Limiting and Excess Reagents
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In an equation, we know that the substances that are used to start a reaction are known as reactants. They might also be referred to as reagents.

Those substances that are formed in a reaction are known as products.

For example:

P4(s) + 6 Cl2(g) → 4PCl3(l)

The P4 and Cl2 are known as the reactants while the PCl3 is the product.

The numbers in front of the products and reactants are the number of moles for that particular substances when the equation is perfectly in balance.

These numbers act as coefficients and mole-to-mole ratios. For example, the ratio between Cl2 and PCl3 is 6 to 4.

Now, that we understand the basics for equations...

Stoichiometry is defined as amount and mass relationships between reactants and products in a chemical reaction.

Stoich allows us to convert from one substance to another or one unit to another. Not all reactions will use all of the given amount of product and reactant. Sometimes, one reactant will be all used up while there is still some left of the other. However, the reaction cannot continue if there is only one reactant because a substance will not react by itself!

Excess Reagent - A reactant that will not be completely used up in a reaction that goes to completion.

Limiting Reagent - A reactant that is consumed completely in a reaction that goes to completion.

Determining Steps:

  1. List what you know. List the amounts and substances that are directly stated in the problem.
  2. Set up the problem.
    • Balance the chemical equation
    • Calculate the molar masses
  3. Calculate and Verify.
    • Change the givens to moles (You will have at least two set-ups for a LR problem)
    • Use a mole ratio to determine the product
    • Convert to the units asked for in the problem
    • The smallest answer is the correct answer!  The given reactant that gave the smallest answer is called the LIMITING REACTANT.  The other given reactant(s) is the EXCESS REACTANT.

 

Sample: Qualitative
Hamburgers

Given - 3 meat patties, 4 hamburger buns (including the top and bottom), 6 cheese slices, and 7 tomato slices.

Make as many cheeseburgers as possible. Each sandwich MUST have: 1 bun, 1 meat patty, 1 cheese slice, and 2 slices of tomato.

You will have ingredients left over.

Question:  How many sandwiches can be made?

               What ingredient(s) are left over (in excess)? How much of each is left?

               What ingredient(s) are used completely (limiting)?