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This article incorporates, in modified form, material from Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture.

 

A catalyst is a substance that increases the rate of a chemical reaction, but is not consumed or changed by the reaction. A catalyst works by reducing the activation energy needed to initiate and sustain the reaction. For example, two molecules of hydrogen peroxide can react to form two molecules of water and one molecule of molecular oxygen gas by the following reaction:

2 H2O2(aq) → 2H2O(I) + O2(g)

At room temperature, this reaction occurs very slowly because few of the collisions between hydrogen peroxide molecules have sufficient energy to activate the reaction. Furthermore, commercial hydrogen peroxide solutions, such as the 3% hydrogen peroxide solution sold in drugstores and the 6% solution sold by beautician supply stores, are treated with stabilizers (sometimes called negative catalysts) that increase the activation energy for the reaction, further inhibiting it from occurring.

If you add a catalyst to a solution of hydrogen peroxide, the effect is immediately evident. The solution begins bubbling, as oxygen gas is evolved. Numerous substances can catalyze the reaction of hydrogen peroxide to water and oxygen gas, including many metal oxides such as manganese dioxide, but the efficiency of catalysts varies. One of the most efficient catalysts for hydrogen peroxide is the enzyme catalase, which is contained in blood. (Catalase functions in the body as a peroxide scavenger, destroying peroxide molecules that would otherwise damage cells.)

One catalase molecule can catalyze the reaction of millions of hydrogen peroxide molecules per second. Immediately after each pair of hydrogen peroxide molecules reacts, catalyzed by the catalase molecule, that catalase molecule is released unchanged and becomes available to catalyze the reaction of another pair of hydrogen peroxide molecules. When all of the hydrogen peroxide has reacted to form water and oxygen gas, you end up with as many catalase molecules remaining as you started with.

In this lab session, we’ll measure the reaction rate of the catalyzed reaction of hydrogen peroxide by adding a fixed amount of catalase enzyme to measured samples of hydrogen peroxide. After allowing the reaction to continue for measured periods of time, we’ll stop the reaction by adding sulfuric acid to denature (deactivate) the catalase and then titrate the resulting solutions with a dilute solution of potassium permanganate to determine how much unreacted hydrogen peroxide remains in each sample. In acidic solution, the intensely purple permanganate (MnO4-) ion reacts with hydrogen peroxide to form the light brown Mn2+ ion according to the following equation:

5 H2O2(aq) + 2 MnO4-(aq) + 6 H+(aq) → 2 Mn2+(aq) + 8 H2O(I) + 5 O2(g)

Because the purple color of the permanganate ion is so intense, the titrant can serve as its own indicator for this titration. As long as hydrogen peroxide remains in excess, MnO4- ions are quickly reduced to Mn2+ ions, and the solution remains a light brown color. As soon as permanganate ions are slightly in excess, the solution assumes a purple color. By determining the amount of permanganate titrant required, we can calculate the amount of hydrogen peroxide that remained in the original samples.

Required Equipment and Supplies

  • goggles, gloves, and protective clothing
  • beaker, 150 mL (6)
  • graduated cylinder, 10 mL
  • graduated cylinder, 100 mL
  • pipette, Mohr or serological, 10.0 mL
  • pipette, Beral(or other disposable)
  • burette, 50 mL
  • funnel (for filling burette)
  • ring stand
  • burette clamp
  • stopwatch
  • catalase enzyme solution (see Substitutions and Modifications)
  • hydrogen peroxide, 3% (50 mL)
  • potassium permanganate solution, 0.1 M (~150 mL)
  • sulfuric acid, 0.1 M (~250 mL)
  • distilled or deionized water

All of the specialty lab equipment and chemicals needed for this and other
lab sessions are available individually from Maker Shed or other laboratory
supplies vendors. Maker Shed also offers customized laboratory kits at special
prices, including the Basic Laboratory Equipment Kit, the Laboratory Hardware Kit, the Volumetric Glassware Kit, the Core Chemicals Kit, and the
Supplemental Chemicals Kit.

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CAUTION

Sulfuric acid is corrosive. Hydrogen peroxide is a strong oxidizer and bleach. Potassium permanganate is a strong oxidizer and stains skin and clothing. Wear splash goggles, gloves, and protective clothing at all times.

Substitutions and Modifications

  • If you don’t have six 150 mL beakers, you may substitute Erlenmeyer flasks, foam cups, or other containers of similar capacity. Alternatively, you can complete this lab session using only one beaker or other container by doing the timed runs sequentially instead of simultaneously.
  • You may substitute a 10 mL graduated cylinder for the 10 mL pipette, at the expense of some loss in accuracy.
  • If you do not have a burette, you may use the alternative titration procedure described in Chapter 5.
  • If you do not have a stopwatch, you may substitute a timer or watch with a second hand.
  • Rather than purchase catalase enzyme, you may substitute a dilute solution of animal blood, which contains catalase. The liquid that leaks from a package of raw ground beef suffices, or you can obtain a small amount of animal blood from a butcher. The exact concentration of catalase in unimportant, as long as that concentration is the same for all of your test runs. One or two mL of animal blood diluted with distilled water to 10 mL works well.

