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JCE Classroom Activity: #10
Chemistry Time: Factors Affecting the Rate of a Chemical Reaction by the Journal’s Editorial Staff
Integrating the Activity into Your Curriculum
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This activity can be used to introduce a unit on chemical kinetics, but it is simple enough to be used in a discussion of chemical reactions or experimental methods/procedures during the first weeks of the semester. The activity involves reacting bicarbonate with acid, a reaction with practical applications in everyday life. The procedure is qualitative and encourages careful observation. Various factors are investigated by changing each separately, with all others constant. Discussion of how or why each variable affects the reaction rate is left to the instructor. If the activity is performed early in the school year, it might be revisited later when kinetics is introduced and those questions addressed at that time. All materials are common household items. The activity can be safely done at home or in the chemistry lab at school.
About the Activity The activity steps can be done independently and in any order, but as written they progress from simpler to more complex procedures. Students should work in pairs or small groups because some procedures need to be done simultaneously. Tap water can be used with this activity. All waste can be disposed of down the drain. 1. The acid and base in the dry tablet do not react (or react so slowly that evolution of gas is not observed). When the tablet is added to water the acid and base both dissolve. The reaction to produce CO2 bubbles begins at once and proceeds rapidly. The dry tablet half will not react unless water is added. 2. The greater the surface area of the solid the more rapidly the reaction occurs—the powder reacts faster than the whole tablet. 3. The rate of reaction is faster at higher temperatures. 4. No reaction is observed when water is added to baking soda. The solid dissolves in water, but no CO2 bubbles form. Baking soda dissolves and reacts with the acetic acid in vinegar to produce CO2 bubbles. A series of solutions with vinegar to water ratios of 1:0 (100%), 1:3 (25%), 1:7 (12.5%), 1:15 (6.25%), 1:31 (3.13%), and 0:1 (0%) show a clear progression of reaction rates when equal volumes are reacted with baking soda. Using an acetic acid solution more concentrated than vinegar is a reasonable experiment, but the reaction with vinegar is so rapid that students are unlikely to be able to notice a difference using a more concentrated solution. Students must be carefully supervised and must observe the necessary safety precautions if they are allowed to attempt such a procedure. Switching to an acid other than acetic acid would introduce a variable other than concentration.
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Answers to Questions 1. State of matter: substances in solution react faster than solids. Temperature: reactions occur faster at higher temperature. Surface area: the greater the surface area, the faster the reaction. Concentration: a more concentrated solution reacts faster. 2. Increase rate: Increase the temperature, grind or dissolve solid reactants, increase concentrations of reacting solutions. Decrease rate: Lower the temperature, use bigger pieces of solid reactants, lower concentration of solutions. 3. Baking soda is sodium bicarbonate. Baking powder is sodium bicarbonate mixed with a dry acid or acidic salt. Both react to produce bubbles of carbon dioxide in dough or batter, causing it to rise. For baking soda to react, an acid must be present (examples: buttermilk, lemon juice). Baking powder requires only a liquid to dissolve the acid and base (examples: water, milk). 4. A catalyst is a substance added to a chemical reaction that increases the rate of the reaction without itself being consumed. There are many examples. Enzymes are catalysts for many important biological reactions.
Additional Information and Related Activities 1. Alexander, A. J.; Zare, R. N. J. Chem. Educ. 1998, 75, 1105–1118. 2. Arce, J.; Betancourt, R.; Rivera, Y.; Pijem, J. J. Chem. Educ. 1998, 75, 1142–1144. 3. Carroll, H. F. J. Chem. Educ. 1998, 75, 1186–1187. 2. Sarquis, A. M.; Sarquis, J. L. Fun with Chemistry, Vol. 1, 2nd ed.; Institute for Chemical Education: University of Wisconsin– Madison, Madison, WI, 1995. 3. Sarquis, A. M.; Sarquis, J. L. Fun with Chemistry, Vol. 2; Institute for Chemical Education: University of Wisconsin–Madison, Madison, WI, 1993. 4. Journal Editorial Staff. How Big is the Balloon? J. Chem. Educ, 1997, 74, 1328A. 5. Newton’s Apple: Bread Chemistry: http://www.pbs.org/ktca/newtons/12/bread.html
This Activity Sheet may be reproduced for use in the subscriber’s classroom. JChemEd.chem.wisc.edu • Vol. 75 No. 9 September 1998 • Journal of Chemical Education
1120A
JCE Classroom Activity: #10
Student Side
Chemistry Time: Factors Affecting the Rate of a Chemical Reaction by the Journal’s Editorial Staff When and how fast will a chemical reaction will occur? This can be very important to know because we often need to speed up or slow down a reaction rate. Some reactions are so slow that they may take thousands of years to produce a measurable amount of product. Others are very fast, and possibly dangerous—explosions. Using a simple reaction you can discover some factors that determine how fast chemical reactions occur.
