Stoichiometry: Limiting Reactants

When preparing to do a chemical reaction, often the reactants are not present in the exact amounts specified by the balanced chemical equation. When there are fixed amounts of reactants, one of the reactants, most likely, will be consumed first. When preparing sandwiches for a party, there is a recipe for specifying the ratio of slices of bread, lettuce, tomato, and ham. If the slices of bread are used up first, despite being plenty of lettuce, tomato and ham present, no more sandwiches can be made.

Model 1 – Making Ham Sandwiches – Identifying the limiting ingredient

Sandwich recipe or sandwich balance equation

2 slices bread + 3 pieces ham + 1 piecelettuce + 4 tomato slices one sandwich

Ingredients in the kitchen

16 pieces bread, 18 pieces ham, 7 pieces lettuce and 16 tomato slices.

Complete the following table

Bread / Ham / Lettuce / Tomato / Sandwiches
Initial / 0
Used
Left over

Clicker Question

  1. How many sandwiches can you make? Show your work.
  1. If you follow the recipe, using only the ingredients available, how many slices of bread are left over?
  1. Which of the ingredients were present in excess? ______
  2. Which of the ingredients were consumed completely in making sandwiches?

______

  1. Which ingredient limits the number of sandwiches you can make? ______
  2. Define the term limiting ingredient.

Model 2 – Limiting Reactant in Particles – Assembling Hydrogen Molecules

Chemical reaction Mg(s) + 2 HCl(aq)  MgCl2(aq) + H2(g)

Representations

# atoms of Mg x 1 molecule of H2 = # of H2 molecules

1 atom of Mg

  1. Refer to the chemical reaction and the particle diagram in Model 2.

a. Complete the following table

Mg / HCl / MgCl2 / H2
Initial / 6 / 6 / 0 / 0
Used (-) or Formed (+)
Final

b. Complete Model 2 by drawing what particles are found in the circle after the reaction occurs. Be sure to obey the law of conservation of mass (atoms). Draw any un-reacted particles in the “after the reaction” circle.

c. How many H2 molecules can form? Show your work.

d. Which reactant is present in excess and remains? Show your work ______

e. Which reactant limits the amount of hydrogen molecules that can be produced?

Model 3 – Limiting Reactant in Particles – Assembling Hydrogen Molecules

Chemical reaction Mg(s) + 2 HCl(aq)  MgCl2(aq) + H2(g)

Representations

# atoms of Mg x 1 molecule of H2 = # of H2 molecules

1 atom of Mg

  1. Refer to the chemical reaction and the particle diagram in Model 2.

a. Complete the following table

Mg / HCl / MgCl2 / H2
Initial / 6 / 6 / 0 / 0
Used (-) or Formed (+)
Final

b. Complete Model 2 by drawing what particles are found in the circle after the reaction occurs. Be sure to obey the law of conservation of mass (atoms). Draw any un-reacted particles in the “after the reaction” circle.

c. How many H2 molecules can form? Show your work.

d. Which reactant is present in excess and remains? Show your work ______

e. Which reactant limits the amount of hydrogen molecules that can be produced?

Model 4 Lecture Demonstration Reaction of magnesium and hydrochloric acid to produce aqueous magnesium chloride and H2 gas

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