Fall 2010, Question #6C (Equilibrium shifts right or left?)

[Savanah Trewman 1H]

The reaction given for this question is N2O4 (g) -->2NO2 (g)

For part C, it states: If additional NO2 is added to the flask how will this change affect the concentration of N2O4?

The answer given is: The addition of 0.010 mol of NO2 will shift the equilibrium to the left and will increase the concentration of N2O4.

My question is more on the terminology/phrasing: Since you will be adding more of the products, why would the answer be that the equilibrium shifts to the left? Wouldn't it be shifting to the right? And then from there we could say that the reaction moves to the left to form more reactants?

[BlakeOwens4B]

The equilibrium needs to shift left because the concentration of N2O4 needs to be increased in order to maintain equilibrium. If it shifts right then it will only stray further from equilibrium.

[Victor Qiu 1C]

According to the Le Chatelier's principle, the reaction would shift to counteract the change. Therefore, if more products are added, the reaction should shift towards the reactants because in this way, there will be less products and more reactants and the reaction counteracts the change of adding more products.

[Arezo Ahmadi 3J]

If you increase the concentration of products at equilibrium, then that means the equilibrium constant needs to remain constant. Since the number in the numerator is now increased, by Le Chatelier's Principle, we need to increase the number in the denominator too, which represents the concentration of the reactants. Thus, we need to shift the equilibrium to the left towards the reactants so we can produce more of them and return to the same K value, as it is a ratio of the products to reactants at equilibrium.

[Xavier Herrera 3H]

I suppose that these types of questions ask more about the long term behavior of the equilibrium. So, initially, there are more products when NO2 is added. Over time, this additional product is used up to produce more reactants in order to return to the equilibrium constant value. So the equilibrium is shifted "to the left," towards the reactants.