[smollenk]

Hello,

I am in my second semester of general chemistry and trying to reconcile 2 (new to me) concepts about limiting reactants and Le Chatelier's Principle.

I understand that a reaction in chemical equilibrium is a reaction that has not stopped but continues to progress with constant concentrations of reactants and products after a good amount of time. I also understand that via Le Chatelier's principle, if the reaction is disturbed by something like a change in reactant concentration, the equilibrium will shift to accommodate the change and establish a new equilibrium point.

My questions:
How does the change in reactant concentration not create a stoichiometry problem because the reactants are no longer in the same proportions needed to react?
Is it not possible to reduce or raise the concentration of one reactant to a point that the reaction stops? Or if the reaction never stops, is there some sort of ceiling where adding more concentration will not have any further effect on the equilibrium?

I am thinking about this reaction:
H+ + OH-  H₂O

If I have some finite quantity of water in my beaker it will already be in equilibrium. If I add H+, according to Le Chatelier's the equilibrium will shift and produce more water. However, if I keep adding H+ it seems to me like the finite number of oxygen available in my beaker would be overwhelmed and there should be a maximum number of waters that I can create because my oxygen in limited.

[Borek]

You can't add just H⁺, you have to add it with a counterion.

Border cases (like the one you are thinking about) are a rather theoretical construct and can't be achieved in practice (so there is no need to worry about them). They sometimes show up when you push rules behind the limit of their application.

[Corribus]

The limiting reagent concept doesn't have a whole lot to do with equilibrium, per se, because it usually applies under situations where reactions are designed to go to "completion". This doesn't mean equilibrium doesn't exist - it does - but rather the reaction is so favorable that at equilibrium almost all of the species are (in principle) the desired products rather than reactants.

E.g., in a reaction

A + B  C

An event in which A and B collide almost always gives C (due to thermodynamics, say), such that, given enough time, every A in the solution reacts with every B to form one C. In this situation, K >> 1, and we would just simplify it as

A + B  C

even though implicitly some C always converts back to A + B. Now, under this simplification, if you have a lot more A than B, eventually all of the B will be consumed and there will be A left over. B is your limiting reagent. In reality, not all of B is truly consumed because there IS an equilibrium, even if it's very far to the right.

You can think of it in terms of probability.

Assume a situation

A + B  C + D

where you start with a lot of A and B. At early times there will be a large probability, per unit time, of an A and B colliding to form a C and D. Conversely, there is not a lot of C and D in solution, so there is a small probability of the reverse reaction to form A and B. The instantaneous forward reaction rate is high and the backward rate is low. As time goes on, there is less and less A and B in solution, so the probability of a forward reaction event decreases; conversely, there is more and more C and D in solution, so the probability of a reverse reaction revent increases. At some point, the probabilities are equal, such that the amount of A + B  C + D and the amount of C + D  A + B is equivalent. This is equilibrium.

This situation doesn't change if you start with an unequal (non-stoichiometric) amount of A and B in your reaction pot, or if you continually add A as a function of time. the concept of probability still applies. If you continually add A, you may raise the probability of a forward reaction event for a while, but eventually the amount of B in solution gets so low that again there is a low probability of a collision between A and B, so further adding more A doesn't do a whole lot. You are still governed by the equilibrium constant, which incorporates the idea of probability and reaction thermochemistry into the balanced forward and backward rates. You may add so much A that it "drives" the reaction forward by maximizing the probability of a reaction event between A and B, so you may consider B to be a limiting reagent in this case, and the reaction essentially "goes to completion". Just so, you can remove your C and D as they are produced, to again change the probability of A + B going forward relative to C + D going backward. These are both strategies used in synthetic chemistry to maximize product formation.

[sunkal]

What do you mean by "create a stoichiometry problem"?