Question

Does it matter what initial concentration of reactants and products are used to determine the equilibrium constant?

Answer 1

No, because the equilibrium constant is found from EQUILIBRIUM concentrations.

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Whenever you use initial concentrations, you can only locate where you are in the progress of the reaction, i.e. what the reaction quotient `Q` is.

For

`aA + bB rightleftharpoons cC + dD`,

`Q = ([C]_i^c[D]_i^d)/([A]_i^a[B]_i^b)`,

where all concentrations are initial (or current, if you will).

The equilibrium constant `K` is defined in the same way, except with `[" "]_(eq)`. The only difference is their values, and the way the numbers are plugged in is the same.

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Using an ICE Table...

• one would be able to start from initial concentrations, already knowing the equilibrium constant, to find the change (`x`) in concentrations, regardless of the initial concentrations chosen (because `x` is dependent on what `K` is).
• one would be able to start from initial concentrations, already knowing the change in concentrations (`x`), to find the equilibrium constant `K` (provided the change in concentrations is correct!), because `K` does not change at the same temperature for the same reaction.