Name Period Date

20. Molar Mass of Copper

Driving QuestionsHow can electricity cause metal atoms to evenly coat and permanently adhere to the surface of objects being electroplated?

BackgroundIn some chemical reactions, electrons are transferred between reactants. Reactions involving the transfer of electrons between atoms are referred to as reduction-oxidation, or redox, reactions. Because of the law of conservation of matter, one reactant must lose electrons (oxidation) in order for the other reactant to gain them (reduction). The separate processes are referred to as half-reactions because they each represent half of the overall reaction.

The reactants of each half-reaction can be physically separated into half-cells and connected by a wire through which electrons are forced to travel. Electricity is a flow of electrons. If the reactants spontaneously react, the flow of electrons through the wire can be measured as electricity. For reactions that are not spontaneous, a supply of electricity from a battery or other power source is required. When electricity is supplied, ions in solution gain electrons and take their elemental form. For many ions, this means they become solid metal, as shown in a reduction half-reaction for copper at the cathode.

Cu2+(aq) + 2e– → Cu(s)

These metal atoms adhere to the source of the electrons, such as a spoon connected to the cathode. This is the electrode where reduction occurs. As the ions plate the cathode, they must be replaced. The anode is the electrode where oxidation occurs. At the anode, metal atoms lose their electrons and become ions that dissolve into the solution. These ions replace those that coated the cathode, as shown in an oxidation half-reaction for copper at the anode.

Cu(s) → Cu2+(aq) + 2e–

The current of electricity refers to how fast the electrons are flowing. The current in amperes A and how long in seconds s the power is supplied to the reaction allows you to determine the charge supplied to the reaction. Using the Faraday constant (96485 mol e–/(As)), allows for the determination of the number of moles of electrons supplied. By knowing the number of electrons transferred, the amount of copper plated can be determined using stoichiometry.

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Molar Mass of Copper

Materials and Equipment

For each student or group:

Data collection system DC power supply

Voltage-current sensor Red patch cord (2), 4-mm banana plug

Balance, centigram Black patch cord, 4-mm banana plug

Beaker, 250-mL Alligator clip (2)

Utility clamp (2), insulated Copper electrode

Ring stand Stainless steel spoon (or other item to electroplate)

Magnetic stirrer 0.5 M Copper(II) sulfate (CuSO4), 150 mL

Stir bar

SafetyAdd these important safety precautions to your normal laboratory procedures:

Use caution when operating the DC power supply. Injury to people or damage to equipment can occur due to electric shock. Refer to the instruction manual of your particular power supply for specific safety precautions.

Never connect electrodes while the power supply is connected to an electrical outlet. Be sure the power supply is unplugged while attaching patch cords and alligator clips to the various components.

Be sure that the power supply, data collection system, sensors, and general area around the experiment remain dry. Do not handle equipment with wet hands.

Sequencing ChallengeThe steps below are part of the Procedure for this lab activity. They are not in the right order. Determine the proper order and write numbers in the circles that put the steps in the correct sequence.

ProcedureAfter you complete a step (or answer a question), place a check mark in the box () next to that step.

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Put the spoon and a piece of copper metal into a beaker of copper(II) sulfate solution.

Measure the mass of a clean, dry stainless steel spoon (or other metal to be plated).

Turn on the power and graph Current versus Time. Determine the area under the curve.

Determine the mass of the copper plated on the spoon and then calculate the molar mass of copper.

Connect the spoon and the copper electrode (that are in the beaker) to the power supply and the voltage-current sensor.

Student Inquiry Worksheet

Note: When you see the symbol "�" with a superscripted number following a step, refer to the numbered Tech Tips listed in the Tech Tips appendix that corresponds to your PASCO data collection system. There you will find detailed technical instructions for performing that step. Your teacher will provide you with a copy of the instructions for these operations.

Set Up

1. Add approximately 150 mL of 0.5 M copper(II) sulfate solution (CuSO4) to a 250-mL beaker containing a magnetic stir bar.

2. Place the beaker on a magnetic stirrer.

3. After cleaning and drying a stainless steel spoon (or other metal to electroplate), measure and record its mass.

Mass of spoon (g): ___________________

4. Why does the spoon need to be clean and dry?

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5. Use the ring stand and utility clamps to position the spoon and the copper electrode in the CuSO4 solution so they are not touching the bottom of the beaker or each other.

6. Turn on the stirrer and set it to a low or medium speed. Make sure the stir bar does not hit the copper electrode or the spoon.

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Molar Mass of Copper7. Why is it necessary to stir the electrolyte solution?

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8. Start a new experiment on the data collection system. �(1.2)

9. Connect the voltage-current sensor to the data collection system. �(2.1)

10. With the power supply unplugged, assemble the electroplating circuit by following the steps below:

Note: It is customary to use a red patch cord for the positive terminal and a black patch cord for the negative terminal of the power supply. The color of the cord, however, does not affect the lab results.The voltage sensor leads are not used during this experiment.

a. Plug a red patch cord into the negative terminal on the voltage-current sensor. Attach the other end of this red patch cord to the copper electrode using an alligator clip adapter.

b. Attach a black patch cord to the negative terminal of the DC power supply. Attach the other end of this black patch cord to the spoon using the second alligator clip.

c. Connect the second red patch cord to the positive terminal on the DC power supply. Attach the other end of this red patch cord to the positive terminal of the voltage-current sensor.