Try This
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You and your partner will need the following: Six effervescent antacid tablets (such as Alka Seltzer); three or more clear glass or plastic containers such as beakers or cups; labels and/or a marking pen; about 1 L of room-temperature water; hot water; ice or a freezer; graduated cylinder or measuring cup; a small plastic bag; baking soda; measuring spoon or scoop; and vinegar. A thermometer is helpful, but not required. Examine the box of effervescent antacid tablets. Two of the ingredients are citric acid and sodium bicarbonate (NaHCO3), an acid and a base. When these substances react, one of the products is carbon dioxide (CO2) gas. You can measure how fast the reaction occurs (the rate of the reaction) by the rate at which carbon dioxide gas is produced. 1. Open a new packet of effervescent tablets. Break one of the tablets in half. Place one half on a clean dry surface. Drop the other half in a container half filled with water. What happens? Which half tablet produces gas more quickly? Set the slower-reacting half tablet aside in a safe, dry place. Check it again when you have completed all parts of the experiment. Has there been any change? How can you get the reaction rate to increase? 2. Place an effervescent tablet in a small plastic bag and crush it into a powder by tapping it with a heavy object, such as a textbook. Label two clean, dry, clear containers “Powder” and “Whole”. Pour about 100 mL (or about 1/2 cup) of room-temperature water into each of the containers. At exactly the same time, pour the crushed tablet into the first container and drop a whole tablet into the other. Which reacts faster? 3. Chill some water to about 10 °C by adding ice or putting it in a freezer or for at least 45 minutes. Heat a sample of water to about 50 °C or let a hot-water tap run until the hottest possible water is obtained. Obtain three clean, dry, clear containers. Label the first container “C” and add 100 mL (or about 1/2 cup) of cold water. Label the second “RT” and add the same volume of room-temperature water. Label the third “H” and add the same volume of hot water. At exactly the same time, drop an effervescent tablet into each of the containers. Which reacts fastest? Slowest? 4. Baking soda is sodium bicarbonate. Vinegar is a solution of acetic acid in water. Acetic acid reacts with sodium bicarbonate to produce carbon dioxide, just as the citric acid in an effervescent tablet does. Place about a teaspoon full of baking soda in each of two clean, dry, clear containers. Pour a few milliliters of room-temperature water into one container and about the same volume of vinegar into the second. What do you observe? Design an experiment to determine the effect of concentration of acetic acid on the rate of its reaction with baking soda. Have your instructor check your procedure. When it has been approved, perform the experiment. Record your observations, then state your conclusion.
__1. Based on the results of your experiments, what factors affect the rate of a chemical reaction? __2. What might you do to speed up a chemical reaction? To slow it down? __3. Baking soda or baking powder is often added to baked goods to make them rise. How are baking soda and baking powder different? How do they make dough or batter rise? What is required to make baking soda work? What is required to make baking powder work? __4. Adding a catalyst is another very important method of changing a reaction rate. Define the term catalyst. Write down as many examples of catalysts as you can.
Information from the World Wide Web 1. Antacid Tablet Race: http://quest.arc.nasa.gov/shuttle/teachers/rockets/act4.html 2. Chemical Kinetics Simulation: http://www.chem.uci.edu/instruction/applets/simulation.html 3. The Inquisitive Cook: http://www.inquisitivecook.com/articles/faceoff.html 4. Baking Soda Experiments: http://www.users.interport.net/~sue/food/sodaexpt.html 5. Bubble Bomb: http://www.exploratorium.edu/science_explorer/bubblebomb.html
This Activity Sheet may be reproduced for use in the subscriber’s classroom. 1120B
Journal of Chemical Education • Vol. 75 No. 9 September 1998 • JChemEd.chem.wisc.edu
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Questions
