WHAT YOU WILL NEED :
1) Knowledge on stoichiometry
(featured on this website)
2) Periodic table
3) Understanding of significant figures
(featured on this website)
4) To know how to balance equations
(featured on this website)
WHAT IS EXCESS AND LIMITING REAGENTS?
The limiting reagent is the substance that is totally
consumed or used up when the chemical reaction is
complete. It is called limiting because there is not enough
of it, it is limiting. There is never extra left.
On the other hand, the excess reagent is the substance
that contains a greater amount than necessary to react
completely with the limiting reagent. When the chemical
equation is complete, there is an excess of that substance
since it had more than enough of what was required to
complete the chemical equation.
LIMITING REAGENTS EXAMPLE
I have 500 nails and 100 pieces of wood, this makes one house. There are
2000 nails and 200 pieces of wood, I want to make 3 houses, what is
the limiting and excess reagent?
The limiting reagent would be the wood because there is missing 100
pieces and the excess reagent would be the nails because I have an
extra of 500 nails.
THE METHOD.
To solve a question of which reactant is the limiting
reagent (given the initial amount for each reactant) is
to calculate the amount of product that could be
made from each amount of reactant, assuming all
other reactants are available in unlimited amounts. In
this case, the limiting reagent will be the one that
produces the least amount of product.
EXAMPLE
4 NH3(g) + 5 O2(g)4 NO(g) + 6 H2O(g)
We have 2.00 grams of NH3 and 4.00 grams O. What is the
limiting reagent and how much excess is remaining?
NEXT:
Use stoichimoetry to determine how much product is formed by each
reactant.
ANS: In this case, the oxygen produces the lesser amount of product,
therefore it is the limiting reagent!!
Next, to find the amount of excess reactant, we need to calculate how much of the
non-limiting reactant (ammonia) actually did react with the limiting reactant
(oxygen), because there is going to be extra left since it is the excess.
We're not finished yet!!! 1.70 g is the amount of ammonia that reacted,
not what is left over. To find the amount of excess reactant left over,
subtract the amount that reacted from the amount in the original sample.
2.00g NH3 (original) – 1.70g NH3 (reacted) = 0.30g NH3 remaining
ANS: 0.30 g NH3
remaining
BIBLIOGRAPHY
• Capraro, Ernest. "Helium." Finding the Limiting
Reactant (n.d.): 1-2. Web. 11 Oct. 2012.
<http://www.helium.com/items/663291-finding-thelimiting-reactant>.
• Teachers class notes, Chemistry class. (Ms. Young)