# Relation between thermodynamic reversible process and reversible reaction

I know it seems to be a weird question. But for long I have been thinking whether there is any relation between thermodynamic reversible process and reversible reaction. Do they have any connection and if so how?

• Perhaps you might first consider whether a reaction is or is not a process. Once you understand that a reaction is a "chemical" process, you will have your answer. – Jeffrey Weimer Sep 30 '14 at 14:59
• Chemical process? Can you please explain me? – user5764 Sep 30 '14 at 15:14

In thermodynamics, a reversible process -- or reversible cycle if the process is cyclic -- is a process that can be "reversed" by means of infinitesimal changes in some property of the system without entropy production (i.e. dissipation of energy). Due to these infinitesimal changes, **the system is in thermodynamic equilibrium throughout the entire process.**

Thermodynamic equilibrium is an axiomatic concept of classical thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by permeable walls. In thermodynamic equilibrium there are no net macroscopic flows of matter or of energy, either within a system or between systems

dO YOU OBSERVE SUCH KIND OF equilibriums IN A REACTION. SO THEY ARE NOT RELATED AT ALL

ACCORDING TO WIKIPEDIA

A reversible reaction is a chemical reaction that results in an equilibrium mixture of reactants and products. For a reaction involving two reactants and two products this can be expressed symbolically as aA + bB ---> 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 reversible process in thermodynamics.

Reversible reaction

According to Merriam Webster, a reversible reaction is:

a reaction that takes place in either direction according to conditions (as the formation of hydriodic acid by union of hydrogen and iodine or its decomposition into these elements)

For this type of reaction, you would use a double-harpoon to write the chemical equation:

$$\ce{H2(g) + I2(g) <=> 2HI(g)}$$

and would be able to write an equilibrium constant expression

$$K = \frac{[\ce{HI}]^2}{[\ce{H2}][\ce{I2}]}$$

with [] denoting activity or fugacity. If you start with pure reactants, the reaction would go forward (decrease in reactants, increase in products). If you start with pure products, the reaction would go backwards (increase in reactants, decrease in products). Once the reaction has reached equilibrium, concentrations would no longer change.

In any case, reactions in both directions would occur at the particular level. When the reaction is at equilibrium, you just won't know at the macroscopic level and would have to disturb the equilibrium to see macroscopic changes again.

Reversible process

Again according to Merriam Webster, a reversible process is:

an ideal process or series of changes of a system which is in complete equilibrium at each stage such that when the process is reversed each of the changes both internal and external is reversed but with the amount of transferred energy unaltered

Applied to chemical reactions, a reversible process is one where the Gibbs energy is constant throughout (or where the entropy of the universe is not increased by it). We would not expect any macroscopic changes (the process is at equilibrium), which why this is an ideal process (not real). The closest real process is one where the system is near equilibrium, and the increase in entropy of the universe is minimal.

What does IUPAC say?

The IUPAC Gold book makes reference to a paper (DOI: https://doi.org/10.1351/pac199466051077) offering a glossary of terms used in physical organic chemistry. In their definition of chemical equilibrium, it says: While the definition of reversible process matches the one I cite above, and they use the same notation for the equilibrium reaction. However, the connection between reversible process and chemical equilibrium surprises me. When a reaction approaches equilibrium, the Gibbs energy does change, and it will not return to the initial state unless work is done on the system. Maybe the confusion regarding reversible process vs. reaction stems in part from this.

• Somehow IUPAC seems to have dropped the ball with that definition. I would not say its is exactly the same as Merriam-Webster's definition, and even though (one would think) IUPAC's should take precedence, I prefer the MW defintion. – Buck Thorn Dec 3 '19 at 20:08