Procedure

  1. If you have not already done so, put on your splash goggles, gloves, and protective clothing.
  2. Transfer about 40 mL of the sulfuric acid to the 100 mL graduated cylinder.
  3. Fill the 10 mL graduated cylinder with the catalase solution.
  4. Label six beakers or other containers from A through F.
  5. Use the pipette to transfer as closely as possible 10.00 mL of the hydrogen peroxide solution to each of the beakers. Record the amount of hydrogen peroxide in each beaker to 0.01 mL on the corresponding line in Table 12-4.
  6. Beaker A is the control, to which we will not add any catalase enzyme. Set it aside for now.
  7. Use the Beral pipette to withdraw 1.0 mL of catalase solution from the 10 mL graduated cylinder.
  8. As close to simultaneously as possible, start the stopwatch or timer and squirt the catalase solution into beaker B. Swirl the beaker to mix the solutions.

Too Fast or Too Slow

If the reaction is so vigorous that the solution foams over the top of the container, repeat step 8 using a more dilute catalase solution, and use that more dilute solution for the following steps as well. If the reaction rate is too slow-not enough bubbles forming quickly enough-use a more concentrated catalase solution or more of it.

  1. With the graduated cylinder of sulfuric acid held ready, when the timer reaches the 15.0 second mark, dump the sulfuric acid quickly into beaker B and swirl to stop the reaction. Record the elapsed reaction time as closely as possible on the corresponding line in Table 12-4.
  2. Refill the 100 mL graduated cylinder with 40 mL of sulfuric acid solution.
  3. Repeat steps 7 through 10 for beakers C, D, E, and F using reaction times of 30 seconds, 60 seconds, 120 seconds, and 240 seconds.
  4. Set up your burette, rinse it with the 0.1 M potassium permanganate titrant, and refill it to near the 0.00 mL line.
  5. Titrate the solution in beaker A by adding titrant to the beaker until a slight purple coloration just persists. Record the volume of titrant required to 0.01 mL on the corresponding line in Table 12-4. (Assuming nominal concentrations, about 35 mL of titrant should be required for beaker A, and correspondingly less for beakers B, C, D, E, and F.)
  6. Repeat step 13 for beakers B, C, D, and E.
  7. For each titration, calculate the number of millimoles (mM) of potassium permanganate required to neutralize the remaining hydrogen peroxide. (One millimole is 0.001 mole. Using mM locates the decimal point more conveniently for calculations.) Enter the number of millimoles of titrant required for each titration in Table 12-4.
  8. Using the balanced equation provided in the introduction, calculate the number of millimoles of unreacted hydrogen peroxide in each beaker, and enter that value in Table 12-4.
  9. For each beaker, calculate the reaction rate in millimoles/second (mM/s) and enter that value in Table 12-4.

Table 12-4. Effect of a catalyst on reaction rate – observed and calculated data

Beaker Hydrogen peroxide Reaction time Titrant volume Titrant millimoles Peroxide remaining Reaction rate
A.
___.__mL
0.0 s
___.__mL
___.__mM
___.__mM
n/a
B.
___.__mL
___.__s
___.__mL
___.__mM
___.__mM
___.__mM/s
C.
___.__mL
___.__s
___.__mL
___.__mM
___.__mM
___.__mM/s
D.
___.__mL
___.__s
___.__mL
___.__mM
___.__mM
___.__mM/s
E.
___.__mL
___.__s
___.__mL
___.__mM
___.__mM
___.__mM/s
F.
___.__mL
___.__s
___.__mL
___.__mM
___.__mM
___.__mM/s

Disposal

All waste solutions from this laboratory session can be disposed of by flushing them down the drain with plenty of water.

Optional Activities

If you have time and the required materials, consider performing these optional activities:

  • Graph your results for the trials to determine if the reaction rate is linear.
  • Repeat the trials, using more concentrated catalase solution (or more of it) and determine the effect of additional catalyst on the reaction rate.
  • Boil a solution of catalase for 30 seconds to one minute and then use the boiled catalase solution to determine if the catalase enzyme is denatured by heat.
  • Test other materials such as manganese dioxide, copper(II) oxide, and zinc oxide to determine if they function as catalysts for the decomposition of hydrogen peroxide and, if so, how efficient each is compared with each other and with catalase.

Review Questions

Q1: What effect did you observe the catalase catalyst to have on reaction rate?

Q2: Based on the data you recorded in Table 12-4, is the effect of the catalyst on reaction rate linear? If not, propose an explanation.

Q3: Would you expect the reaction rate to increase, decrease, or remain the same if you increased the amount of catalyst? Why?

Q4: When you begin a titration of a reacted hydrogen peroxide solution, you find that the first drop of potassium permanganate titrant causes the solution to assume a purple color. What has happened?

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