Kinetics Page 1 of 4 Clock Reaction KINETICS OF A CLOCK ...

KINETICS OF A CLOCK REACTIONKinetics(Adapted from David R. Ehrenkranz and John J. Mauch. “Reaction Rates-Determining the Order of a Reaction”, inChemistry in Micro Scale: Book 1; Kendall/Hunt Publishing Company:1993.)Introduction: Chemical KineticsKinetics is the study of the rates of chemical reactions as well as the study of the sequences of steps that reactions gothrough in proceeding from reactants to products. The steps of a chemical reaction are collectively called the reactionmechanism. In order for a reaction to occur, a collision must occur; the collision must be of sufficient energy tobreak the necessary bonds and be of proper orientation. This is the basis of reaction rates.Factors that Affect Rate:• The concentrations of the reactants used in a reaction usually affect the observed rate. Generally, the moreconcentrated the reactants, the faster the reaction proceeds.• The temperature at which the reaction occurs also affects the rate. Reactions increase in rate as thetemperature is increased.• A catalyst is a substance that, when added to a chemical reaction, increases the rate of the reaction whilenot being consumed itself. Consequently, catalysts are not present in the balanced chemical equation for thereaction.• For solid or liquid reactants or catalysts, the rate of the reaction increases as the exposed surface area of thesubstance increases.Collision Theory• For a reaction to occur, molecules must collide with each other.• Molecules must possess at least a certain minimum energy necessary to break the bonds.• Molecules must have the proper orientations.• Increased concentration of reactants results in greater numbers of collisions per unit time.Goals: During the lab students will:1. Apply the Collision Theory to factors that change the rate of a reaction.2. Measure reaction time when changing the concentration of a reactant, hydrogen peroxide.3. Generalize the effect of changing the concentration of hydrogen peroxide on reaction time.4. Graph the reaction time and concentration (drops) of hydrogen peroxide.Pre-lab Questions:1. What is the Collision Theory?2. List three factors that may affect the rate of a chemical reaction. How does the Collision Theory explaineach factor's impact on reaction rate?Page 1 of 4Clock Reaction

Kinetics3. During this activity you will change the concentration of the reactant hydrogen peroxide. Predict how therate will change by decreasing the concentration of the hydrogen peroxide.4. Sketch the graph showing your predicted change in reaction time as the hydrogen peroxide concentrationdecreases.Safety: Always wear safety goggles on the MCL.Experimental Procedure:It is very important that you put exactly the right amount of drops in the wells. To get the feel of it, practice with yourwater pipette in one of the wells on the bottom row.Practice with the stopwatch so you know how to start it, stop it and clear it without looking. The person with thestopwatch must look at the reaction!Figure 1. Example of a 96-well plateRow 1 !Row 2 !Place your well plate on a piece of white paper to help with observing the color change during the reaction. The solutionsare already in the pipettes ready for you to use. You have a 96 well plate.1. Add water and solution A (hydrogen peroxide) to the wells in the first row of your well plateaccording to the number of drops given in the following diagram:Row 1# drops of solution A 4 4 4 3 3 3 2 2 2 1 1 1#drops of water, W 0 0 0 1 1 1 2 2 2 3 3 3Page 2 of 4Clock Reaction

2. Add solutions B and C to the wells in the second row of the well plate according to the numberof drops given in the following diagram:KineticsRow 2# drops of solution B 1 1 1 1 1 1 1 1 1 1 1 1# drops of solution C 4 4 4 4 4 4 4 4 4 4 4 43. Make sure the stopwatch is ready to use. Make sure your “water” pipette is empty. Using theempty water pipette, draw up the entire solution out of the first well in row 2. Add it to the first well in row 1.The person with the stopwatch must start the stopwatch the instant you mix the solutions. When the firstfaint change in color appears, stop the watch and record the reaction time in the data table provided. (DataTable for Run # 1.)DO NOT ROUND OFF THE TIMES!4. Use the distilled water in the small beaker to rinse out the pipette you used to move the solutionfrom row 2 to row 1. Squirt the rinse water into the empty beaker. Repeat the rinsing twomore times. Rinse the pipette three times after each use.5. Using the clean, rinsed pipette, pull the entire solution out of well 2 in row 2 and add it to well2, row 1. Start the stop watch as soon as the solutions are mixed and stop the watch when thefirst sign of color appears. Record the reaction time in the data table below.Data Table for Run # 1reactiontime (s)Avg. time6. Continue to move the contents of corresponding wells from row 2 to row 1, measuring and recording each reactiontime in the table above.7. After you have recorded all your times, go to a computer station, and plot a graph of your raw data usingMicrosoft Excel. (Remember that your independent variable is on the x-axis and your dependent variable is on they-axis. Record your dependent variable in the first column and your independent variable in the second column.)An explanation of graphing with excel can be found A member of the MCL staff will help you. After looking atyour graph go back to your experimental station and repeat the experiment, using the wells in row 4 and 5. Youwill repeat steps 1 through 7.Data Table for Run # 2reactiontime (s)Avg. timePage 3 of 4Clock Reaction

9. Find your average times for each run and put the results in the appropriate box.KineticsPostlab Questions:1. The pipettes in this reaction are aqueous solutions containing the following substances.Pipette A: hydrogen peroxide (H 2 O 2 )Pipette B: starch and sodium thiosulfate (Na 2 S 2 O 3 )Pipette C: potassium iodide (KI), sodium acetate (NaC 2 H 3 O 2 ), and acetic acid (HC 2 H 3 O 2 )Pipette W: distilled waterIn general, did the reaction time increase or decrease as the concentration of hydrogen peroxide was decreased?2. How does this compare with your prediction in prelab question 3?3. Using the Collision Theory explain the change in reaction rate as the concentration of hydrogen peroxidedecreased.4. Copy the average times you found in # 9 into the chart below. Use these values to plot another graph by hand,using graph paper.Run # 1Run # 2Average Times5. How do your experimental graphs (the Excel graphs you generated on the computer) compare to your predictedgraph in prelab question 4?6. Compare your graph of the averages to the other graphs you have generated.Page 4 of 4Clock Reaction