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Factors That Affect the Rate of a Reaction Purpose: 1. To study the motion of solutions 2. To investigate the effect stirring, temperature, and particle size have on the rate of a chemical reaction. 3. To explain the behavior of solutions and reactions using the Kinetic Molecular Theory Materials: one measuring cup, preferably Pyrex 1 pan boiling water 1 pitcher (or bowl) of ice water 1 pitcher (or bowl) of room-temperature water 6 effervescent tablets (for example, Alka Seltzer or generic equivalent) stop watch or clock/watch with second hand plastic sandwich bag hammer or mallet dark food coloring teaspoon Discussion: Have you ever considered why all reactions do not take place instantaneously? You know, for example, that rusting occurs over an extended period of time. Likewise, methane (natural gas) and oxygen, left undisturbed are essentially unreactive. However, one a match is lit or a spark produced the reaction proceeds explosively. The answer must lie in understanding the way reactions occur and what is necessary for a reaction to proceed. First, in order to have a reaction the reacting particles must collide with each other. In addition, these collisions must be strong enough to begin breaking the bonds in the reactant molecules. In order to have good molecular motion, reactions are usually run in the gas phase or in solution. While some reactions can occur in the solid state the lack of motion of the particles produces very little mixing of the materials. In addition, because the point-to-point movement (translational motion) of particles in the solid state is minimal, the molecules do not typically have enough collision energy to break chemical bonds, a necessary first step in any reaction. In this experiment you will be examining the rate at which an effervescent tablet dissolves. Then you will investigate how changing conditions, such as stirring, increase or decrease in temperature, and particle size affect the rate of that reaction. Finally, you will use the Kinetic Molecular Theory to help explain what is happening on the molecular (particulate) level to produce the observed effects. Procedure: Part I: Diffusion within Water Fill a clear glass about ¾ full of water and set it in a spot where it will not be disturbed. Add 2 or 3 drops of a dark food coloring (red, blue, or green, for example) and observe what happens for about five minutes. Record your observations. Let the glass sit undisturbed while you complete the rest of the experiment. When you have finished the other parts of the experiment, observe what has happened in the glass. Record your observations. If you are not sure whether or not the change is complete leave the glass sit undisturbed for 8 to 24 hours and then make a final observation and record what you see. Part II: Stirred vs. Not Stirred The first set of observations will be made using room-temperature water (about 25C) one whole effervescent tablet without any stirring. This reaction will serve as your reference point. You will compare all the other reactions you perform to this one. Begin by adding 1 cup of room-temperature water to the measuring cup. Be careful in adding the correct amount of water and fill the cup to the same mark for each experiment. Begin timing the reaction as you drop in the effervescent tablet. When the final bit of tablet dissolves, stop timing. Record the time it took to complete the reaction. Empty and rinse the cup. Repeat this procedure. If the results differ by more than 10% repeat it a third time. Select two times that lie within 10% of each other to determine the average. Pour out the solution in the cup and rinse it with clean water before proceeding to the next experiment. Repeat the above procedure, except this time stir the solution once you have added the effervescent tablet. Once again record the time it took from the introduction of the tablet until all of it had dissolved. Repeat this procedure until you get two times that lie within 10% of each other. Empty and rinse the cup. Part III: Water Temperature In this part we will use water at two different temperatures and leave the sample unstirred. For a hot water sample we will use boiling water (about 100.0ºC) and for the cold water sample we will use ice water (about 0.0ºC). Pour 1 cup of boiling water into the measuring cup. Before you add the effervescent tablet predict what will happen by making the appropriate choice on the Report Sheet. Add an effervescent tablet and record the time it took for complete reaction. Empty and rinse the cup. Repeat this procedure until you get two times that lie within 10% of each other. You should have two values that lie within 10% of each other. Repeat the procedure using 1 cup of ice water. Record the time it took for complete reaction. Empty and rinse the cup. Repeat this procedure until you get two times that lie within 10% of each other. Part IV: Particle Size Place one effervescent tablet in the sandwich bag and crush using the hammer or mallet. (You can crush it with the back of the teaspoon if you are strong enough.) Pour 1 cup of room temperature water into the measuring cup. Add the crushed tablet all at once and begin timing. Stop timing when the entire tablet has dissolved. Record the time. Empty and rinse the cup. Repeat this procedure until you get two times that lie within 10% of each other. Break a tablet into 4 to 6 pieces. Fill the cup with room temperature water. Add the pieces all at once and begin timing. Record the time it takes for the tablet to dissolve completely. Empty and rinse the cup. Repeat this procedure until you get two times that lie within 10% of each other.
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