1. 9.2 Limiting Reactants It's one of earth's greatest mysteries! Why do hot dogs come in packages of ten but hot dog buns come in packages of eight? It makes no sense, does it? (Unless you want those extra two dogs to spice up to your macaroni and cheese.) And what does this have to do with chemistry? Well, it's a perfect example of a limiting reactant problem. In this case, the limiting reactant is hot dog buns. See, we can only make eight hot dogs because we only have that many buns. Well, it's a little harder in chemistry, but don't make it any harder than it has to be.

2. 9.2 Limiting Reactants 4/5/2016

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4. Limiting Reactant vs Excess Reactant Limiting Reactant- the reactant that is used up in the chemical reaction and controls the amount of product that will form Excess Reactant- the reactant that is not used up completely in a reaction, there is some left over

5. How to Solve for the Limiting Reactant 1 st - Set up two stoichiometry problems to solve for the product in question 2 nd – Compare the two mass of the product. Whichever reactant produces the LEAST amount of product is the limiting reactant. The other reactant is the excess reactant, meaning; not all of the reactant was used up.

6. 4 PH 3 + 8 O 2 → P 4 O 10 + 6 H 2 O If 5.30 g of PH 3 and 6.80 g of O 2 are combined, what is the theoretical yield (in grams) of P 4 O 10 ? What is the limiting reactant? What is the excess reactant? 1 st - Set up two mass/mass problems to solve for grams of P 4 O 10 1 mol PH 3 34 g PH 3 1 mol P 4 O 10 4 mol PH 3 283.88 g P 4 O 10 1 mol P 4 O 10 = 11.1 g P 4 O 10 1 mol O 2 32 g O 2 1 mol P 4 O 10 8 mol O 2 283.88 g P 4 O 10 1 mol P 4 O 10 = 7.54 g P 4 O 10 5.30 g PH 3 6.80 g O 2

7. 2 nd – Whichever reactant would produce the LEAST amount of product is the limiting reactant. Limiting reactant is O 2 Theoretical yield is 7.54 g P 4 O 10 Excess reactant is PH 3

8. How much excess reactant is left over? 1 st - Calculate how much of the excess reactant (PH 3 ) actually reacted with the limiting reactant (O 2 ). 6.80 g O 2 1 mol O 2 32 g O 2 8 mol O 2 4 mol PH 3 34 g PH 3 1 mol PH 3 3.61 g PH 3 reacted

9. 2 nd - Subtract the amount that reacted from the amount in the original sample. 5.30 g PH 3 (original amount) ─ 3.61 g PH 3 (reacted) = 1.69 g PH 3 of excess reactant (left over)

10. Practice Al 2 (SO 3 ) 3 + 6 NaOH  3 Na 2 SO 3 + 2 Al(OH) 3 If 10.0 g of Al 2 (SO 3 ) 3 is reacted with 10.0 g of NaOH, determine the theoretical yield (in grams) of Al(OH) 3. What is the limiting reactant? What is the excess reactant? Theoretical yield is 5.30 g of Al(OH) 3 Limiting reactant is Al 2 (SO 3 ) 3 Excess reactant is NaOH