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Yes you would also leave out solids and liquids because their concentrations do not change. Also just to refresh, Q is the reaction quotient when the reaction is NOT at equilibrium and if Q <K, reaction favors the forward direction, and if Q>K, then reaction favors the backwards direction, and if Q=K, then the reaction is at equilibrium.
In the reaction, whether it's at equilibrium or not, solids and liquids are pure substances. This means, that no matter how the reaction goes, the overall reaction will not change the substance significantly enough for its concentration to change. That is why at equilibrium (for K constant) and the reaction quotient (Q), the concentration of that pure substance has no impact on the constant values -- because these substances have such an insignificant change in concentration that they practically cancel out when put into that ratio.
Megan Kirschner wrote:I understand the fact that you omit solids and liquids when calculating the equilibrium constant, but why? What's the logic behind it?
Solids and liquids aren't included because they do not have a significant effect on the concentration. Lavelle also mentioned during lecture that solids do not have concentrations, so it wouldn't make sense to include them.
briannam_1F wrote:When calculating for Q do we omit solids and pure liquids as well?
Yes you would leave them out because Q is pretty much K, with the difference being that Q is at any time during the reaction but you would calculate it the same way as K
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