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Student Inquiry Worksheet

11. Double check that your circuit is set up correctly. A black cord connects the negative terminal of the power supply to the spoon. A red cord connects the copper electrode to the negative terminal of the sensor and a second red cord connects the positive terminal of the sensor to the positive terminal of the power supply.

Collect Data

12. Create a graph of Current (A) versus Time (s). �(7.1.1)

13. Start recording data. �(6.2)

14. Plug the power supply into a power outlet and set the power supply to output 0.4 amperes.

CAUTION: Do not exceed 0.4 A. This much current can be dangerous!

15. Adjust the scale of the graph so that you can clearly see the data being collected. �(7.1.2)

16. Why do you have to begin collecting data before you turn on the power supply?

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Power supply

Spoon

Electrolyte

Copper

Black

Red

Red

Circuit diagram

Molar Mass of Copper___________________________________________________________________________________________

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17. After 15 minutes, turn off and unplug the power supply.

18. Stop recording data. �(6.2)

19. Remove the spoon from the electrolyte solution and allow it to air dry.

20. Measure and record the mass of the spoon that is now plated with copper.

Mass of copper plated spoon (g):

___________________

21. Save the data file and clean up the lab station according to your teacher’s instructions, including any special instructions for the disposal of the copper(II) sulfate solution. �(11.1)

Data Analysis

1. Sketch or print a copy of the graph of Current (A) versus Time (s). Label the overall graph, the x-axis, the y-axis, and include units on the axes. �(11.2)

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Student Inquiry Worksheet2. Determine the area under the curve on the graph of Current (A) versus Time (s).

�(9.7)

Area under the curve (As):

___________________

3. Using the area under the curve, calculate the moles of electrons transferred.

4. Give the balanced chemical equation for the reduction half-reaction used in this experiment.

5. Using the stoichiometry of the reduction half-reaction, calculate the number of moles of copper plated onto the spoon.

6. Calculate the mass of copper that was plated onto the spoon.

7. Calculate the molar mass of copper using the number of moles of copper and the mass of copper determined in the above calculations.

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Molar Mass of Copper

Analysis Questions

1. What is the percent error for the molar mass of copper calculated?

2. What are some possible combinations of amperage and time that would produce the same amount of copper plating as observed in the experiment?

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3. The electrolyte solution used in this experiment is 0.5 M copper(II) sulfate. What happens to the concentration of this electrolyte solution during the course of the reaction?

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Student Inquiry Worksheet

Synthesis QuestionsUse available resources to help you answer the following questions.

1. If you stop collecting data before you turn off the power supply, where would the error show up? Would the moles of copper or the mass of copper you used to calculate the atomic mass be incorrect? Explain.

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2. If you stop collecting data before you turn off the power supply, would your calculated atomic mass be too big or too small? Explain.

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3. Which electrode is the anode and which electrode is the cathode during this electroplating experiment? How do you know?

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Multiple Choice QuestionsSelect the best answer or completion to each of the questions or incomplete statements below.

1. In the reaction between aluminum and chlorine, which substance is undergoing oxidation?

2Al(s) + 3Cl2(g) → 2AlCl3(s)

A. Aluminum (Al)B. Aluminum chloride (AlCl3)C. Chlorine (Cl2)D. Not enough information

2. What process is occurring at the cathode?

A. Oxidation B. PrecipitationC. Redox

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Molar Mass of CopperD. Reduction

3. Using a table of standard reduction potentials, what is the calculated voltage associated with the spontaneous reaction between nickel and tin sulfate?

Ni(s) + SnSO4(aq) → Sn(s) + NiSO4(aq)

A. –0.39 VB. –0.11 VC. +0.11 VD. +0.39 V

4. What is the calculated number of moles of electrons transferred if the area under the curve is found to be 5566 A·s.

A. 5.769 x 10–2 mol e– B. 1.733 x 101 mol e–

C. 5.566 x103 mol e–

D. 5.370 x 108 mol e–

5. Imagine another electroplating lab with an unknown metal. You know the metal ion has a 3+ charge. If the mass of the metal deposited is 0.500 g and the area under the curve is 5566 A·s, what is the identity of the metal?

A. AluminumB. ArsenicC. ChromiumD. Iron

Key Term ChallengeFill in the blanks from the list of words in the Key Term Challenge Word Bank.

1. In a ________________________ reaction, electrons are transferred between reactants. In the ________________________ half-reaction, ions ________________________ electrons to become solid metal. In the ________________________ half-reaction, metals ________________________ electrons to become dissolved ions in solution. Because electroplating is a ________________________ process, electricity must be supplied to the reaction. The object to be plated is connected to the ________________________ where ions are reduced. The ions are replaced at the ________________________ where metal atoms are oxidized.

2. ________________________ is a flow of electrons. The rate of flowing electrons (current) is measured in ________________________. Knowing the current and ________________________ the electricity is applied, you can determine the charge supplied to the reaction. Incorporating ________________________, allows you to determine the

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Student Inquiry Worksheet

number of moles of electrons transferred in the reaction. Using the ________________________ of the reduction half-reaction then assists you in calculating the number of moles of metal electroplated.

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Molar Mass of Copper

Key Term Challenge Word BankParagraph 1 Paragraph 2anode

cathode

gain

lose

neutralization

non-spontaneous

oxidation

precipitation

reduction

reduction-oxidation

spontaneous

amperes

Avagadro’s constant

electricity

ohms

opposite

Plank’s constant

stoichiometry

temperature

the Faraday constant

time

volts

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