How does an equilibrium reaction respond to the addiof extra reactant?

Answer 1

The concentration or activity of the product(s) will increase, and if there is at least one other reactant than the added one that is required for the completion of the reaction, the concentration of such an unadded reactant will decrease. (If there were no available unadded reactant, the reaction would not technically have been in equilibrium at the start, even though it may have reached a steady state that can persist for a long time in the absence of changed conditions.)

Answer 2

Adding extra reactant to an equilibrium reaction will shift the equilibrium towards the products side, in order to consume the added reactant and establish a new equilibrium. This occurs to maintain the equilibrium constant and keep the reaction mixture stable.