1. 7.2 – The Limiting Reactant

2. The Limiting reactant
The coefficients of equations are stoichiometric coefficients.
Reactants present in the mole ratio predicted by a balanced chemical equation, are known as stoichiometric amounts.
Limiting reactant: reactant that is completely used up in a chemical reaction.
Excess reactant: reactant that remains after a reaction is over.

3. Identifying the limiting reactant
To find the limiting reactant:
Find out how much product would be produced by each reactant
Reactant that produces the least amount of product is the limiting reactant

4. Sample Problem
Consider: Li3N(s) + 3H2O(l) → NH3(g) + 3LiOH(aq) If 4.87 g of lithium nitride reacts with 5.80 g of water, find the limiting reactant.

5. Sample Problem
Find moles of given reactants: n mol Li3N = 4.87 g Li3N 34.8 g/mol = mol Li3N n mol H2O = 5.80 g H2O 18.0 g/mol = mol H2O

6. Sample Problem
Calculate moles of NH3 produced based on amount of Li3N : 1 mol NH3 : 1 mol Li3N x mol NH3 : mol Li3N moles of NH3 would be produced

7. Sample Problem
Calculate moles of NH3 produced based on amount of H20: 1 mol NH3 : 3 mol H20 x mol NH3 : mol H moles of NH3 would be produced

8. Sample Problem
With the given quantities of water and ammonia, the water would produce less ammonia. Therefore the limiting reactant is water.

9. Sample Problem
Consider: P4(s) + 5O2  P4O10(s) A 1.00 g piece of phosphorus is burned in a flask filled with 2.60 x 1023 molecules of oxygen gas. What mass of tetraphosphorus decaoxide is produced?

10. Sample Problem
Find moles of given reactants: n mol P4 = 1.00 g P g/mol = 8.07 x 10-3 mol P4 n mol O2 = 2.60 x 1023 molecules O x 1023 molecules/mol = mol O2

11. Sample Problem
Calculate moles of P4O10 produced based on amount of P4 : 1 mol P4O10 : 1 mol P4 x mol P4O10 : 8.07 x 10-3 mol P x 10-3 moles of P4O10 would be produced

12. Sample Problem
Calculate moles of P4O10 produced based on amount of O2: 1 mol P4O10 : 5 mol O2 x mol P4O10 : mol O x 10-2 moles of P4O10 would be produced

13. Sample Problem
Since P4 is the limiting reactant, find the mass of P4O10 produced by converting the mole amount produced: Mass of P4O10 = (8.07 x 10-3 moles of P4O10)( g/mol) = 2.29 g Therefore, the mass of P4O10 is 2.29g.

14. Home work
P # 1 – 7 McGraw Hill
P 330 # 1 – 7 Nelson