wish to explore the limit of low friction and high efficiency.

Reversibility in the sense of a chemical reaction is different; it means we have a hope of running the reaction in the opposite direction. But whatever way the net reaction is run, it is in practice thermodynamically irreversible.

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    $\begingroup$ Reversibility is also a statement that both reactions are happening at the same time. The forward and backward reaction rates are equal at equilibrium, and any reaction happening at equilibrium is, by definition, ideally reversible. Ice in water at 0 C will stay that way and (ideally) no entropy is gained no matter how long you continue this stasis. But in fact it is continuously melting and freezing at an equal rate. $\endgroup$
    – Stian
    Commented Mar 16, 2023 at 18:27
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    $\begingroup$ Good point; edited to emphasize the net process. $\endgroup$ Commented Mar 16, 2023 at 18:32

Reversible chemical reactions are not really processes from thermodynamics point of view, i.e., it doesn't make sense to apply to them term reversibility in its physical meaning. It is worth adding that reversible chemical reaction is a case of thermodynamic equilibrium between different components of the system (i.e., between different types of molecules.)

On the other hand, irreversible chemical reactions do correspond to irreversible processes in thermodynamics. Thus, salt depositing from a solution results in overall entropy increase. This could be viewed as a relaxation of the system (salt+solution) to thermodynamic equilibrium, where the rate at which the salt molecules are diluted becomes equal to the rate of deposition.

Rusting might be considered as another example. Which brings us to a different example of oxidation reaction: combustion. E.g., reaction $2H_2+O_2\leftrightarrow H_2O$ might be extremely slow to consider hydrogen and oxygen mixture being in equilibrium with war. However, once ignited, the reaction takes a special path (not just dissociation into ions), which leads to nearly complete conversion into water, with great amount of heat released - definitely the case where entropy increases. Such processes are better seen as phase transitions rather than relaxation. See my answer to Can we call rusting of iron a combustion reaction? for more discussion.

Definition of a reversible chemical reaction is:

A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously. $$aA + bB\leftrightarrow cC + dD$$ 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.

See also Chemical equilibrium.


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