1. Limiting & Excess Reactants
How do you know which one is which?

2. What does limiting & excess mean?
Limiting Reactant - the reactant that runs out first in a chemical reaction, thus determining the amount of product produced
Excess Reactant - the reactant that there is a quantity of left over after a chemical reaction
*The excess reactant should be the cheaper reactant since we do not like to waste unused chemical
*The excess reactant should be easy to isolate from the product(s) once the reaction is complete since we do not want contamination

3. Identifying Limiting and Excess Reagents
WHY DO WE CARE??
It is often desirable to know how much excess reagent is required to ensure that a reaction goes to completion.
Plus, when you run out of your limiting reactant, you wont make any more product!
When you know the quantity of more than one reagent, you may need to know which one will limit the reaction.

4. Steps:
1. Convert all quantities to moles 2. Compare via mole ratio 3. Look back at original quantities

5. Practice Problem 2NaI + Cl2  2NaCl + I2
1. You are given 22.1 g of NaI and 4.13 g of Cl2. What is the limiting reactant?
Pick one of the values given for your reactants and through stoichiometry find out how much you need of the other reactant.

6. You need 5.23 g to react completely with 22.1 g NaI
1 mol NaI
1 mol Cl2
70.90 g Cl2
5.23 g Cl2
g NaI
2 mol NaI
You have 4.13 g Cl2 available
You need 5.23 g to react completely with 22.1 g NaI
Therefore, Cl2 is the limiting reactant which will run out first
in the reaction and determine the amount of product produced.

7. How much NaCl is produced?
*Remember that you must always start with the limiting reactant quantity that you have because this is what determines the amount of product produced.
4.13 g Cl2
1 mol Cl2
2 mol NaCl
58.44 g NaCl
6.81 g NaCl
70.90 g Cl2
1 mol NaCl

8. How much excess reactant will be left over?
*Remember that you must always start with the limiting reactant quantity that you have because this is what determines the amount of the excess reactant you will need.
4.13 g Cl2
1 mol Cl2
2 mol NaI
g NaI
17.5 g NaI
70.90 g Cl2
1 mol NaI
You have 22.1 g available
You need 17.5 g
22.1 g NaI – 17.5 g NaI = 4.6 g NaI will be left over
after the reaction is complete

9. C2H4(g) + 3O2(g)  2CO2(g) + 2H2O(g)
If 2.7 mol of ethane is reacted with 3.6 mol of oxygen: a) Identify the limiting and excess reactants b) find the amount of both products produced c) find how much of my excess reactant is left over at the end of the reaction

10. Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
When 6 g of magnesium react with 5 g of hydrochloric acid: a) Identify the limiting and excess reactants b) find the amount of both products produced c) find how much of my excess reactant is left over at the end of the reaction

11. % Yield Chemists like to know how right we are 
% yield = (actual/theoretical) x 100
Actual  lab results
Theoretical  mathematical prediction through stoichiometry
% error = |theoretical – experimental|/theoretical x 100
% error + % yield = 100

12. Independent Practice