
Skills to build:

Performing an oxidation-reduction reaction that generates a gas

Using the ideal gas law to solve a stoichiometry problem

Using stoichiometry to determine the composition of bleach
Gases
The gaseous phase is a physical state in which molecules have random motion and negligible intermolecular interactions. Gases are easily compressed and flow from areas of high pressure to areas of low pressures. Four physical quantities are used to describe a gas: pressure (P), volume (V), temperature (T), and moles (n). The ideal gas law gives the relationship among these four variables:
R is the universal gas constant. For this experiment, R = 62.36 L
 torr/mol

K is used. T is expressed in Kelvin units. This equation states that there is a direct relationship between the volume of a gas and the number of moles of gas present. At standard temperature and pressure (0

C and 1 atm), one mole of a gas occupies 22.4 L, known as the standard molar volume.
Bleach
Bleach is a common household product used to disinfect and remove stains. Commercially, it is used to treat waste water, disinfect drinking water, and as a bleaching agent in the textile and paper industries. It is commercially prepared by passing chlorine gas through a column of aqueous sodium hydroxide.
The chlorine and sodium hydroxide react to produce sodium hypochlorite, NaOCl, the active ingredient in bleach.
Cl
2
(g) + 2 NaOH(aq)

NaOCl(aq) + NaCl(aq) + H
2
O( l )

Bleach
NaOCl partially hydrolyses in water solution to hypochlorous acid and sodium hydroxide.
NaOCl(aq) + H
2
O( l )  HOCl(aq) + NaOH(aq)
HOCl and OCl
- are strong oxidizing agents that produce the bleaching power. HOCl diffuses through bacterial cell walls and inactivates an enzyme necessary for cell growth, thereby destroying the bacteria, which also makes bleach a good sterilizing agent.
Household bleach sold for laundry use contains 3-6% m/v sodium hypochlorite.
The strength of the solution changes over time due to the slow decomposition of hypochlorite.
2 OCl
-
(aq)

2 Cl
-
(aq) + O
2
(g)
High-test hypochlorite (HTH), used for chlorination of swimming pools, contains calcium hypochlorite in a crystalline form that yields 65-70% available chlorine by mass.
Oxidation-Reduction
When hydrogen peroxide is added to bleach, an oxidation-reduction reaction occurs in which oxygen gas is released. Sodium chloride and water are also produced.
NaOCl(aq) + H
2
O
2
(aq)

O
2
(g) + NaCl(aq) + H
2
O( l )
Bleach is a basic solution with a pH range of 11-12. In basic solution, the oxidation halfreaction, where H
2
O
2
is the reducing agent, may be written
2 OH
-
(aq) + H
2
O
2
(aq)

O
2
(g) + 2 H
2
O( l ) + 2 e
-
The reduction half-reaction, where bleach is the oxidizing agent, may be written
2 e
-
+ H
2
O( l ) + NaOCl(aq)

NaCl(aq) + 2 OH
-
(aq)
Note that there is a transfer of two electrons during the chemical reaction between the oxygen atoms in H
2
O
2
and the Cl atom in NaOCl. The balanced chemical reaction shows the amount of oxygen produced is in a 1:1 molar ratio with NaOCl.
In this experiment, the oxygen produced by the reaction is collected over water.
According to Dalton's law, the total pressure inside the collection tube is the sum of the partial pressure of water plus the partial pressure of oxygen. To calculate the number of moles of oxygen gas produced from the ideal gas law, the vapor pressure of water at the temperature inside the tube must first be subtracted from the atmospheric pressure to give the pressure of oxygen. The Handbook of Chemistry and Physics gives water vapor pressure at a given temperature. The moles of NaOCl needed to produce the oxygen can be calculated and then converted into grams to determine the % m/v of
NaOCl in the bleach.
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Bleach
show different views and properties of the molecule. Answer the questions based on the manipulation of the molecule.
Go to the web site and click on the Hydrogen Peroxide link. Use the radio buttons to
Safety considerations
Wear suitable protective clothing, gloves, and eye/face protection!
You should read the online MSDS for:
Bleach Hydrogen Peroxide
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Bleach
Determination of H
2
O
2
volume
To insure that NaOCl is the limiting reagent and totally consumed during the redox reaction, the volume of hydrogen peroxide needed for an excess to be present must first be determined. Assemble the apparatus as shown in Figure 1 . Add tap water to the buret until the leveling bulb is half-filled with water. Adjust the leveling bulb so the level of water inside the buret and leveling bulb are equal. Record the initial level showing on the buret. Using a 1000-

L micropipettor, add 1.0000 mL of bleach to the test tube. Cover with the septum. Fill a syringe with 0.50 mL 3% m/v H
2
O
2
. Wipe the outside of the syringe with a Kimwipe. Insert the syringe into the septum without touching the plunger. Relevel the water if necessary. Push the plunger on the syringe to add the H
2
O
2
to the bleach. When all bubbling has stopped, measure the volume on the buret. Subtract to find the volume of oxygen generated. Remove the syringe.
Repeat the procedure with 1.00 mL, 1.50 mL, 2.00 mL , and 2.50 mL of H
2
O
2
.
Determine the volume of H
2
O
2
where NaOCl (bleach) is the limiting reagent.
Figure 1. Set-up for bleach and peroxide reaction. All tubing and connections should be airtight.
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Bleach
Determination of bleach composition
Clean and dry the test tube from the previous experiment. Add 1.0000 mL of bleach to the test tube. Record the brand of bleach and its m/v% NaOCl. Assemble the gas collection apparatus again. Using the excess volume of H
2
O
2
determined in the previous section, add it to the bleach. Repeat two more times. Using gas stoichiometry, compute the grams of NaOCl in the bleach samples. Determine the m/v% for each of the four samples collected at the volume where H
2
O
2
is in excess (see Gas
Stoichiometry online for an example calculation). Average the m/v% values. Does the average agree with the label on the bottle?
Laboratory Report
Answers to the following question should appear in the discussion section of your report:
1. Calculate the class average and standard deviation for the NaOCl m/v%.
2. Calculate the percent experimental error in the value you obtained for the bleach composition and then the class average. Give a possible explanation for any errors.
References
Knutsen, R. Chem 140 General Chemistry. http://www.ivygreen.ctc.edu/knutsen/ chem140/peroxblc.html (accessed August 2005)
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