Intro to Stoichiometry, p. 5 of 5

Introduction to Stoichiometry:

The reaction of bleach and hydrogen peroxide1

Introduction:

Stoichiometry is the quantitative study of chemical reactions. The word stoichiometry comes from Greek words that mean matter and counting, so stoichiometry is counting matter. Stoichiometry is used to determine how much of one chemical will react with another chemical and also to predict how much of a desired product will be produced from the starting materials.

The reaction that you will study in this lab is the reaction between two common household items, hydrogen peroxide (H2O2) and bleach (NaOCl). Both chemicals are found in the home in diluted form, bleach is a solution that is 5.25% NaOCl, or 5.25 grams of NaOCl in 100 mL of solution. Hydrogen peroxide typically comes in a 3% solution, which means there are 3 grams of hydrogen peroxide in 100 mL of solution. The reaction between the two chemicals releases oxygen gas. Many household chemicals can be used for studying chemical reactions but you must be careful because some produce unexpected or dangerous results. If you mix bleach and ammonia (NH3) then poisonous chlorine gas is produced.

1Adapted from "Stoichiometry and the Chemical Equation" in Laboratory Manual for Blei and Odian's General, Organic, and Biochemistry: Connecting Chemistry to Your Life, by Sara Selfe. Published by W. H. Freeman.
PROCEDURE

You will run two sets of reactions for this experiment. In Reaction Set A, the volume of bleach will remain constant (3 mL) and the volume of hydrogen peroxide will vary. In Reaction Set B, the volume of hydrogen peroxide will remain constant (3 mL) and the volume of bleach will vary. The volumes of reactants to be used for each set of reactions are given below. This may seem like a lot of runs but each run takes just minutes to perform.

Set A / Volume of bleach / Volume of H2O2
Run l / 3 mL / 1.0 rnL
Run 2 / 3 mL / 2.0 mL
Run 3 / 3 mL / 3.0 mL
Run 4 / 3 mL / 4.0 mL
Run 5 / 3 mL / 5.0 mL
Run 6 / 3 mL / 6.0 mL
Run 7 / 3 mL / 7.0 mL
Set B / Volume of bleach / Volume of H2O2
Run 1 / 1.0 rnL / 4 mL
Run 2 / 2.0 mL / 4 mL
Run 3 / 3.0 mL / 4 mL
Run 4 / 4.0 mL / 4 mL
Run 5 / 5.0 mL / 4 mL
Run 6 / 6.0 mL / 4 mL
Run 7 / 7.0 mL / 4 mL

1. Fill a basin water with tap water. Completely immerse a 100-mL graduated cylinder (if necessary, remove the plastic bottom 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 ring stand positioning it high enough to allow you to slip a piece of rubber tubing into the mouth of the cylinder. The up-ended graduated cylinder should be full of water (NO AIR BUBBLES) and securely fastened onto the ring stand. 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. Then you will be able to read the volume displaced directly from the graduations on the cylinder. The overall setup for the gas-collecting apparatus is shown below.


2. Label two clean, dry small beakers, one for bleach and one for hydrogen peroxide. Obtain approximately 50 mL of bleach and 50 mL of hydrogen peroxide.

3. With the aid of a pipet, transfer the designated amount of bleach (3 mL for Set A, reaction 1) into the side-arm Erlenmeyer flask.

8. With the aid of a different pipet, transfer the designated amount of hydrogen peroxide (1.0 mL for Set A, reaction 1) into the small vial, and then use tweezers to lower the vial into the sidearm flask, taking care not to knock over the vial.

5. Again being careful not to knock over the vial, stopper the flask with the rubber stopper. Push it in firmly to form a good seal. Place the loose end of the tubing into the mouth of the up-ended graduated cylinder (refer to diagram above). You may have to hold the tubing to prevent it from flopping around. Don’t worry if a bubble or two escapes from the tubing into the cylinder.

6. Once the setup is complete, jostle the reaction flask until the vial tips over and spills the hydrogen peroxide into the bleach. Gently swirl the flask to ensure complete mixing. Oxygen should begin forming immediately. 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. Record the amount of gas that was produced by noting the level of the liquid-gas interface in the graduated cylinder. Remember that the cylinder is upside down when you read the markings. The volume of oxygen produced should be recorded in the data table in the Data section.

7. Empty the flask by pouring the reaction solution into the large beaker that will serve as 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.

8. Repeat all of the reactions noted above for Reaction Set A and Reaction Set B. Record the volume of oxygen produced in each run.

DATA

Ø  In your lab book record your data for each reaction set in a table like that below.

Set A / Volume of H2O2 used / Volume of bleach used / Volume of oxygen produced
Run 1
Run 2
Run 3
Run 4
Run 5
Run 6
Run 7

Ø  Plot the volume (mL) of oxygen produced versus the volume (mL) of hydrogen peroxide used for Reaction Set A. Remember that a constant volume of bleach was used. You may chose to use Excel or another computer spreadsheet program for your graphs (Excel is available on the lab computers).

Ø  Plot the volume (mL) of oxygen produced versus the volume (mL) of bleach used for Reaction Set B. Remember that a constant volume of hydrogen peroxide was used.

Ø  The lab report will consist of a Purpose, Conclusion, the data tables from A and B, the graphs from A and B, and answers to the questions on the following page.


QUESTIONS

1. What happens as more hydrogen peroxide is added in Reaction Set A? What happens as more bleach is added in Reaction Set B?

2. What reason can you give for the trends you saw in Question #2? Why does it happen that way?