Excess and Limiting Reactants

This entry focuses more on the reactant side of a chemical equation
*aside: after all there's no reaction without them!!

A balanced equation assumes that there is a perfect amount of reactants in order for the reaction to occur; but it's theoretical. Say what?

In real life however, this is of course, impossible! In order for a reaction to occur there must be too much of one reactant to ensure that enough of the reactants will combine.

Therefore, one reactant will be fully used and be the limiting reactant (because it limits how much of the other reactant is reacted) and the other reactant will have some molecules left over and be the excess reactant.

The Four Steps to Success! OR The Four Steps to finding the excess reactant and its amount...

1. Write a balanced equation! Simple eh? If an equation is given but is unbalanced - balance it. And even if there is an equation that looks suspiciously balanced - check it anyway.

2. Convert each mass of the reactants to the mass of the same product.

3. Take the limiting reactant (the mass of the reactant that results in a smaller mass value of the product) and convert that into the excess reactant (the mass of the other reactant).

4. Subtract your accurately approximated and brilliantly solved for value from step 3 from the theoretical mass  given.

OR

Watch this video, if you prefer to have someone teach it vocally to you!

Voila!

YOU  CAN NOW FIND THE AMOUNT OF EXCESS REACTANT!!

 

I suppose you may want to test this theory for yourself,

Very well... Example 1!:

Okay seriously awkwardly dancing and cheering people cut it out!

A 2.00 g sample of ammonia is mixed with 4.00 g of oxygen.  Find which reactant is in excess and by how much. You may begin:


4 NH_3(g) + 5 O_2(g)4 NO_(g) + 6 H₂O_(g)











If your awesomeness is as above average as your IQ (or not) you may want to try a more advanced method.
This is of course assuming that you are a chemistry wiz (or not) and are more concerned about how much time you spend doing chemistry, then doing a slower and more certain process (or not). Well I can certainly see that you all fit the bill.

LET US PROCEED!


Example 2: 90.0 g of FeCl₃ reacts with 52.0 g of H₂S.  What is the limiting reactant?

2FeCl_{3 +} 3H₂S ---> 6HCl + 1Fe2S3


52.0 g H₂S x 1mole H₂S  x 2 moles FeCl_{3  x 162.3 g} FeCl_{3  =  165 g} FeCl₃
                            34.1 g H₂S     3 moles H₂S      1 mole FeCl₃ 


The limiting reactant is FeCl_3. 


This website, will help you to master your stoichiometry skills.  It has stoichiometry questions that involve every day situation too!

http://www.chemteam.info/Stoichiometry/Limiting-Reagent.htm