3.2D Theoretical and Percent Yield- Pre AP Chemistry Study Notes - New Syllabus.

3.2.D.1 — Theoretical Yield and Percent Yield

In a chemical reaction, the balanced chemical equation and the mole concept are used to predict how much product should form. This predicted amount is called the theoretical yield.

In real reactions, the amount of product actually collected may be less. Comparing the predicted amount to the measured amount allows chemists to calculate the percent yield of a reaction.

Theoretical Yield

The theoretical yield is the maximum amount of product that can be formed from given reactants assuming:

• The reaction goes to completion
• The limiting reactant is completely consumed
• No side reactions occur
• No product is lost during collection

Theoretical yield is calculated using stoichiometry and depends only on the limiting reactant.

Actual Yield

The actual yield is the amount of product actually obtained from an experiment.

• Measured experimentally
• Usually less than the theoretical yield

Differences arise due to incomplete reactions, product loss, or competing reactions.

Calculating Theoretical Yield

Steps to calculate theoretical yield:

1. Write and balance the chemical equation
2. Identify the limiting reactant
3. Convert the given amount of limiting reactant to moles
4. Use mole ratios to find moles of product
5. Convert moles of product to mass (if needed)

The result represents the maximum possible product.

Percent Yield

Percent yield compares the efficiency of a reaction by measuring how much product was actually formed relative to the theoretical maximum.

Percent yield reflects how effective a reaction is under real conditions.

Interpreting Percent Yield Values

• 100% → ideal reaction (theoretical case)
• Less than 100% → expected for real reactions
• Greater than 100% → indicates experimental error

Yields above 100% often result from impurities or measurement mistakes.

Particle-Level Explanation

At the particle level:

• The limiting reactant controls the number of product particles formed
• Not all collisions produce product
• Some product particles may be lost during handling

This explains why actual yield is often lower than theoretical yield.

Summary of Yield Concepts

Evaluating Yield Calculations

Correct yield calculations must:

• Use a balanced chemical equation
• Base calculations on the limiting reactant
• Use correct mole ratios and molar masses
• Clearly distinguish actual vs theoretical yield