What happens to a reaction at equilibrium when more reactant is added to the system?

Provided all other conditons remain constant (e.g. volume and temperature), the addition of more reactant means the increase of the concentration of the reactant, and so, at constant temperature, more product must be produced to compensate such addition of more reactant.

An equlibrium reaction may be represented by the general expression:

aA + bB ⇄ cC + dD

And the equilibrium constant is given by:

[tex]Keq=\frac{[C]^c.[D]^d}{[A]^a.[B]^b}[/tex]

Thus, since at constant temperature Kea is constant, the increase of a reactant concentration (A or B are in the denominator) means that the concentration of the products (C and D in the numerator) must increase.

Since, the molecular point of view what happens is that the increase of the concentration of reactions increase the rate of the direct (forward) reaction yielding to the production of more products.