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chemistry A-level impossible question????? watch

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    (Original post by K-Man_PhysCheM)
    While this makes sense, for A-level we've been taught that solids/solvents are not included in the equilibrium expression because they are in massive excess and so their concentration remains essentially constant. The effect of the solid/solvent in excess is thus included in the equilibrium constant.

    A solid can have a concentration: number of moles of atoms per unit volume. But this would be a constant anyway, assuming the solid doesn't change its packing pattern during the reaction. Though I suppose this is usually described by density for solids and pure liquids...
    It's nothing to do with massive excess.

    A solid or a liquid may react, or be formed, during the course of a reaction, but the density of their atoms per unit volume will never change.

    This makes them constant in terms of activity and hence never included in the law of mass action, from which equilibrium and rate expressions are derived.
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    (Original post by charco)
    It's nothing to do with massive excess.

    A solid or a liquid may react, or be formed, during the course of a reaction, but the density of their atoms per unit volume will never change.

    This makes them constant in terms of activity and hence never included in the law of mass action, from which equilibrium and rate expressions are derived.
    That's fair enough, but I gave a similar explanation in class (without mentioning activity) and my teacher said that it was actually because pure solids and liquids have much higher concentrations than any solutes so are essentially "in excess". Your explanation makes more sense though, thanks
 
 
 
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