PERCENT YIELD

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Investigation: Percent Yield

Description

Students design and conduct a laboratory process to recover copper from aqueous copper sulfate solution and determine the percent yield of the chemical reaction. The context of this learning experience is after students have a good understanding of chemical reactions and stoichiometry including limiting reactant conditions.

Learning outcome

• Plan and implement a laboratory investigation using safe practices

• Apply concept of stoichiometry and process skills to recover copper from copper sulfate solution using a single replacement reaction with zinc

• Determine the percent yield of the reaction. • Make measurements with accuracy and precision • Use significant digits in measurement and calculation

Lab Management

1. Collect lab equipment and materials. Set equipment on each table for each lab group or instruct students to pick up materials from a central location.

2. Prepare CuSO4 solution, 1M by dissolving 249.61 g of CuSO4 .5H2O in 1000 mL of

solution. Small quantities of zinc powder reagent may be dispensed to students in vials to avoid spillage. Acetone, needed for drying out the precipitate may be

dispensed in small dropper bottles. (Keep the acetone aside till students understand its use)

3. Provide chemicals for each student group.

4. After completing investigation, ask students to clean and put away the equipment in the proper location.

Safety

Students will review the laboratory safety precautions based on their plan. They will also review MSDS of the chemicals used in the investigation and identify hazards and safe handling/disposal of the chemicals.

Attention to chemical laboratory safety is essential as students will be using chemicals, glassware and heat. Instruct students to wear protective aprons and goggles throughout the investigation.

Materials

for each student group

Balance

Gas burner or Hot plate Beaker tongs

Ring stand with ring support

Beakers – 50 mL, 150 mL, 250 mL Spatula

Graduated cylinder 10mL Stirring rod

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Zinc powder reagent grade; 0.6 g Copper (II) sulfate solution 1M; 10 mL Acetone; 10mL

Calculator

MSDS of Zinc powder

MSDS of Copper sulfate solution MSDS of Acetone

Note: Some equipment may vary according to the student plan. (Example: Funnel and filter paper may be needed if students plan to filter the copper from solution.)

Instructional Process

Design

1. Present the challenge gradually – to recover copper from copper sulfate solution using the given materials. Allow think time at every stage before soliciting student responses.

2. Allow students to discuss and come up with a plan.

3. If students are unable to come up with a plan, some prompting questions may help with the design/problem solving process. Some suggested questions are –

• What is the reaction between CuSO4 and Zn? Write the equation.

Single Replacement reaction where Zn replaces Copper CuSO4 (aq) + Zn (s) Cu (s) + ZnSO4 (aq)

• How can we use this reaction to recover copper?

Add Zn powder to CuSO4 solution, filter out the precipitated copper, and dry it At this time conduct a demonstration by adding a small quantity of Zn to CuSO4 solution to

provide a visual prompt to student thinking. They will also see the color change as the reaction proceeds to completion.

• Which reactant should be the limiting reactant? How do you know?

We want all of the Zn to react and precipitate the Cu from solution. We do not want any Zn mixed with the Cu. This means that we should have Zn as limiting and CuSO4 in excess.

• For 1.00 g of zinc, how many moles of CuSO4 will be needed? Ask students to

show calculations using stoichiometric ratios.

1.00 g Zn 1 mol Zn 1 mol CuSO4

65.38 g Zn 1 mol Zn

• If 1000 mL of CuSO4 solution contains 1 mol of CuSO4, what volume of the

solution will be needed to react completely with 1.00 g of Zn? Show calculations.

0.0152 molCuSO4 1000 mL of CuSO4 solution

1mol CuSO4

• How can we ensure that we have an excess of CuSO4 solution?

We can add extra amount

= 0.0152 molCuSO4

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Answers will vary

• Tell students that we will add about 10% extra volume of solution. How many mL will that be?

15.2 mL + 1.52 ml = 16.7 mL of solution will ensure excess reactant without making the process cumbersome.

Instruct students to use a mass of 0.30 to 0.60 g of Zn. (This range will ensure a successful outcome) Remind students that they will have to use the volume of CuSO4

solution corresponding to their mass of zinc powder (that is between 0.3 and 0.6 g) 4. Allow students enough time to develop and record their plan for investigation. 5. Monitor students as they work but allow students to create their own plan.

6. Allow students to share their plan for investigation. Ask students to list and explain the safe use of laboratory equipment needed for the laboratory process. Ensure that students follow correct terminology.

7. Instruct students to review the MSDS for the chemicals used and discuss appropriate safety measures for handling and disposal of the chemicals. Allow time for students to record the chemical safety measures to be followed.

