Chemistry Help Please : Othor Games

» Fri Feb 18, 2011 8:45 pm

I'm having a hard time understanding limiting reactant and excess reactant. I know what they means but like i don't get how to find it and stuff.
So an example problem:

4NH3 + 5O2 = 4NO + 6H2O
If a container were to have 10 molecules of O2 and 10 molecules of NH3 initially, how many total molecules (reactants plus products) would be present in the container after this reaction goes to completion?

» Sat Feb 19, 2011 9:21 am

I'm having a hard time understanding limiting reactant and excess reactant. I know what they means but like i don't get how to find it and stuff.
So an example problem:

4NH3 + 5O2 = 4NO + 6H2O
If a container were to have 10 molecules of O2 and 10 molecules of NH3 initially, how many total molecules (reactants plus products) would be present in the container after this reaction goes to completion?

Well, you know that O2 is the limiting reactant in this, because it requires more of it, but you have the same amount of it as NH3. When you've used 10 molecules of O2, you will have only used 8 molecules of NH3. For every 5 molecules of O2, you're going to get 10 total molecules of products (4+6), and since you used 10, there are 20 molecules of products, plus the 2 leftover molecules of NH3.

» Fri Feb 18, 2011 11:00 pm

Well, you know that O2 is the limiting reactant in this, because it requires more of it, but you have the same amount of it as NH3. When you've used 10 molecules of O2, you will have only used 8 molecules of NH3. For every 5 molecules of O2, you're going to get 10 total molecules of products (4+6), and since you used 10, there are 20 molecules of products, plus the 2 leftover molecules of NH3.

I still don't understand, why is O2 the limiting reactant?

» Sat Feb 19, 2011 6:50 am

I still don't understand, why is O2 the limiting reactant?

Because it is used up first. Hence, it limits the reaction.

» Fri Feb 18, 2011 11:01 pm

I still don't understand, why is O2 the limiting reactant?

Because for every 5 O2 molecules that are used, only 4 NH3 molecules are used, so the O2 will be used up first, since you start with equal amounts of both.

» Sat Feb 19, 2011 12:02 am

Easiest way to do it is to compare the ideal molar ratio with the actual one. The ideal molar ratio for this reaction would be 5/4 or 1.25 (coefficient of oxygen divided by the coefficient of ammonia). The actual ratio you get there is 1 (remember n = N/N_a, thus n(O₂)/n(NH₃) = N(O₂)/N(NH₃) = 1). Since the actual ratio is less than the ideal one means you either need more oxygen or less ammonia i.e. oxygen's the limiting reactant. All oxygen molecules will thusly be used, leaving 2 ammonia molecules along with 8 nitrous monoxide and 12 water molecules.

» Sat Feb 19, 2011 9:20 am

I'm sorry for asking too much, but can you guys show step by step in a way that's simple.

» Sat Feb 19, 2011 1:26 am

You have 10 O2 and 10 NH3. To gain one set of product, you use 5 O2 and 4 NH3. You are now left with 5O2 and 6 NH3. When you run it again, you are left with no more O2 and 2 NH3. Because O2 is used up before NH3, it is the limiting reagent.

It's basic arithmetic.

Now figure out how much products there are. Remember, you had to run this twice.

Or hell, think of the problem as this

You have 10 A and 10 B. When you use 5A and 4B, you make 4X and 6Y. Will you use up A or B faster? How much of A or B will you have left? How much X and Y will you have after you use as much A and B as you can?

I assume this is high school chemistry, right?

» Sat Feb 19, 2011 9:06 am

You have 10 O2 and 10 NH3. To gain one set of product, you use 5 O2 and 4 NH3. You are now left with 5O2 and 6 NH3. When you run it again, you are left with no more O2 and 2 NH3. Because O2 is used up before NH3, it is the limiting reagent.

It's basic arithmetic.

Now figure out how much products there are. Remember, you had to run this twice.

Thank you, i get it now but im not sure about the products
Also, can you use this method for every problem because in my book, it shows that you need to do all these equations and stuff.

I assume this is high school chemistry, right?

Yes it is

» Sat Feb 19, 2011 6:52 am

reread what I wrote. It takes 5A and 4B to make 4X and 6Y
You have initially 10A, 10B, 0X and 0Y. All you need to do is find out how much A, B, X, and Y you have when you use up as much A and B.

