Limiting

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
Vinegar (5 by mass acetic acid) and Baking Soda
Demo

• NaHCO3 (s) HC2H3O2 (aq) ? NaC2H3O2 (aq) H2O
  (l) CO2 (g)

Flask 1 Flask 2 Flask 3
Mass of Baking Soda 1.50 g 3.50 g 5.50 g
Mass of Vinegar 50.00 g (2.5 g HC2H3O2) 50.00 g (2.5 g HC2H3O2) 50.00 g (2.5 g HC2H3O2)
4
Outcome from Demo

• Flask 1 showed the smallest amount of CO2
  produced in the balloon with a clear solution in
  the flask.
• Flask 2 showed the same amount of CO2 produced
  in the balloon as in flask 3, which was a larger
  amount compared to flask 1. The solution in the
  flask was still clear like in flask 1.
• Flask 3 showed the same amount of CO2 produced
  in the balloon as in flask 2. However, the
  solution in the flask was cloudy.

5
Which reactant was limiting and which reactant
was excess in each flask based off observations?
Flask 1 Flask 2 Flask 3
Baking Soda limiting theoretical amount excess
Vinegar excess theoretical amount limiting
6
The proof is in the stoichiometry
50.0 g vinegar 5 g HC2H3O2 1 mol HC2H3O2 1 mol NaHCO3 84.01 g NaHCO3 3.50 g NaHCO3
100 g vinegar 60.06 g HC2H3O2 1 mol HC2H3O2 1 mol NaHCO3
The theoretical amount of baking soda needed to
react completely with the 50.0 grams of vinegar
is 3.50 g.
7
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.

8
22.1 g NaI 1 mol NaI 1 mol Cl2 70.90 g Cl2 5.23 g Cl2
149.89 g NaI 2 mol NaI 1 mol Cl2
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.
9

• 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 Cl2 1 mol NaCl
10

• 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 149.89 g NaI 17.5 g NaI
70.90 g Cl2 1 mol 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
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