Concentration of solids and liquids in Chemical Equilibrium

[zorro]

Homework Statement

Why don't we take into consideration the concentration of pure solids and pure liquids in equilibrium constant expression?

I read that their concentration per unit volume does not change.

If the concentrations (moles per litre) don't change then it won't undergo any reaction i.e. it has no use in the reaction. How is it possible?

If we keep the reactants (say all gases) in a closed container, then their volume does not change. But no. of moles change so we can say that the concentration changes.
But this is not the case in solids(pure) and (liquids)

Please provide detailed explanation

 

[Borek]

Technically concentration of solid that is in contact with the solution is zero in the solution, isn't it? However, it can react on the surface. As long as the surface exists, reaction is possible, and amount of solid doesn't matter. We assume its concentration to be 1, but more correct term to use is "activity" - and activity of the solid is 1.

Note that reaction quotient, which is usually expressed using concentrations, should be in fact expressed using activities.

Activity = activity_coefficient * concentration

For diluted solutions activity coefficient is 1, for more concentrated solutions it initially gets lower, later gets higher. This is quite convoluted and details are still not fully understood, even if the phenomenon is researched for well over 100 years.

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[zorro]

What about a pure liquid?