Stoichiometry and the Chemical Equation

 (Reaction of Hydrogen Peroxide and BleacH)

 

 

          When hydrogen peroxide (H2O2, sold as a 3% (weight/weight) solution in water as a disinfectant for cuts) is mixed with bleach (active ingredient is sodium hypochlorite, NaOCl, sold as a 6% (w/w) solution in water), bubbles of oxygen gas are formed.  In this experiment, you will measure how much oxygen is produced when known amounts of hydrogen peroxide solution are mixed with known amounts of Clorox bleach.  Based on the amount of reactants used (bleach and hydrogen peroxide) and the amount of product formed (oxygen) you will determine the stoichiometry of the chemical reaction.    

          You may have encountered similar sorts of problems outside the lab–for instance, figuring out how many cookies (products) can be made from a given amount of ingredients (reactants) based on a specific recipe (equation).  If a recipe requires that 2 cups of flour be mixed with 4 eggs to make 20 cookies, how many cookies can be made from 1 cup of flour and a dozen eggs?  Which one of the ingredients acts as the limiting reagent?

 

 

PROCEDURE

 

                You will run two sets of reactions for this experiment.  In Reaction Set A, the volume of bleach will remain constant (4 mL) and the volume of hydrogen peroxide will vary.  In Reaction Set B, the volume of hydrogen peroxide will remain constant  (4 mL) and the volume of bleach will vary.  Listed below are the volumes of reactants to be used for each set of reactions:

 

Set A

Volume of  Bleach

Volume of Hydrogen Peroxide

Run 1

4 mL

2.0 mL

Run 2

4 mL

2.5 mL

Run 3

4 mL

3.0 mL

Run 4

4 mL

3.5 mL

Run 5

4 mL

4.0 mL

Run 6

4 mL

4.5 mL

Run 7

4 mL

5.0 mL

 

Set B

Volume of Bleach

Volume of Hydrogen Peroxide

Run 1

2.0 mL

4 mL

Run 2

2.5 mL

4 mL

Run 3

3.0 mL

4 mL

Run 4

3.5 mL

4 mL

Run 5

4.0 mL

4 mL

Run 6

4.5 mL

4 mL

Run 7

5.0 mL

4 mL

 

1.     Fill a water trough with tap water.  Completely immerse a 100 mL graduated cylinder and a large beaker in the water trough, filling the cylinder with water.  Turn the cylinder upside down in the beaker, keeping the mouth below the surface of the water.  Slip a piece of rubber tubing into the mouth of the cylinder, then secure the apparatus by clamping the graduated cylinder to a ringstand.  At this point the up-ended graduated cylinder should be full of water with NO AIR BUBBLES!  This is the oxygen-measuring vessel; gas formed by the reaction will bubble into the up-ended cylinder, where it will displace some of the water, and you can read the volume displaced directly from the graduations on the cylinder.

2.      Label two clean, dry 150 mL beakers, one for bleach and one for hydrogen peroxide.  Obtain approximately 60 mL of bleach and 60 mL of hydrogen peroxide.

3.   Obtain two 10 mL graduated cylinders, label one for bleach and one for hydrogen peroxide.  With the aid of a pipet, transfer the designated amount of bleach (4 mL for Set A, reaction 1) into the 10 mL graduated cylinder.  Since the experiments are to be semi-quantitative, it is necessary to measure the quantities exactly.  Pour the bleach into the Erlenmeyer flask, then attach the rubber tubing to the vacuum arm.

4.   Measure out the designated amount of hydrogen peroxide (2.0 mL for Set A, reaction 1) into a 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 knock down the vial.  Carefully stopper the flask with the rubber stopper; push it in firmly to form a good seal.

5.   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.  DO NOT SWIRL TOO HARD OR THE VIAL MAY BREAK!  Some reactions may 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. 

6.       Empty the flask by pouring the reaction solution into your ‘waste container.’  Rinse the flask and vial several times with water (the flask and the vial do not have to be dry for the subsequent runs).

7.       Repeat all of the steps noted above for Reaction Set A and Reaction Set B.  You may wish to repeat runs to ensure the reliability of your data.

8.       Plot the volume of oxygen produced vs. the volume of reactant used for both Reaction Set A and Reaction Set B.  Based on your data, perform one or two more runs for both Reaction Set A and Reaction Set B.  You decide what volume of bleach and hydrogen peroxide to mix.  (Keep in mind that you are trying to determine the exact volume of bleach that will completely consume the hydrogen peroxide). 

 

 

CALCULATIONS

 

1.   Graph A:  Plot the volume (mL) of oxygen produced versus the volume (mL) of hydrogen peroxide solution used for the reaction set where bleach is held constant.

 

2.   Graph B:  Plot the volume (mL) of oxygen produced versus the volume (mL) of bleach solution used for the reaction set where hydrogen peroxide is held constant.

 

3.   From each graph determine at what point does adding more reagent fail to produce more oxygen.  For example (and not a correct one), when you are reacting bleach with 4 mL of peroxide, you find that after 8 mL of bleach the curve levels off and you get the same amount of oxygen no matter how much additional bleach is added.

 

4.  Follow the steps below to calculate the mole ratio of NaOCl to H2O2 for each graph.

 

   NOTE:  Assume the density of bleach and the density of hydrogen peroxide solution to be 1.00 g/mL.

 

a) Calculate the number of moles of NaOCl in 4 mL of bleach [bleach is 5.7% (w/w) NaOCl]. 

b)  Calculate the number of moles of H2O2 (use the above volume recorded from graph A) required to consume 4 mL of bleach [hydrogen peroxide solution is 3% (w/w) H2O2]. 

c)  Calculate the mole ratio of these reactants. 

d)  Repeat these calculations for the data obtained from Graph B.   

 

5.   Indicate on Graph A where hydrogen peroxide is the limiting reagent in the reaction.  Indicate on Graph A where hydrogen peroxide is in excess (not all of the reactant is consumed).