LIMITING REACTANTS (February 1st)

LIMITING REACTANTS
- usually one reactant gets used up first
- one reactant is completely consumed
- this determines how much product is produced
- "guess" which reactant is limiting & check how much of the other is required


EXAMPLES:

1) what is the limiting reactant when 125g of P4 reacts with 323g of Cl2 to produce Phosphorus trichloride?

first step is to write a balanced eq'n: P4 + 6 Cl --> 4 PCl3

125g of P4 x 1 mol P4/124g P4 x 6 mol Cl2/1 mol P4 x 71g of Cl2/1 mol Cl2 = 431g of Cl2

*This makes the limiting reactant is Cl2*

Now, determine the theoretical yield of the previous question..

323 g mol Cl2 x 1 mol Cl2/71 g Cl2 x 4 mol PCl3/6 mol Cl2 x 137.5 PCl3/1 mol PCl3 = 417g PCl3

2) in the formation of water, 50g of oxygen gas reafts with 20g of hydrogen gas. determine the limiting reactant, the theoretical yield, and excess reactant left over.

To determine the limiting reactant (LR):

50g O2 x 1 mol O2/32g O2 x 2 mol H2/1 mol O2 x 2g of H2/1 mol H2 = 6.25 g of H2

For the theoretical yield:

50g of O2 x 1 mol O2/32g O2 x 2 mol H2O/1 mol O2 x 18g H2O/1 mol H2O = 56g H2O

Excess reactant left over:

20g - 6.25 g H2 = 13.7 g = 14 g H2 is E.R.