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How Much Copper is in the Brown Sample?

Learning Objectives: At the end of this lab you should be able to…

1. Demonstrate an understanding of stoichiometry in calculations.

2. Observe the progression of a chemical reaction through color change and product formation.

3. Knowing the limiting reactant, conduct measurements, mathematical calculations and conversions

to calculate theoretical yield and percent yield to determine the effectiveness of experimentation.

Introduction

Sarah and Olivia, two chemistry lab assistants, found some unlabeled vials containing a brown powder in their chemistry stockroom. “I remember that it is copper (II) chloride, anhydrous solid.” said Olivia. Sarah asked: “If it is copper (II) chloride, it should dissolve in water and form a blue solution. Can we perform a quantitative analysis to find how much copper is contained in every gram of this solid?” Their idea was supported by the lab coordinator, Ms. Fambro, who gave them several chemical materials and apparatus, and left the whole investigation to them.

Solid copper (II) chloride dissolves in water, forming a green or blue solution depending on

concentration. The copper contained in the solution can be reduced to metal copper (as a brown precipitate) when it reacts with a reducing agent such as zinc and aluminum. Ms. Fambro provided aluminum wire for the analysis. The balanced equation between copper(II) chloride and aluminum is:

3CuCl2(aq) + 2Al(s) → 3Cu(s) + 2AlCl3(aq) (Equation 1) This type of reaction is known as an oxidation-reduction reaction, since the oxidation states of

both metals involved changes. The oxidation state (also called oxidation number) of a metal ion is equal to the charge of the ion. As you can see, copper in CuCl2 has an oxidation number of +2, while copper solid on the product side has an oxidation number of zero. Therefore, copper had to gain electrons during this change. Aluminum had an initial oxidation number of zero and lost electrons to become aluminum

ion (Al3+) in AlCl3. These processes can be summarized by the chemical equations below:

Cu2+ + 2e – → Cu0 (Equation 2)

Al0 → Al3+ + 3e – (Equation 3)

The transfer of electrons from one species to another is a distinct characteristic of oxidation-

reduction reactions, which is also called redox reactions. You will learn more about this type of reaction at the end of the semester and it is not the focus of the current lab.

In this lab, you and your partner will perform the same experiment as Sarah and Olivia did. You

will obtain a sample of copper(II) chloride (CuCl2) and record its exact mass. You will dissolve it in water and add a piece aluminum wire. The amount of aluminum is in excess. When the reaction start, aqueous copper ions, Cu2+, is reduced to metallic copper, Cu0 or Cu(s), while the aluminum Al(s) is oxidized to aluminum ion, Al3+(aq). Both reduction and oxidation occur simultaneously. According to the balanced

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equation (stoichiometry), the number of moles of copper ions with which you started equals the number of moles of metallic copper to be obtained. Therefore, you can estimate the mass metallic copper that should be recovered. This is called theoretical yield. You want to add an excess amount of aluminum so you need only be concerned with copper, the limiting reactant, in your calculations of theoretical yield.

The theoretical yield calculated from the limiting reactant gives the amount of product in an expected and ideally performed experiment. However, not everything goes according to your plan. There is no flaw with the chemistry, but there are various experimental techniques that will prevent you from obtaining the ideal amount of copper that you predicted. Often there are incomplete reactions, which can easily be seen in this reaction if there is a greenish color still persisting in solution. Reduced copper is sometimes collected incompletely during filtration and drying, resulting in loss of product.

Percent Yield is a percent ratio of amount obtained to amount expected. Below is the equation to let you evaluate how well your experiment was performed by comparing what you recovered to the theoretical yield. This result is called percent yield:

Experimental Yield Theoretical Yield

Before you start the lab, it will be helpful to practices a few related problems. The information here is helpful when you complete your lab submission. 1. Find the atomic masses of copper and chlorine, and the calculate the molar mass of

CuCl2. Keep THREE decimal places. Show all work and units:

Molar mass of Cu: __________

Molar Mass of Cl: ____________

Molar mass of CuCl2: __________

2. If Sarah and Olivia used 2.000 grams CuCl2(s) and excess amount of aluminum, what’s

the theoretical yield of copper according to Equation 1? Show all work.

