The equilibrium constant and solids

In summary, the concentrations of solids are not included in the equilibrium expression because the concentration of a pure solid is considered constant, regardless of its size or the amount that dissolves. This is because the reaction only takes place on the surface of the solid, and for solids the activity is always considered to be 1. Therefore, even if only a small percentage of the solid remains, its concentration is still considered constant. Other approaches to this problem have also been discussed before.
  • #1
whsmt85
4
0
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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  • #2
You drop a nail into the solution.

Does it react?

What is its "concentration"?
 
  • #3
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
 
  • #4
It is not the size that matters, what is important is the fact that reaction takes place only of the surface.
 
  • #5
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?..
 
  • #6
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.
 

1. What is the equilibrium constant?

The equilibrium constant, also known as Keq, is a numerical value that represents the ratio of products to reactants at equilibrium in a chemical reaction.

2. How is the equilibrium constant calculated?

The equilibrium constant is calculated by taking the concentration of the products raised to their stoichiometric coefficients, divided by the concentration of the reactants raised to their stoichiometric coefficients. This can also be expressed using partial pressures for gas-phase reactions.

3. How does the presence of solids affect the equilibrium constant?

The concentration of solids does not affect the equilibrium constant because they are not included in the calculation. Solids are considered to have a constant concentration and do not participate in the equilibrium reaction.

4. Can the equilibrium constant change?

The value of the equilibrium constant is constant at a given temperature and does not change unless the temperature changes. However, the composition of the reaction mixture can change, which would result in a change in the equilibrium constant.

5. How does the magnitude of the equilibrium constant relate to the position of equilibrium?

The magnitude of the equilibrium constant is related to the position of equilibrium. A larger Keq value indicates that the reaction favors the products, while a smaller Keq value indicates that the reaction favors the reactants. The position of equilibrium can also be affected by factors such as temperature and pressure.

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