A = O2
B = NH3
X = NO
Y = H2O

» Sat Feb 19, 2011 10:50 am

reread what I wrote. It takes 5A and 4B to make 4X and 6Y
You have initially 10A, 10B, 0X and 0Y. All you need to do is find out how much A, B, X, and Y you have when you use up as much A and B.

A = O2
B = NH3
X = NO
Y = H2O

Correct me if im wrong
So i used 10 molecules of O2 which is 2 times. So i multiply 4 from NO by 2 and 6 from H2O by 2, then 8 and 12?
So then 2A+8X+12Y = 22 molecules total?

» Fri Feb 18, 2011 7:30 pm

yes.

» Sat Feb 19, 2011 10:20 am

Thank you

» Fri Feb 18, 2011 8:30 pm

Another question. How come your not supposed to use gram quantities to determine the limiting reactant, but mole instead?

» Sat Feb 19, 2011 3:28 am

Another question. How come your not supposed to use gram quantities to determine the limiting reactant, but mole instead?

Because you first need to convert the grams to moles. The ratios in the equation are by moles, not mass, so you can't use the ratios with mass. 5 grams of O2 isn't the same number of moles as 5 grams of NH3, so you first have to convert it.

» Sat Feb 19, 2011 5:42 am

Another question. How come your not supposed to use gram quantities to determine the limiting reactant, but mole instead?

Because grams don't explicitly say how many molecules of the thing there are whereas moles do and what determines what's the limiting reactant is the number of molecules.

» Sat Feb 19, 2011 2:43 am

gram is just weight, and each element has a different weight. Using moles uses a system of accounting of how many molecules there are, molarity for the concentration.

» Sat Feb 19, 2011 2:17 am

gram is just weight, and each element has a different weight. Using moles uses a system of accounting of how many molecules there are, molarity for the concentration.

Pretty much this ^

Grams is weight and there can be any number of molecules within that item. However using the mole denotation allows for a more precise measurement with a reaction. Still don't understand moles myself and took 1 course of chemistry at my Community College. As Hellmouth said though using moles allows for more precise calculation of a chemical reaction.

You'll love it OP when you start getting into Molarity rate reaction with temperature between 2 and possibly 3 different chemical compounds.

» Sat Feb 19, 2011 3:55 am

Thank you guys for your help!
and also, what if you're given a balanced equation and are asked to find out the limited reactant, but they don't give you the grams of any of the reactant to convert it to mole to find which is limited or excess. So for instance, lets say:
4Fe + 3O2 = 2FE2O3
what should i do?

» Fri Feb 18, 2011 10:44 pm

In this kind of problem, assume there is 10 moles of reagent for the both of them. Meaning, you assume you have 10 moles of Fe and 10 moles of O2. NOW SOLVE IT!

» Sat Feb 19, 2011 2:54 am

In this kind of problem, assume there is 10 moles of reagent for the both of them. Meaning, you assume you have 10 moles of Fe and 10 moles of O2. NOW SOLVE IT!

So Fe is limited and O is excess cause i start off with 10 moles in both reactant and keep subtracting them to the corresponding coefficient till one runs out?

» Sat Feb 19, 2011 7:33 am

This is correct, Fe is the limiting reagent. Also, 10 moles of O2 mean there will be 20 moles of O. With each reaction, you need 4 things of Fe and 6 things of O. By the second run, you'll run out of Fe (or at least you won't be able to run a third), while there is more than enough O2 to continue running.

» Fri Feb 18, 2011 7:30 pm

I have another question. Let's say you have Pb(OH)2 and you're trying to find the molar mass. Do you distribute the 2 and get PbO2H2 or just PbOH2?

» Sat Feb 19, 2011 7:54 am

PbO2H2.

When you have something in brackets followed by a subscript number it means multiply everything within the brackets by the number.

» Sat Feb 19, 2011 10:54 am

Actually, gram is a measurement of mass. Force equals mass times acceleration; Newtons = Grams * Meters per Second. Semantics.

I remember chemical equations... A bunch of confusing short-hand because scientists don't have time to waste writing everything out. Plus, the Post-It collage is more impressive as a crucible of esoteric epiphanies than if it were just parts of a whole canvas for a single thought.