Limiting Reactants

[Lillie Yazdi 3J]

I know this is a simple question but I just wanted clarification. There can only be one limiting reactant because once that's used up, no more product can be formed, correct? Because on the module quizzes it says there can't be two because they won't react with each other which does provide an explanation to my question but doesn't explain exactly why.

[Karla Vega]

The reason they wouldn't react with each other is because there is not enough for the reaction to continue. You have the right idea just worded a bit differently!

[KyleNagasawaDisc3C_Chem 14B2022W_]

Hey Lillie,

Karla did a good job answering your response. For reactions that tend towards completion, with the prototypical example being hydrocarbon oxidation, the extent of the reaction can only occur so long as reactants are available for reaction. You also asked about limiting reagents being limited to only 1 component of a reaction. While you could theoretically have multiple limiting reagents, this is less likely because it would require that at least two reactants are present in exactly the same quantity, which is very difficult to measure out in a laboratory setting.

[Jiayin Yola Yan 1G]

I think Kyle did a great job answering your question! The likelihood of having two reactants with identical moles is extremely low in the laboratory setting, and the reaction has to stop whenever one of the reactants is used up (aka the limiting one!).

[Phillip Ma 1J]

Jiayin did a good job explaining the low probability of two reactants being the limiting reactant! When comparing two reactants, make sure you are thinking about it in moles because that is the way in which you can compare the quantity of two reactants. Do not compare the reactants in grams because they all have different molar masses. Usually, one reactant runs out before the other, which is why there is one limiting reactant.

[Emily Widjaja 3A]

Yes, there can only be one limiting reactant!

[Helen Ringley 2E]

I was also confused by that statement in the module quizzes, but the way I think about it is like making a cake. If you need three eggs and one cup of sugar for every cake, it's unlikely that you'll have the right ratio of sugar and eggs so that you use up all of your ingredients and have none left over. It's more likely that you have like a dozen eggs but a bulk Costco bag of sugar, so you run out of eggs before you run out of sugar. In this case, the eggs are the limiting reactant and the sugar is the excess reactant. Just think of your reactants like ingredients you use to make the product, and the limiting reactant is the ingredient that runs out first.

- Helen Ringley 2E

[Neelaj Das 3I]

This is correct. One reactant will almost always run out faster than the others and therefore it will inherently determine how far a reaction can go. Because think about, once one reactant runs out, what is the other reactant supposed to react with?

[Neelaj Das 3I]

This is correct. One reactant will almost always run out faster than the others and therefore it will inherently determine how far a reaction can go. Because think about, once one reactant runs out, what is the other reactant supposed to react with?

[Syrah Tamola 3C]

From my understanding it's that once the limiting reagent runs out, there will no longer be a reaction with the excess.

[Jason Ho 2L]

Yes, there's usually one limiting reactant. It is highly unlikely that 2 (or more) reactants are presented in such amounts that they would all be used up at exactly the same time, but it isn't impossible. I understand your confusion about there only being one limiting reactant as explained by the module though!

[Chloe Fuson]

Hey, I know this has absolutely nothing to do with the topic of this question, but I am unable to figure to figure out how to post a new topic on this site (I can only find the option to post a reply, hence my current method), and I was wondering if someone might be able to tell me how? It might be super obvious, but I can be quite technologically inept sometimes so I'd really appreciate the help.

[Megan Bundy 2K]

Hi Chloe! If you are currently on another person's post, you can look above where it says Chem 14A—Review of Chemical and Physical Principles and click on Review of Chemical and Physical Principles. There, you can select the section you want to ask a question in and when you click on that there should be a button at the top that says "New Topic" where you should be able to make your own post.