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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: zibb3r on March 14, 2011, 10:44:29 am

Title: ~\\\Excess and limiting reagents///~
Post by: zibb3r on March 14, 2011, 10:44:29 am

I have this question:

If you put equal weights of sodium metal and chlorine gas into a reaction vessel, which is the limiting reagent?
 
I said it was the Na but the answer says its the Cl. :-\. I thought that the Na would be because the balanced reaction equation indicates that two Na atoms would react with two Cl atoms or one Cl2 molecule. Thus, if you have 6 Na atoms, 3 Cl2 molecules will be required. If there is an excess number of Cl2 molecules, they will remain unreacted. We can also state that 6 moles of sodium will require 3 moles of Cl2 gas. If there are more than 3 moles of Cl2 gas, some will remain as an excess reagent, and the sodium is a limiting reagent. It limits the amount of the product that can be formed.

THanks :D

Title: Re: ~\\\Excess and limiting reagents///~
Post by: Water on March 14, 2011, 10:52:49 am

Hey . Zibbr, take this hypothetically.


2Na                    +                      Cl2 -> 2NaCl

m(Na) = 100grams                      m(Cl2) = 100 grams
M(Na) = 23grams/mol                  M(Cl2) = 70.90grams/mol


n(Na) = 100g/23gram/mol           n(Cl2) = 100g/70.90g
        = 4.35 mol                                = 1.41mol

Mole Ratio 2:1

You only need 4.35mol Na -> 2.175 mol Cl  provided we actually have 2.175mol requisite.


But clearly, Cl2 is way below the required mol ratio if Na was too be all used up


Therefore, Cl2 would be the limiting, cause when they react. There would be excess Na


Mol Ratio for Na : Chlorine (As limiting reagent)

           2.82 mol : 1.41 mol



Na would have excess of 4.35mol - 2.82mol
                                 =1.53mol

Title: Re: ~\\\Excess and limiting reagents///~
Post by: zibb3r on March 14, 2011, 11:37:05 am

so why does this site say different???

http://www.science.uwaterloo.ca/~cchieh/cact/c120/limitn.html :-\

and do we use mole ratios or stoichiometric ratios

Title: Re: ~\\\Excess and limiting reagents///~
Post by: Water on March 14, 2011, 11:50:08 am

It doesn't say different, it said the same thing as I said.

It represents a reaction of a metal and a diatomic gas chlorine. This balanced reaction equation indicates that two Na atoms would react with two Cl atoms or one Cl2 molecule. Thus, if you have 6 Na atoms, 3 Cl2 molecules will be required. If there is an excess number of Cl2 molecules, they will remain unreacted. We can also state that 6 moles of sodium will require 3 moles of Cl2 gas. If there are more than 3 moles of Cl2 gas, some will remain as an excess reagent, and the sodium is a limiting reagent. It limits the amount of the product that can be formed.




So it says that

2Na + Cl2 ---> 2NaCl


It said that, for a complete reaction, you would have 6 moles of Na to every 3 Cl

Thats established.


However, then it goes to say. If you had More than 3 Moles of chlorine, Then Na would be limiting. That would stand correct


For instance, you had 4 Chlorine to 6 Sodium.



That you go beyond the ratio of 2:1



Because you only need 3 Chlorine to react with 6 Sodium.

But because you have an additional Chlorine, it would be in excess. Therefore, Sodium would be the limiting. Because, it is stopping all of Chlorine to react, because it doesn't have enough to react with Chlorine

Title: Re: ~\\\Excess and limiting reagents///~
Post by: zibb3r on March 14, 2011, 11:52:48 am

ok i get it now ;D

thanks Water