Friday, November 20, 2009

Moles of limiting reactant?

Hi there,


I'm wondering how to determine what compound/element is the limiting reactant in a chemical reaction. I'm not asking for answers, but I'm asking for an explanation. For example, if I have a problem which states that I have 2.5 moles of copper and 5.50 moles of silver. Is copper is the limiting reactant, because there's less of it? Or is there a method to determine this? How can I determine the moles of the excess reactant? Any help will be greatly appreciated. Thanks!

Moles of limiting reactant?
You need to use stoichiometry to find out how many moles of the product you have. You can pick any product but if one product is water it would best not to pick that.


So you would go from moles of copper to moles of a product and then a seperate problem to go from moles of silver to moles of the same product.


Whichever reactant creates the least amount of product is your limiting reactant.
Reply:The limiting reactant is the reactant which is consumed first. So yes, once you have carried out the reaction, you use stoichiometry to determine the moles or grams of the amounts of the products that are left. Whatever one is smaller is the limiting reactant because it limits the amount of product that can be formed.
Reply:Try and set up a ratio between the moles of silver and the moles of copper. It would really help to have a balanced equation, but I'll make one up and show you.





2 AgCl +CuSO4 - - - CuCl2 + Ag2SO4





Set up a ratio. I have 2.5 moles of copper, so . . .





2.5 mol copper X 2 mol silver/1 mol copper (because I have two moles of silver for every 1 mole of copper.)





I would need 5 moles of silver to complete this reaction. I have 5.5, so the copper is my limiting reactant.





Hope that helped!
Reply:Hello!


okay firstly, u can't determine which is the limiting or the excess just by looking at the number of moles of reactant. Because, in a chemical reaction, it is not always that the reactants are in the ratio 1:1.





For eg, in the reaction of say, hydrogen and oxygen to produce water:





2H2 + O2 ---%26gt; 2 H2O





as you can see, in a balanced equation as such, 2 units of hydrogen are required to react with 1 unit of oxygen.





Hence, if 4 moles of H2 and 2 moles of O2 are available, there will be no excess/limiting reactant because the reactants are in the correct ratio.





However, if 5 moles of H2 and 2 moles of O2 are available,


the O2 becomes the LIMITING FACTOR because 2.5 moles of O2 are required to react with 5 moles of H2. and the H2 is now in EXCESS.





The number of moles of the excess reactant will be 5 moles. And the number of moles of the reactant that is in excess, (not participating in the reaction and will be left over in the products) is hence 1 mole of H2.





get it?





:D


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