8. Instruct students to review the plan for investigation and identify safety precautions that will be necessary during the investigation. Allow time for students to record the laboratory and personal safety measures to be followed.

9. Model and discuss correct laboratory procedure for this activity. A brief procedure or task list is shown -

Procedure:

• Ensure personal safety by wearing aprons and goggles.

• Find the mass of clean, dry, empty 150-mL beaker and record it. Add about 0.3-0.6 g of zinc powder to the previously massed beaker and record the mass of the zinc accurately.

• Calculate the volume of 1M copper (II) sulfate solution needed for mass of zinc used.

• Measure out the required amount of copper (II) sulfate solution using the graduated cylinder and pour into a 50-mL beaker. Heat the copper (II) sulfate solution using the support stand set-up but do not boil the liquid.

• Using tongs, pour the hot copper (II) sulfate solution slowly into the beaker containing zinc. Rapid addition of the liquid may result in frothing and loss of reactants and products. Stir the reacting mixture with the stirring rod during the addition of copper (II) sulfate solution.

• While the reaction is taking place, prepare a hot

water bath by heating about 80 mL of water contained in 250 mL beaker.

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• Rinse the copper metal by adding about 10 mL of water, stirring, settling, and finally decanting the supernatant liquid. This process may be repeated to ensure thorough cleaning of the copper. (If filtration is used, water will be poured gently over the filter paper containing the copper, taking care not to make holes in the filter paper.)

• The final rinse should be performed in a well-ventilated area using about 5 mL of acetone. (If filtration is used, the acetone will be poured over the filter paper in a well-ventilated area.)

• Place the 150-mL beaker containing the copper into the 250-mL beaker containing hot water below boiling temperature for a few minutes to allow the copper to dry out. This must be done in a well ventilated area. The residual acetone will evaporate quickly and the dry copper may be recovered.

• The mass of the copper recovered is measured and recorded.

10. Ask the question, “How can we ensure accuracy and precision in the experiment?” Tell students that due to limitations of time and materials only one trial will be conducted so accuracy of procedure and measurement is very important.

Investigate

11. Ask students to perform the investigation, record data, then discuss and write down calculations and conclusions. Monitor students and the learning process.

12. Allow student groups enough time to complete their investigations and analyze their results and analysis. Encourage small group discussions and collaborations.

13. In the applications and extensions stage students may need time and resources to research areas of their interest. In this investigation, students may examine

applications of limiting reactant and percent yield in industry or in real life. An example is in mining of where a comparison may be made between recovering iron from the same mass of two kinds of ores; magnetite Fe3O4 or hematite Fe2O3.

Scientific as well as financial comparisons may be made. Percent yield is often used to make economic and financial decisions in industry.

Report

14. Allow class time for student groups to share the results of their investigation, analyses, extensions, and applications. Peer student groups may be coached to assess the presentations.

15. Student work may be displayed physically or digitally for other students’ review. 16. At closure, students should demonstrate understanding of applications of percent

yield of chemical reactions in scientific research, industry and in the real world. This may be facilitated through questioning and discussion. Students should find percent yield and discuss how they can increase the yield of the reaction. Sample scaffolding questions and student responses shown below may be used as a guide. However, class data and responses will vary.

• What mass of Zn was used?

0.44 g Zn

• What volume of CuSO4 solution was used for this mass of Zn? (Show work)

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For 0.44 g of Zn,

0.44 g Zn 16.7 mL CuSO4

1.00 g Zn

• What mass of copper will be theoretically produced from 0.44 g Zn and excess of CuSO4 solution? (Using the chemical equation,

CuSO4 (aq) + Zn (s) Cu (s) + ZnSO4 (aq)

0.44 g Zn 1 mol Zn 1 mol Cu 63.54 g Cu 65.38 g Zn 1 mol Zn 1 mol Cu

• How can we use this information to calculate percent yield?

The actual mass of Cu recovered was 0.39 g. Percent yield is calculated as –

0.39 g Cu 100 0.43 g Cu

• How can we improve percent yield? Or what are some of the sources of error?

The actual mass of Cu is less than the theoretical mass of Cu that should be obtained. We may have lost some Cu while rinsing and decanting. (If actual mass was greater than theoretical mass, the Cu may not be dry. The moisture may increase the measured mass. Or unreacted Zn may have increased the mass; since molar mass of Zn is greater than Cu.)

= 7.35 mLCuSO4 solution

= 0.43 g Cu

Figure

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References