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My book says that the batteries we have considered so far (Daniell cells) have a low life span because they reach equilibrium too quickly.

In the next paragraph, it states they are also made by irreversible reactions and we call them "primary batteries."

How can a reaction reach equilibrium without being reversible?

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  • $\begingroup$ The path to an equilibrium state does not need to be reversible. What do you think would stop it from reaching equilibrium? $\endgroup$ – Buck Thorn Oct 6 at 20:38
  • $\begingroup$ So what is an equilibrium state? I thought the only way to reach it is being a reversible reaction ( the book also says that the Daniell cells are in dynamic equilibrium Zn+Cu2+ <------> Zn2+ + Cu). Plus, here( siyavula.com/read/science/grade-12/chemical-equilibrium/…) says that an equilibrium is a state of a reversible reaction. $\endgroup$ – Sonim Blenim Oct 6 at 21:42
  • $\begingroup$ Sorry, I have deleted it. $\endgroup$ – Sonim Blenim Oct 7 at 21:56
  • $\begingroup$ So is my book wrong? $\endgroup$ – Sonim Blenim Oct 7 at 21:56
  • $\begingroup$ Maybe you can include the passage from the book, or a link to the text as an image (in whatever language) as a temporary solution. It is not possible to pass judgement based on the info you have provided. You have now two questions asking the same thing, no? If so please remove one of them. $\endgroup$ – Buck Thorn Oct 8 at 5:47
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Your confusion probably stems from the definition of a reversible path between two states: an infinite sequence of steps along a continuum of equilibrium states, such that at each step the equivalence condition of the second law of thermodynamics, $\mathrm dS_\text{total}=0$, is satisfied.

However, there is no requirement that a path between two states must be reversible. Many textbook problems are based on comparing the entropy changes associated with reversible and irreversible paths. For the reversible paths, $\Delta S_\text{total}=0$, for the (spontaneous) irreversible ones, $\Delta S_\text{total}\gt0$.

Your textbook explains that the discharging process of a Daniell cell is considered irreversible. The usage of the term irreversible in the book is rather similar to the common usage, which means "impossible to bring back to its original state, for practical reasons.". The answer that Karl has given you is correct. Basically it is not possible to bring the cell to its original state. The chemical and physical integrity of the recharged electrodes and electrolyte are altered compared to the original structure. There are entropic costs associated with cycling the cell.

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