Experiment 14 - Grade: A PDF

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Lab 14...

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Brady Rolfes CHE 113-008 Henry Pruett 6 October 2020 Hydrogen Peroxide Decomposition Using Potassium Iodide Introduction This experiment’s main purpose is to find the rate constant of a reaction consisting of hydrogen peroxide (H2O2), potassium iodide (KI), and water. The reaction is mainly focused on the decomposition of hydrogen peroxide, which the chemical formula can be seen as followed (French et. al, 83): 1.

2 H 2 O 2 (aq)⇋2 H 2 O (l)+O 2 (g)

This equation shows the basic formula for the decomposition of hydrogen peroxide, but since this reaction takes place over a long time, the use of a catalyst is needed, which would be potassium iodide. A catalyst simply is added to a chemical equation to speed of the reaction by lowering the activation energy. The catalyst is never consumed by the reaction, which does not cause the overall product of the reaction to change, but rather just lowers the activation energy (French et. al, 83). In order to find the rate constant, one must know the rate the reaction takes place, which can be seen as the equation 2 below, and the rate law equation, which can be seen as the equation 3 below (French et. al, 83): 2. 3.

rate=

Δ [ C] Δt

rate=k [ A ]a [ B]b

For the rate of the reaction, the change of concentration is divided by the time the reaction takes place. For the rate law equation, the concentration of A and B is raised to the order of A and B respectively. The order of A and B are not the same as the coefficient in the balanced equation, although it could be. The order of the reaction must only come from experimentally found values (French et. al, 83). These numbers times the rate constant would in theory equal the same as equation 2. To solve for K, one would have to rearrange the equation so that K is isolated, or solve algebraically for K. The initial value for concentration of hydrogen peroxide is 3% by mass. This number will have to be converted into molarity in order to find the rate constant for the reaction (French et. al, 84). Molarity is simply moles of solute divided by the liters of solution. But since the moles of hydrogen peroxide is not given, one must be able to use the mass percent equation to convert into mass, which then can be used to get moles. The mass percent formula is mass of solute divided by total mass of solution times one hundred (French et. al, 84). The ideal gas law is used in this experiment to find the rate constant in another way. The ideal gas law can be rearranged so that moles can be found using the ideal gas law. This equation is as followed (French et. al, 84): n P = v RT

4.

Since n is moles and v is volume, the left side of equation 4 is equal to molarity. The R value used for this equation is going to be 0.08206 (L*atm)/(mol*K) in order to cancel out all the units and leave one with the correct units for molarity. The Arrhenius Equation shows one how temperature can affect the rate constant. The Arrhenius base equation can be seen below (French et. al, 85): 5.

k =A e−E / RT

But by rearranging the equation, the equation can be set up that the activation energy can be solved by using the slope of a graph, which will be graphed in the experiment, and the temperature of water. The R value will be different for this equation than that of equation 4. This R value will be 8.314 (J/(K*mol)). Methods 1. Prepare and calibrate MeasureNet by setting atmosphere pressure at 760 torr. Set system to record for 200 seconds. 2. Obtain 150-200 mL of water in a beaker and record water temperature with thermometer. Record temperature in the lab notebook. 3. Using graduated cylinders, obtain chemicals, Hydrogen Peroxide and Potassium Iodide, and set them in separate test tubes. Record exact volumes in lab notebook. The following amount volumes should be used from Table 14.1 (French et. all, 86): Trial

Volume of

Volume of

Volume of Water

Hydrogen Peroxide

Potassium Iodide

(mL)

(mL) 1 0.5 1 1

0 0.5 2 0

(mL 001 4 002 4 003 2 004 4 4. Allow contents to sit for two minutes.

5. Pour chemicals for trial one into one test tube and plug immediately with pressure probe. Start recording on MeasureNet as soon as the plug is inserted. After 200 seconds, save file as “001” and dispose of solution. 6. Clean off the probe with DI water. 7. Repeat step 5 and 6 for trials 002 and 003. Saving the files as respect to trial number.

8. Heat a beaker of water to about 10°C hotter than room temperature and record the exact temperature in the lab notebook. 9. Have the test tubes for trial 004 sit in the hot water for about 2 minutes to warm up. 10. Quickly do step 5 for trial 4, keeping the test tube in the warm water. Let reaction take place for 200 seconds. Save file as “004” after reaction is complete. 11. Clean up the station and materials used for the lab, disposing of all chemicals correctly. Analysis This experiment was set in place in order to find the rate constant for the decomposition of hydrogen peroxide using a catalyst of potassium iodide. For the order of the reaction in respect to hydrogen peroxide, the value I got was 3, and the same goes for potassium iodide. These do not support the purpose of the lab because in reality the order should be 1 for hydrogen peroxide and 2 for potassium iodide. For solution 1, I got a rate constant of 2.4 m^-1*s^-1. For solution 2 and 3, I got the same rate constant of 2.8 m^-1*s^-1. For solution 4, I got a rate constant of 5.5 m^-1*s^-1. These support the purpose of the lab because I was able to find the rate constant of all the solutions. Although the rate order was off, I was still able to find the rate constant of the order I found. I was also able to find the activation energy for the overall reaction using the rate constant of solution 1 and 4. Using the rate constant and the temperature, I found the activation energy to be 39 kJ/mole. This also supported the main purpose of the lab because the activation energy was found through experimental data collected. There are multiple reasons why the results could have been off. One reason was not having the pressure probe tightly fitted into the test tube. This could have thrown off the data because the pressure inside the test tube could have been altered from the outside atmospheric

pressure which would mess up the graphs. This would throw off the slope of the graphs leading to calculations being skewed. To prevent this from happening next time I could make sure to firmly hold down the probe making sure there are no possible ways for outside pressure to get inside or allowing some pressure from escaping the test tube the probe messing up the reading. Another source of error could have been not properly cleaning out the pipet leading to some cross contamination. This would throw off the volume of the amount of either potassium iodide or hydrogen peroxide. This would lead to incorrect calculations of molarity, which would ultimately lead to a wrong rate constant. In order to prevent this problem in next experiment, I could clean the pipet more thoroughly while also taking my time in making sure that I fully clean out the pipet. One other source of error could have been having extra DI water in the bottom of a graduated cylinder. This would dilute the sample of either hydrogen peroxide or potassium iodide. This would lead to a weaker reaction which would throw off the graphs and the slope. This would be fixed by drying out graduated cylinders after each use instead of moving on quickly and skipping over a thorough cleaning. Conclusion This lab has taught me how to solve for the rate constant using experimental data. This lab also sought out to teach how to use different equations to enable the ability to solve for the K value and the activation energy, and successfully did so. This lab finally taught how a catalyst is utilized in the chemistry setting and how important they are in different reactions. This lab shows how these concepts can be used in real life. For example, like baking a cake, the higher the temperature the faster the cake will bake. Also adding a catalyst, like baking soda, can be used to also speed up the time a cake takes to bake. To improve this lab, the lab can provide more graduated cylinders to avoid cross contamination throughout the experiment.

Work Cited French, April, Allison Soult, M. Mearl Savas, Francois Botha, Carolyn Brock, Charles Griffith, Darla Hood, Robert Kiser, Penny O’Connor, William Plucknett, Donald Sands, Diane

Vance, William Wagner. “Experiment 14: The Decomposition of Hydrogen Peroxide.” General Chemistry II Laboratory Manual. Plymouth, MI: Macmillan Leaning Curriculum Solutions, 2019. 81-86. Web. 6 October 2020. https://www3.chem21labs.com/labfiles/36194_45_Exp%2014_FrenchA%2021871%20W20..pdf?rf=7164...