Limiting Reactants

- usually 1 reactant is used up before the other
- this reactant is called the limiting reactant (or reagent) ----> it stops or limits the reaction.
- the limiting reactant determines the amount of products produced.
- assume one reactant is the limiting reactant, and determine what quantity of the other is needed.

example

what is the limiting reactant when 25 g of p₄ reacts with 64.6 g of Cl₂ forming phosphorus trichloride?
P₄ + 6 Cl₂ ---> 4PCl₃
P₄ = 123.6 g/mol
Cl_{2 = 71 g/mol}


Since you need more than the given amount of Cl₂ to fully react the P₄, Cl₂ is the limiting reagant.


Determine the theoretical yield of the above reaction.
PCl<sub>3 = 137.4 g/ mol

when we figure out the theoretical yield, we have to calculate the amount of the product that will be produced from the limiting reagent, because that is how much is being used before the reaction is over.</sub>







83.3 g of PCl3 will be produced. (we used the limiting reagent in the equation because that is the amount that will be used in the reaction.


example.
in the formation of water, 25g of oxygen reacts with 10g of hydrogen. determine the limiting reactant, theoretical yield and amount of excess reactants.

2O2 + 2H2 -----> 2 H2O

H2 = 2 g/ mol
O2 = 32 g/mol

For 10g of hydrogen, we need 80 g of oxygen, so
O2 is the limiting reagent.



H2O = 18 g / mol
THEORETICAL YIELD
28.1 g of water will be produced.


EXCESS - we plug in the calculation so we know exactly how much hydrogen we need, then we subtract this amount from the amount that is given to know how much excess there is.

10g - 3.13g = 6.87 g will be left.

***** Notice that the amounts you need (25 g O2 + 3.13g H2 = 28.1 which is the theoretical yield!!!)



here's a worksheet

and heres a video for further help: