Question

So I had a test question that asked the following, "What will happen when a reactant is added to a system at equilibrium?"

The answers are
A. more reactants will form
B. The system will remain at equilibrium
C. Only the concentration of the reactant will change
D. The equilibrium of the system will be upset.

I answered with B, as Le Chatelier's principle, states that the reaction will shift to remain at equilibrium. However my test says the the equilibrium of the system will be upset. I just want to know why this is/ what am I not understanding about Le Chatelier's principle.

Answer 1

The correct answer is D.
Answers A, B and C are incorrect.
Your understanding of Le Chatelier's principle is correct; however, I think you simply misstated it. It will NOT REMAIN at equilibrium. You are right that if a reactant is added to the system which is at equilibrium then the system will shift to the right in order to reach a new equilibrium in which the concentrations of reactants and products have changed in order to keep Keq constant. The B answer is incorrect. It will NOT REMAIN at equilibrium but it will SHIFT to form a NEW equilibrium. I think your problem is with the word "remain". That implies that there will be no change. Yes, the system will "remain" at equilibrium but not before it shifts to reach a new equilibrium in which reactants and products have changed so that Keq is constant. Answer B clearly says that no change will occur. Had B said that I suspect you would not have picked that answer.

Answer 2

Ok thank you, this clears it up. :)