Unit 2, Topic 5c:

Percent Yield

Lesson Instructions

Review the learning goals and success criteria for Topic 5c: Percent Yield.
- Review the minds on, action, and consolidation sections of the lesson, completing any tasks as outlined.
- Complete the fill-in-the-blanks handouts that have been provided to you. They have been linked below.
- Practice and apply what you've learned by completing the practice tasks outlined at the end of the lesson.

Learning Goals

I am learning to ...

- Calculate the percent yield of a chemical reaction.

Success Criteria

I am able to ...

- Determine the percent yield of a chemical reaction.

Minds On

It is important that chemical manufacturers keep a careful watch on the limiting and excess reactants in their processes. Controlling which chemical is the limiting or excess reactant can affect the profitability of the process.

For example, the pain reliever Aspirin can be made by reacting salicylic acid with acetic anhydride. The chemical equation for this reaction is:

C₇H₆O₃ + C₄H₆O₃ → C₉H₈O₄ + C₂H₄O₂

(salicylic acid) + (acetic anhydride) → (Aspirin) + (acetic acid)

Watch this video to see the synthesis of Aspirin in a lab!

Salicylic acid is a much more expensive reagent than acetic anhydride. To make sure that salicylic acid is completely used up and not wasted, Aspirin manufacturers react salicylic acid with an excess of acetic anhydride. This method helps the manufacturer produce the most product while minimizing the cost of the reactants.

The quantity of product expected in a reaction is called the theoretical yield and, in practice, is rarely achieved. Rather, the amount of product collected is often less than what we expected. There are a number of factors that contribute to this difference in yield. The amount of product that is actually collected during a chemical reaction is called the actual yield. Our goal is to get the actual yield as close to the theoretical yield as possible.

Chemists use the term percent yield to indicate how close the actual yield is to the theoretical yield. It is a good indicator of the efficiency of the conversion from reactants to products. A reaction with a high percent yield successfully converts almost all of its reactants into products.

Maintaining a high percent yield is important for any company manufacturing chemicals or drugs. In the example above, the chemical process that makes Aspirin is most profitable when the percent yield is high. This means that the maximum possible amount of Asprin is produced from the reactants.

Figure 1. The chemical structure of Aspirin, a common drug used for the relief of pain. The IUPAC name for Aspirin is 2-acetyloxybenzoic acid

Action

Theoretical Yield

- The maximum mass of product predicted from stoichiometry.

Actual Yield

- The mass of product that is measured from the chemical reaction in an experiment.
- This quantity is usually different from theoretical yield.
- Sources of error – purity of chemicals, products escape during heating, filtering precipitates, transferring solutions.

Percentage Yield

- Ratio of the actual mass of the product and the mess predicted from stoichiometry.

How to Calculate Percentage Yield

Example 1.

When 245 g of arsenic ore (FeSAs) was processed industrially, 95.3 g of pure arsenic was obtained. Calculate the percentage yield of arsenic in the process.

Example 2.

In an experiment, when 16.1 g of FeS reacted with 10.8 g of oxygen gas, 14.1 g of iron (III) oxide was produced. Calculate the percentage yield of iron (III) oxide in the experiment.

Consolidation

(1) Practice Questions

Using the textbook, complete the following textbook questions:

- Page 233 #5, 6

This lesson is now complete. Return to D2L - Brightspace and complete the assigned tasks to consolidate your learning.

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