The equilibrium constant and solids

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SUMMARY

The discussion centers on the concept of the equilibrium constant and the treatment of solids within equilibrium expressions. It is established that the concentration of a pure solid is considered constant because its activity is defined as 1, regardless of the amount present. Participants clarify that reactions involving solids occur only at the surface, and even significant dissolution does not alter the solid's concentration in the equilibrium expression. The conversation emphasizes that the reaction quotient is based on activities, where solids maintain a constant activity value.

PREREQUISITES
  • Understanding of chemical equilibrium principles
  • Familiarity with the concept of reaction quotients
  • Knowledge of activities in chemical reactions
  • Basic grasp of solid-state chemistry
NEXT STEPS
  • Research the concept of chemical activity in equilibrium
  • Explore the differences between solids and aqueous species in equilibrium expressions
  • Study the implications of surface area on reaction rates for solids
  • Learn about the role of concentration in reaction quotients
USEFUL FOR

Chemistry students, educators, and professionals interested in chemical equilibrium, particularly those focusing on the behavior of solids in reactions.

whsmt85
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Why are the concentrations of solids not incorporated into the equilibrium expression? All the books say it's because the "concentration of a pure solid is constant", but that doesn't make sense to me... If the solid is dissociating to produce a given product, why doesn't the concentration change?

Thanks for your help
 
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You drop a nail into the solution.

Does it react?

What is its "concentration"?
 
Borek said:
You drop a nail into the solution.

Does it react?

What is its "concentration"?


So I'm guessing that because the nail is so big, whatever ions it loses to the solution don't cause a significant change to its "concentration".

But aren't there any situations where you have a small enough amount of solid that the ions it produces does detract significantly from the solid's original mass?...

Thanks
 
It is not the size that matters, what is important is the fact that reaction takes place only of the surface.
 
Borek said:
It is not the size that matters, what is important is the fact that reaction takes place only of the surface.


Alright, but say you have a solid that dissolves 99% into solution, and you are left with only 1% of the original solid, how can one consider the solid's concentration to be constant if only 1% of it is left?..
 
Concentration is already an approximation. Reaction quotient is built using activities of all substances. For diluted solutions activity equals concentration, for solids activity is always 1.

There are several other ways of approaching the problem. They were discussed here on many occasions.
 

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