Equilibrium problem TBR 3.17

[Buzzin]

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The problem is not the issue, the issue is that I don't understand the EXPLANATION of one of the wrong answers.

3.17
What is the effect of adding CaO (s) to the following equilibrium mixture?

CaCO3 (s) <----> CO2 (g) + CaO (s)

a. the products will decrease.
b. the reactants will increase
c. the equilibrium constant will incrase
d. there is no change in the equilibrium.

Solution
Adding a solid to a reaction mixture that is already at equilbrium has no effect on the equilibrium. This means that the correct answer is choice D.
A solid affects a reaction mixture ONLY if the reaction is not yet at equilibrium. This is to say that the solid is involved only when it is the limiting reagent for the reaction.

Can someone explain that last statement, that "This is to say that the solid is involved only when it is the limiting reagent for the reaction". I understand solids are never included in the equilibrium expression of [products] / [reactants] bc the concentration of solids and liquids is [1.0] since they are pure so can be left out.

Does this mean I should be checking if my equilibrium concentrations are even possible based on the initial concentration and balanced equations before I calculate Keq?

 

[quicheduty]

So, even though solids don't really "count" in your equilibrium expressions, they're still part of the reaction itself (if you had no CaCO3 you cannot react it to get to the right side of the equation). I feel like most problems will either indicate if there's excess or very little, and so far in my studies I haven't had to worry about calculating to make sure the solid isn't the limiting reagent. In this case you start off with an equilibrium mixture, so you can safely assume CaCO3 isn't "limiting."