3. If they actually collected 0.8177 g copper, what’s the percent yield in their experiment?

× 100% = Percent Yield% (Equation 4)

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Lab Procedure

Before attending lab, review the procedure that follows carefully, and outline the observations and numerical values that should be recorded in your laboratory notebook to allow you to successfully perform the calculations at the end of the lab. Prepare a table for all the experiment data that will be collected.

Tare (zero) the mass of a clean and dry 250-mL beaker on a balance. Remove it from the balance

and add slightly >3 g copper(II) chloride to it. Return the beaker to the balance and record the mass to the precision of the balance. Add approximately 60-mL of water to the beaker. Stir until the copper(II) chloride is completely dissolved. What’s the color of the resulted solution? This color is characteristic of Cu2+ ions in water.

Obtain a precut piece of aluminum wire and record its mass. Twist one end of the aluminum wire

into a coil; bend the other end into a hook to make it easier to hold. Immerse the coiled end of the wire in the copper(II) chloride solution.

Record all observations as the reaction proceeds. Shake the wire nearly continuously to loosen

the copper that accumulates on the surface of the wire. The reaction has reached completion when the blue color in the solution has disappeared and when the exposed aluminum wire has no more copper forming on it (minimum 20 minutes).

When the reaction is complete, remove the aluminum wire and shake the aluminum so that all

the copper falls back into the beaker. Note that if aluminum accidentally falls or the wire breaks into pieces and some are remained in the beaker, you will be isolating impure copper (copper mixed with aluminum), so be sure that only the copper remains in the beaker. Use a wash bottle of deionized water to rinse the aluminum wire over the beaker to remove any residual copper. You might find it helpful to scrape the copper off the aluminum with the rubber tip on a glass stirring rod. Carefully inspect the wire for any changes in appearance of the wire and record your observation; use a paper towel to wipe any residual copper off the wire, dry it, and record its mass. Discard the aluminum in the appropriate solid waste container, which is separate from the liquid waste.

Obtain a piece of filter paper that fits into a Buchner funnel and a watchglass, and label one of

them with your initials. Record the mass of this filter paper along with the watchglass to the precision of the balance.

Assemble a vacuum filtration apparatus including a vacuum trap to catch any accidental overflow

from the filtration process (Figure 1). First place two clamps on a ring stand and secure the side-arm flask and an Erlenmeyer flask with the clamps to the stand. Place a filter adapter (identified in Figure 1) into the mouth of the side-arm flask and place the Buchner funnel in the filter adaptor. Place the stopper of the provided tubing system in the Erlenmeyer flask, connect one end of the hose to the side-arm flask, and the other end to the vacuum nozzle with yellow tab, which can be found on each benchtop.

With the pre-weighed filter paper placed covering the flat surface of the Buchner funnel, turn the

vacuum on and moisten the paper with some DI water to ensure a good vacuum seal. Isolate the copper by vacuum filtration, carefully rinsing all the copper from the beaker into the Buchner funnel using a deionized water wash bottle. When the copper has been transferred, wash the copper with about 20-mL of acetone. The acetone will wash the water away and evaporate more quickly than water, hastening the drying of the copper. Allow the vacuum to dry the copper in the funnel for five minutes.

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Carefully transfer the filter paper and copper to the watchglass using a spatula. Use the spatula to

break up any large clumps and then wipe any copper clinging to the spatula onto the filter paper. Place the watch glass and contents into the oven to dry for 10-20 minutes. Allow it to cool to room temperature for a minimum of 10 minutes. Record its mass.

Data Analysis and Results (This will be completed in your notebook and on your final lab submission)

Show complete calculations with each blank.

1. Mole of CuCl2 dissolved

2. Mole of copper, Cu(s), expected to be formed (Eq.1 or 2)

3. Theoretical yield of copper in grams (expected mass to be formed)

4. Percent yield of copper metal

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5. What experimental errors occurred during your experiment that may have caused something other

than 100% yield of copper metal? Use complete sentences and discuss at least two possible causes of errors and how they would affect the actual yield. (Simply saying human error is not a valid error).

6. In complete sentences, summarize observations during the experiment, including any changes to the

solution and aluminum wire as a result of the reaction?

Further inquiry of aluminum

7. Using the mass of copper recovered (in data section) and the stoichiometry given in Eq. 1, predict

the theoretical mass of aluminum that reacted. Show work.

8. Compare the amount of aluminum you predict in question #8 with the amount you determined experimentally (see data section); find the percent error.

Percent error = experimental value value − expected value

expected value × 100%