Chem CLass- February 1, 2010

Well, the first thing that we did was get our filter paper with the precipitate from the dry oven to finish the lab. Then we were introduced to limited reactants. Mr. D lighted a paraffin wax candle and then put a cup on top. The lack of oxygen made the flame slowly die out. In this case, the limited reactant or the limited reagent was Oxygen (O2).

Limited 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 and check how much of the other is required

Examples:
What is the limiting reactant when 12.5 g of P4 reacts with 323 g of Cl2 to form Phosphorous trichloride?
First, come up with the balanced equation: P4 + 6Cl2 -----> 4 PCl3
12.5 g x 1 mol/124 g x 6 mol Cl2/ 1 mol P4 x 71 g/ 1 mol = 431 g of Cl2
Cl2 is the Limiting Reactant

Determine the theoretical yield of the previous reaction
323 g = theoretical yield
431 g = Actual yield
323 g x 1 mol Cl2/71 g x 4 PCl3/6Cl2 x 137.4/1 PCl3 = 417 g

In the formation of Water, 50 g of Oxygen gas reacts with 20 g of Hydrogen gas. Determine the LR,the theoretical yield and how much excess reactant will be left.
2H2 + 2O2 -----> H2O
50 g x 1 mol.32 g x 2 mol H2/1 mol O2 x 2 g/ 1 mol = 6.25 g H2
LR is O2
Theoretical: 50 g x 1 mol/32 x 2 mol H2O/1 mol O2 x 18.0/ 1 mol H2O = 56 g (theoretical yield)
20 g - 6.25 g H2 = 14 g (excess)

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Here's a site with a few notes and an example on the limiting reactant:

http://www.chem.tamu.edu/class/majors/tutorialnotefiles/limiting.htm

Here's a video on identifying the limiting reactant: