Reaction Rates - Catalysis, Concentration, Surface Area, and Temperature
|Kenneth Schug||Illinois Institute of Technology|
|Chicago IL 60616|
[Can be adapted to wide range of grade levels.] To observe factors affecting the rate of a chemical reaction.
For each experiment: One packet dry yeast, one pint of 3% hydrogen peroxide, four clear kitchen storage bags (quart size), four small vials with caps, matches, cigarette (or other object to produce glowing embers, e.g. punk, rope, wood splint). Can be done as a classroom demonstration experiment or by students (individually or in groups). [A single vial could be "recycled" if necessary]
CONCENTRATION. Place ¼ packet of yeast in vial and attach cap firmly. Pour 100mL hydrogen peroxide into bag #1, add the vial of yeast, push out most of the air and seal (ziplock or twistem). Fill bag #2 with air by swooping through the air and seal. Keeping bag sealed, unscrew cap of vial and mix contents. [Foaming will occur as oxygen gas is formed by the reaction 2H2O2 = 2H2O + O2 and the bag will inflate.] Light the cigarette (etc.) and press against bag 2; the plastic will melt to form a small hole. Repeat with bag one (placed on a fireproof surface). The bag will burst into flame illustrating the effect on combustion of the higher oxygen concentration (about 100% compared with 20% in air).
TEMPERATURE. Prepare two bag #1s (see CONCENTRATION). Cool one by immersing in cold water (or an ice bath) for several minutes, then uncap both vials and observe rate of gas formation. Gas forms much slower at lower temperature. [Optional extension: repeat but heat one of the bags by immersing in boiling water for several minutes. Keep in mind that high temperatures will eventually "destroy" the ability of the enzyme to catalyze the reaction; a phenomenon which seldom happens with] non-biological enzymes such as manganese dioxide or iron (III) ["ferric] salts.
SURFACE AREA. Prepare two "#1 bags" (see CONCENTRATION) but in one case grind the yeast before putting it in the vial. [Results pending but expect a faster reaction with "ground up" yeast because a larger surface area of yeast is available for the reaction to occur, the usual behavior of solids and liquids.]
CATALYSIS. The effect of the yeast on the hydrogen peroxide, which otherwise shows no sign of reaction, is due to the presence of biological catalysts or enzymes (one of which is called catalase}
Ask students to predict effect on chemical reactions they observe in everyday life, or read about in the newspapers. Some examples are: A) no reaction occurs when the gas supply to a bunsen burner is turned on unless a source of high temperature (e.g. a match) is present; B) a pile of flour will not burn very readily, but in a grain elevator the same flour dispersed in air (which contains oxygen) will explode or burn if a spark or flame is present because a much larger surface area is in contact with oxygen; C) hospital rooms in which patients are receiving oxygen have NO SMOKING signs because the increased concentration of oxygen makes combustion much more likely; D) the starch in our diet would not provide us with energy if our bodies did not have contain many different enzymes to first break the large starch molecules into glucose for absorption into the bloodstream and later to release the energy of the glucose in our cells to give us "energy".