Limiting Reagents

[spring]

I am having trouble with the idea of limiting reagent. In a balanced equation, can I just look at which one is present in the least number of moles to determine which one if the limiting reagent. Also, if questions state that one of the two reactants is present in excess, do I not need to worry about the limiting reagent concept to determined that moles of product produced? I get the general idea that limiting reagents are what there is less of which restricts the degree to which products will be made but I get confused when doing questions on this topic.

[Savac]

Perhaps you're just wording it poorly, but you're a bit off. The limiting reagent is not necessarily the reactant that is present in a smaller molar quantity. It is actually the reactant that is present in the smallest equivalent molar quantity based on a balanced chemical equation.

 

Let's look at the combustion of propane:

 

C3H8 + 5O2 --> 3CO2 + 4H2O

 

Let's say that we have 44g of propane, and 44.8L of oxygen at STP.

 

This is 1mol of propane, and 2mol of oxygen. If you look at the balanced chemical equation, every 1 molecule of propane reacts with 5 molecules of oxygen. Therefore oxygen, the reactant in higher molar quantity, is in fact the limiting reagent.

 

I hope this helps you out. If you have any questions let me know.

[Gametime24]

Perhaps you're just wording it poorly, but you're a bit off. The limiting reagent is not necessarily the reactant that is present in a smaller molar quantity. It is actually the reactant that is present in the smallest equivalent molar quantity based on a balanced chemical equation.

 

Let's look at the combustion of propane:

 

C3H8 + 5O2 --> 3CO2 + 4H2O

 

Let's say that we have 44g of propane, and 44.8L of oxygen at STP.

 

This is 1mol of propane, and 2mol of oxygen. If you look at the balanced chemical equation, every 1 molecule of propane reacts with 5 molecules of oxygen. Therefore oxygen, the reactant in higher molar quantity, is in fact the limiting reagent.

 

I hope this helps you out. If you have any questions let me know.

 

Well said. Post back if you have more questions about this concept, or any other chem concepts!

[spring]

Thanks! That helps clarify my confusion. I tend to look at it in this way: based on the balanced equation, for every mole of propane, we need 5 moles of oxygen, so we will run out of oxygen first and so that would be the limiting reagent. Also, what if it states that the reaction takes place with excess oxygen? In this case, do I not need to worry about the fact that it is a limiting reagent? Can I determine the moles of product output primarily based on molar ratio with propane?

 

I also have a question from the TPR review book about solubility constants (Chapter 9, FSQ 6):

 

Which is least soluble? and the ksp values are given

 

I get that the lower the ksp value, the least likely it is to dissolve and the more likely it is to form precipitate. In this question all compounds are in the form AB2 (eg. PbI2, PbCl2 etc). I am wondering if all compounds are not in the same format, then how can we solve such problems?

[Savac]

Thanks! That helps clarify my confusion. I tend to look at it in this way: based on the balanced equation, for every mole of propane, we need 5 moles of oxygen, so we will run out of oxygen first and so that would be the limiting reagent. Also, what if it states that the reaction takes place with excess oxygen? In this case, do I not need to worry about the fact that it is a limiting reagent? Can I determine the moles of product output primarily based on molar ratio with propane?

 

Yes, you are correct.

 

I also have a question from the TPR review book about solubility constants (Chapter 9, FSQ 6):

 

Which is least soluble? and the ksp values are given

 

I get that the lower the ksp value, the least likely it is to dissolve and the more likely it is to form precipitate. In this question all compounds are in the form AB2 (eg. PbI2, PbCl2 etc). I am wondering if all compounds are not in the same format, then how can we solve such problems?

 

I don't think they'll be this mean to you on the MCAT, but you would have to calculate it.

 

For example, if it is AB2:

 

AB2 --> A+ + 2B-

 

Ksp=[A+][b-]^2

=x(2x)^2

=4x^3

 

However, if it is AB3 then Ksp=27x^4 (Calculate this by yourself to make sure).

 

Basically at this point you must solve for x and compare them. The lower value for x is the less soluble compound.

[Gametime24]

Thanks! That helps clarify my confusion. I tend to look at it in this way: based on the balanced equation, for every mole of propane, we need 5 moles of oxygen, so we will run out of oxygen first and so that would be the limiting reagent. Also, what if it states that the reaction takes place with excess oxygen? In this case, do I not need to worry about the fact that it is a limiting reagent? Can I determine the moles of product output primarily based on molar ratio with propane?

 

Remember that only works in some circumstances. If you are told you have 2 moles of propane and 13 moles of oxygen to react, then propane is the limiting reagent. All you do is look at the balanced equation and see that you need 5 moles of oxygen for every mole of propane, like you said. And then look at how much of each you have, we have 2 moles of propane, which means we would NEED 10 moles of oxygen. If we have more than that (which we do) then propane is the limiting reagent because we run out of it first. If we had less than 10 moles of oxygen (with 2 moles of propane) then oxygen would be the limiting reagent.

 

Correct about whenever there is a excess of something you can do the question based on the other compounds present.