Question

A system at equilibrium is placed under stress by adding more reactant. If this reaction has a small equilibrium constant (Keq), how will the addition of this stress affect the equilibrium of this system?

Answer 1

I can't tell you the multiple choice answer, but that should not matter...

Since `Q < K_(eq)` after the stress, `Q uarr` to resolve the stress by making more products.

---

Recall that an equilibrium constant for the reaction

`aA + bB -> cC + dD`

is

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

where `a,b,c,d` are the stoichiometric coefficients of `A,B,C,D`, respectively, and `[" "]` indicates molar concentration.

If an equilibrium constant is small, i.e.

`K_(eq) < 1`,

then that means there are more reactants than products before the equilibrium is disturbed.

(Note that in principle, the actual size of `K_(eq)` does not affect which direction the equilibrium shifts given a certain induced stress.)

Adding more reactants initially decreases the reaction quotient `Q` so that `Q < K_(eq)`. This is the stress that was induced.

Since `Q < K_(eq)`, in accordance to Le Chatelier's principle, the equilibrium shifts so that `Q` increases to equal `K_(eq)` again, going against the disturbance. The equilibrium always tries to undo a given disturbance.

That means it will shift to consume more reactants to generate more products.