General Chemistry w/Lab I
Experiment: Reaction of Hydrogen Peroxide and Bleach Stoichiometry, Limiting Reagents and Percent Yield
In this laboratory you will use the reaction of hydrogen peroxide with bleach to gain experience in the concepts of limiting reagent, theoretical yield, experimental yield and percent yield, and use MS Excel to graphically represent these concepts.
When hydrogen peroxide (H2O2, sold as a 3.0 % w/v solution in water as a disinfectant for cuts) is mixed with bleach (the active ingredient is sodium hypochlorite, NaClO, sold as a 5.25% w/v solution in water) aqueous sodium chloride, water, and bubbles of oxygen gas are formed. In this experiment, you will measure the volume of oxygen produced when known amounts of hydrogen peroxide solution are mixed with known amounts of bleach. You will predict, and then observe, how the volume of oxygen gas formed varies with the quantity of bleach or peroxide used. You will also calculate the expected (theoretical) yield of oxygen gas and compare that to the actual (experimental) yield.
The concentration units used above may be new to you. When a solution concentration is expressed as a w/v percent, it refers to the mass of solute per 100 mL of solution. For example, if a solution has a concentration of 5% NaCl (w/v), then there are 5 grams of NaCl per 100 mL of total solution. Notice that the denominator refers to the total solution volume (solute plus solvent) and not just to the solvent. It is understood that the solvent in this case is water.
You will complete two sets of experiments for this lab. In the first set, you will maintain a constant volume of hydrogen peroxide and vary the amount of bleach. In the second set, you will keep the bleach volume constant and vary the peroxide. How will the volume of oxygen gas produced vary in each case? Take a few minutes to think about this ... (This is where you are thinking ... thinking some more ... thinking a lot ...) ... then
... Before beginning the lab, sketch two different graphs in your lab notebook. The first should show the volume of O2 produced (yaxis) as a function of the volume of peroxide added (xaxis). The second should show the volume of oxygen produced as a function of the amount of bleach added. The graphs do not need to be precise since they are simply a visual reflection of your hypotheses about the experiment. Below each graph, write a few sentences explaining your reasoning. You should include this work in the introduction section of your lab report.
The following tables indicate the volumes of bleach and peroxide to be used in each set of experiments:
|Run 1||4.0 mL||1.5 mL
|Run 2||4.0 mL||2.5 mL
|Run 3||4.0 mL||3.5 mL
|Run 4||4.0 mL||4.5 mL
|Run 5||4.0 mL||5.5 mL
|Run 6||4.0 mL||6.5 mL
|Run 7||4.0 mL||7.5 mL
|Run 1||1.5 mL||4.0 mL
|Run 2||2.5 mL||4.0 mL
|Run 3||3.5 mL||4.0 mL
|Run 4||4.5 mL||4.0 mL
|Run 5||5.5 mL||4.0 mL
|Run 6||6.5 mL||4.0 mL
|Run 7||7.5 mL||4.0 mL
Before beginning, obtain the following items from the lab cart:
- Small vial
- Rubber tubing
- Rubber stopper (onehole #7) with glass tubing
- 250 mL Erlenmeyer flask
- Water trough
In this laboratory you are to work with a partner.
- Fill a water trough with tap water. Completely immerse a 100 mL graduated cylinder (remove the plastic end from the cylinder) in the water trough, filling it with water. Turn the cylinder upside down, keeping the mouth below the surface of the water in the trough. Clamp the cylinder onto a ringstand with enough room beneath it to slip a piece of rubber tubing into the mouth of the cylinder. At this point the upended graduated cylinder should be full of water (NO AIR BUBBLES) and securely fastened onto the ringstand. This is the oxygenmeasuring vessel; gas formed during the reaction will bubble into the upended cylinder, where it will displace some of the water. You can then read the volume displaced directly from the graduations on the cylinder.
- Label two clean, dry 150 mL beakers, one for bleach and one for hydrogen peroxide. Obtain approximately 45 mL of bleach and 45 mL of hydrogen peroxide.
- Label two 10 mL graduated cylinders, one for bleach and one for hydrogen peroxide. Using a disposable pipet, transfer the designated amount of bleach (4.0 mL for Set A, reaction 1) into the 10 mL graduated cylinder. Since the experiments are to be semiquantitative, it is necessary to measure the quantities exactly. Pour the bleach into the Erlenmeyer flask.
- Measure out the designated amount of hydrogen peroxide (1.5 mL for Set A, reaction 1) into the second 10 mL graduated cylinder. Pour the hydrogen peroxide into the small vial, then use tweezers to lower the vial into the Erlenmeyer flask, taking care not to tip over the vial.
- Again, taking care not to tip over the vial, stopper the flask with the #7 rubber stopper with glass tubing. Push it in firmly to form a good seal. Attach the rubber tubing to the glass tubing, and then place the loose end of the tubing into the mouth of the upended graduated cylinder. You may have to hold the tubing to prevent it from flopping around. Dont worry if a bubble or two escapes from the tubing into the cylinder.
- Once the setup is complete, jiggle the reaction flask until the vial tips over and spills the hydrogen peroxide onto the bleach. Swirl the flask to ensure complete mixing. Most reactions will finish quite rapidly, others may take several minutes. Wait until the mixture in the flask stops fizzing and oxygen stops bubbling into the graduated cylinder, then record the amount of gas that was produced by reading the graduated cylinder. Remember that the cylinder is upside down when you read the markings. Your objective is to read the volume of gas (not water) in the cylinder.
- Empty the flask by pouring the reaction solution into your own waste container at your desk. (Use a 250 mL beaker to collect the waste from the various reactions. When your beaker is full transfer the waste into the waste container in the hood.) Rinse the flask and vial several times with water (the flask and the vial do not have to be dry for the subsequent runs).
- Repeat all of the reaction steps above for Reaction Sets A and B.
Analysis and Calculations
- Begin by writing the balanced equation for the reaction. Then prepare a spreadsheet using Excel that will calculate the mass and then moles of each reactant for each run of the experiment. (Sets A and B)
- By looking at the balanced equation and the number of moles of each reactant, determine the theoretical yield (in moles) of oxygen gas for each run. You will not be able to enter a simple formula to do this since you must pay attention to which reactant is limiting. You can either enter two separate formulas, or enter the values manually.
- Calculate the theoretical yield of oxygen gas in mL by assuming that every one mole of oxygen gas produced will occupy a volume of 24 L. Well learn why this is true (almost) later. For now, just use this value to calculate the expected volume of oxygen produced in each run.
- Next to the column containing the theoretical yield of O2, enter the actual yield obtained from reading the graduated cylinder. Then, in the next column, calculate the percent yield. Dont expect great results for the percent yield, as there are a number of places for error in this experiment. Still, it will be interesting to see if the actual results follow the trend that is predicted by the theoretical results.
- Lastly, create two graphs that you can use to compare with your initial hypothesis. In one graph, plot the experimental volume of O2 produced vs the volume of bleach used. For the second graph, use the peroxide volumes instead of bleach.
As always, include a detailed, thoughtful discussion of your results in the Results/Analysis section of your lab report.
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