[Edher]

Saludos,

        I am trying to understand why we don't take into consideration pure liquids and solids when calculating the equilibrium constant. According to my book it simply says because the concentrations of a pure solid or liquid cannot change. That, however, I don't quite understand. I was doing my own search on line and I came across a website where they try to explain it using units and they concluded with something like this: The units would be illogical for a pure solid because you would have mg/cubic centimeter. That didn't help either because they are explaining it mathematically, I want to understand it conceptually and concretely.
     
The book along with other sites, give the example of

CaCO₃(s) <> CaCO(s) + CO(g)₂

They put the CaCO₃(s) in a flask where it's heated and it emits CO(g)₂. Well, isn't this a change in the concentration? The fact that a secondary substance is being derived from the original, isn't necessary to take some molecules of the original substance thus changing the concentration of it?

If someone could just explain it to me in the most simple condescending, patronizing terms I would strongly appreciate it,
Edher

[Mitch]

CaCO3(s) <> CaCO(s) + CO(g)2 is not an aqueous example of the aforementioned concept. So you would see a change in mass. In aqueous solutions CaCO3(s) concentration can be considered constant since its usually saturated in the solution.

[AWK]

There are a plenty of equilibriums in chemistry. Students of general and physical chemistry on undegraduate level are taught only the simplest ones, eg: law of mass action, Henry's law, Raoult's law and so on.

[Edher]

Saludos,

       Perhaps I haven't expressed myself correctly. Right now I'm learning the principles of Chemical Equilibrium. Therefore, I am learning how to calculate the equilibrium constant of a chemical equation by using the following set up:

    K_C = [Product]
          [Reactant]

However, one of the rules, according to my book, in the aforementioned expression is that one cannot take into consideration either pure liquids or pure solids. This is the part that I do NOT understand.

The book simply says that the reason why pure solids or pure liquids are not taking into consideration is because their concentrations do not change during a chemical reaction.

Yet the book gives the following chemical equation:

   C_(s) + H₂O <> CO_(g) + H_2(g)

Now this is my reasoning, how is it possible that the concentration of C_(s) doesn't change when you can clearly see that it has reacted with water to yield CO_(g). CO_(g) must have gotten its carbon from C_(s) therefore the concentration of carbon must have changed. Yet carbon was not included in the equilibirum constant expression.

Why aren't  pure liquids and pure solids taken into consideration in a equilibrium constant expression?

Thank You,
Edher

[cofi]

this explanation is taken from Silberberg's "Chemistry: The Molecular Nature of Matter and Change":

"A pure solid, however, such as CaCO₃ or CaO, always has the same concentration at a given temperature, that is, the same number of moles per liter of the solid, just as it has the same density at a given temperature. Moreover, since a solid's volume changes very little with temperature, its concentration also changes very little. For these reasons, the concentration of a pure solid is constant, and the same argument applies to the concentration of a pure liquid."

and in one of my Physical Chemistry books, I've found what to use to express the amount of a certain part of mixture or solution when calculating the equilibrium constant:

mixture:
component:
- solid -> x = n(component) / n(all components in mixture)
- liquid -> x
- gaseous -> partial pressure (p)

solution:
- dissolved compound -> concentration (c)
- solvent -> x

so, you see, in cases when you use x, it is always ~constant, so you can just forget about it