Lesson 10-11: Limiting Reactant and Percentage Yield

When a chemical reaction occurs,

the reactants combine in specific ratios according to the balanced chemical equation.

The limiting reactant (also called limiting reagent) is:

The reactant that is completely used up first, stopping the reaction from continuing.

Chemical reactions are stoichiometric —

meaning they follow mole ratios set by the balanced equation.

If the ratio of reactants you have in real life does not match the stoichiometric ratio, at least one will run out before the others. The first to run out is the limiting reactant.

Excess Reactant (ER):

The reactant(s) left over after the reaction stops.

Theoretical Yield:

The maximum amount of product that can be made from the limiting reactant.

Actual Yield:

The amount of product you actually get in real life.

Identifying the Limiting Reactant (Conceptually, no math yet)

There are three mental routes to thinking about limiting reactants:

1. Mole Ratio Perspective

   • Compare the available mole ratio of reactants to the mole ratio required by the equation.

   • Whichever reactant is “short” compared to what’s needed is the LR.

2. Reaction “Simulation”

   • Imagine “running” the reaction in your head, consuming reactants in the required proportions, and see which runs out first.

3. Recipe Analogy

   • Think of the balanced equation as a recipe.

   • Ingredients with less than the required proportion are the “shortest supply” — the LR.

The theoretical yield is always?

based on the LR.

Percentage yield

is a measure of how much product is actually made in a reaction. It can vary from 0-100% depending on how much product is made or lost during the reaction.

Low Percentage Yield

A reaction can have a low percentage yield when product is lost through spillage or filtering, side reactions occur and reactions do not go to completion.

theoretical yield (ideal product)

how much of the product should be performed based on the stoichiometry. The maximum amount of product that can be produced from the limiting reactant, assuming perfect reaction conditions (no loss, complete reaction). Found by doing stoichiometry based on the limiting reactant.

The actual yield

is the actual product collected and measured or experimentally determined. Actual Yield. The measured amount of product you actually obtained from the reaction in real life (from experiments). Often less than the theoretical yield because of real-world factors.

The percent yield

is the ratio of actual yield and theoretical yield.