# Concentration of excess reactant

If increasing/decreasing the concentration of an excess reactant influenced the percentage yield of a product experimentally, how could this be explained?

I know that in theory, the excess shouldn't affect the yield- since it's the limiting that decides how much is produced. But is there any possible explanation for this result?

• RE: I know that in theory, the excess shouldn't affect the yield... Says who?!? – MaxW Jul 23 '18 at 20:28
• @MaxW Says almost every other high school textbook that I've read. If the given statement in the question is true, I personally think it would make for a good question to clear the misconception of high schoolers. – Gaurang Tandon Jul 24 '18 at 1:23
• See Le Chatelier's principle courses.lumenlearning.com/boundless-chemistry/chapter/… – DrMoishe Pippik Jul 24 '18 at 2:31
• Yes, but Le Chatelier's principle only applies to systems at equilibrium. If I am reacting some organic goo there is no guarantee that the system reached equilibrium. Kinetics could play a big factor in the yield. – MaxW Jul 24 '18 at 2:52
• But even if we use Le Chatelier's principle, wouldnt it still result in the same amount of yield since the amount of limiting reactant is kept the same?? – chemqss Jul 24 '18 at 8:52

A very typical situation in organic chemistry is the following:

$$\ce{A -> undisired products}$$

$$\ce{A + B -> target product}$$

Here it is easy to see that increasing the concentration of $$\ce{B}$$, we will get a higher yield of the target product.

The other reaction can be of many kinds. $$\ce{A}$$ can decompose in the reaction medium, can react with itself, with the solvent, with byproducts form the desired reaction, etc.

• Thank you! But does the "increasing conc of B results in higher yield of target product" take into account that there is a limiting and excess reactant? – chemqss Jul 25 '18 at 14:52
• Here the idea is that A is limiting and B in excess – Raoul Kessels Jul 25 '18 at 17:46
• Doesn't: A⟶undesired products A+B⟶target product just mean that the addition/inclusion of B would produce the target product? If there is an equal amount of B with A, this would make the same amount as when there is an excess of B. How could this prove that increasing concentration of B would result in a higher yield? – chemqss Jul 25 '18 at 18:51