Bombardier Beetle Lab
The bombardier beetle (Coleptera: Carabidae) is a ground beetle found in North and South America, Europe, Africa and Australia. This insect possesses a unique method of defense: it sprays predators with hot corrosive liquid that is under pressure. Within its abdomen the beetle has two chambers. One contains a mixture of hydrogen peroxide and hydroquinone; the other contains a mixture of the enzymes catalase and peroxidase. When threatened, the beetle mixes the contents of these two chambers. The enzymes catalyze the decomposition of hydroquinone and hydrogen peroxide to water, p-quinone and oxygen. The heat generated from the reaction brings the mixture to boiling, building up pressure that results in the mixture being expelled explosively from the chamber. The oxygen generated by the reaction acts as a propellant, causing the mixture to pop out of the abdomen through the abdominal tip, which can be aimed by the beetle. A single beetle can discharge up to 29 consecutive times and can spray at a distance of up to 4 times its body length.
Activities:
Before you begin the lab, you must figure out the relative amounts of each chemical that you will need. First, write out and balance the overall equation:
+ à + +
Hydroquinone hydrogen peroxide quinone oxygen water
The beetle typically has solutions of 0.25 micromol hydroquinone and 7.4 micromol hydrogen peroxide per milligram aqueous solution. Based on the results of balancing the above equation, which compound is the limiting reagent in the beetle?
If we will be using 10 mg of hydroquinone, how much hydrogen peroxide will you need? The molecular weight of hydroquinone is 110.1 g/mol. The molecular weight of hydrogen peroxide is 34.01 g/mol.
Catalase is sold by Sigma and other chemical companies in quantities of units. One unit is defined as the amount of catalase that will decompose 1 micromole of hydrogen peroxide per minute at 25 Celsius. Our formulation contains 2,000 to 5,000 units per milligram. Given the number of moles in our reaction, how much catalase will you need?
Peroxidase is sold in units that correspond to a chemical we aren’t using, so I’ve figured out the amount you’ll need experimentally in the lab. Each of you will obtain an ependorf tube containing the proper amount of peroxidase.
After you’ve had Ms. Camasta or Katy check your calculations, weigh out the correct amounts of each chemical. In this experiment, we will allow hydroquinone to be the limiting reagent, so round your amount of hydrogen peroxide up to the nearest milliliter. Obtain two test tubes and a thermometer. Place the thermometer in the test tube. Add the hydroquinone and hydrogen peroxide. Does anything happen? Why or why not? In a separate test tube, combine the two enzymes. Switch your thermometer to this test tube. Carefully, add the mixture of hydroquinone and hydrogen peroxide to the test tube containing the enzymes and thermometer. Remember this is an explosive exothermic reaction! Be careful! Also, one of the end products is irritating to skin, so don’t touch the foam with your bare skin. Measure the change in temperature. Do you think this would be an effective defense?