AP CHEMISTRY LAB RATES OF CHEMICAL REACTIONS (II)

Transcription

1 PURPOSE: Observe a redox reaction. AP CHEMISTRY LAB RATES OF CHEMICAL REACTIONS (II) Apply graphing techniques to analyze data. Practice computer skills to develop a data table. Determine the order of a reaction. Determine the activation energy for the reaction. Determine the frequency factor for the reaction. DISCUSSION: You will notice if you look at the purpose for this lab, that we are really being ambitious with this lab. Since we are looking at so many different aspects of chemical kinetics, we will be doing this work over several lab periods. The reaction under consideration is a redox reaction. It involves crystal violet (a dye) and sodium hydroxide. Those two reactants involved in the reaction are crystal violet, C H 0 N and sodium hydroxide, NaOH. The good thing about this reaction is that the crystal violet will change colors as it is decomposed by the sodium hydroxide, so we will be able to monitor the reaction as it proceeds. The reaction is slow enough so that we can observe the color change with the spectrophotometer. Since our spectrophotometers are linked to the classroom computers, we have a quick and easy way to manipulate our data and construct graphs with the Logger Pro program. We will record our data in table form by using the Excel spread sheet. There are several important concepts that will be addressed in this lab. The methodology for each part will be somewhat different, but there will be ties between the different sections. We will look at each separately. Order of the reaction: When you look at the equation, you will notice that there are two chemical species that and consumed in the reaction; crystal violet and the hydroxide ion.

2 This reaction is dependent on the fact that the reaction is run in an basic environment, we will keep the concentration of NaOH large relative to the crystal violet. In that way, the concentration of NaOH will remain constant throughout a trial. That leaves the crystal violet for us to worry about. Our first trial for each chemical will be conducted by mixing the stock solutions of the reactants. The crystal violet will have a concentration of.0x0 - M and the [NaOH] = 0.0 M. We will run our first trial to get our feet wet. Each lab group will then be assigned a different concentration of NaOH at which to run another trial. You will mix the crystal violet with your NaOH and watch the color change in the spectrophotometer. By knowing the absorbance of the crystal violet, we should be able to calculate the concentration of that material at any time during the run. We will then compile data to arrive at the bigger picture of the lab and develop the rate statement for the reaction. This work will consume a day of lab work. Activation energy and frequency factor: Day two of the lab will involve keeping the concentration of the chemicals constant and then running the reaction at different temperatures. We will use equal volumes of the solutions, heat or cool them to a specific temperature, and then mix them at that temperature. We will keep a record of the time of reaction. Once we collect our data, we need to analyze the data. We will be generating a graph for our data. The Arrhenius equation in its log form is: lnr = (E act /R)(/T) + lna. The rate (r) α /t so this equation becomes ln(/t) = (E act /R)(/T) + lna. Here t = time and E act = the energy of activation for the reaction, T = temperature and A = the frequency factor. This equation is again in the form of a linear equation so if we plot ln(/t) vs /T, we should get a straight line whose slope is E act /R. Since R = the gas constant and has a value of 8. kj/mole, you should be able to calculate the activation energy of this reaction. Finally, the frequency factor is an indication of the number of effective collisions that occur between the crystal violet and the hydroxide ion. The y-intercept for your graph will allow you to calculate A. All of this seems like your lab work and calculations will be daunting, but you will find that we can collect good data and analyze it well. Let s get started!

3 PROCEDURE: Of course, it goes without saying, you will record your data in well organized data tables and everything will be labeled! If you don t, you will get confused easily! Part I: ) Measure out two 0.0 ml samples of crystal violet solution and place each in a ml Erlenmeyer flask. Record the concentration of the crystal violet. It should be close to.0x0 - M. ) Place a stir bar in the flask, and set the stirrer so that it mixes the solution at a moderate rate. ) Measure out 0 ml of the stock NaOH. the concentration of that solution should be close to 0.0 M. Make sure you record its concentration! ) You need to make sure that your computer has the correct program loaded (Logger Pro, in the Advanced Chem file, lab Rate & Order.cmbl), and that the spectrophotometer is connected to the LabPro and functioning properly. ) Now, be ready to work quickly and efficiently! Add the NaOH to the crystal violet, start your stop watch as soon as the first bit of NaOH comes in contact with the crystal violet. Quickly pipette some of the mixture into a cuvette. 6) Insert the cuvette into the spectrophotometer and collect data until the absorbance approaches 0. Save your data to you flash drive. 7) Dilute a sample of the stock NaOH to your assigned concentration and repeat steps # &. (This part of the lab will take the first day!) Running sample set (B) Day # 8) Measure out a sample of crystal violet. Again, its volume should be 0 ml. Place your sample in a ml Erlenmeyer flask. 9) Measure out a 0 sample of NaOH. (Remember, we decided which concentration of NaOH would work best for today s work!) Place your sample in a X0 mm test tube. 0) Prepare a water bath with your assigned temperature and place it in a QT cup. If you have an assigned temperature that is warmer than room temperature, start the bath at a couple degrees above your assigned value. If you have an assigned temperature below room temperature, have a starting temperature a couple degrees below your assigned value. (The temperatures will be ~ o C, o C, 0 o C, o C, and 0 o C.) ) Place an Erlenmeyer flask containing the crystal violet and a test tube containing the NaOH in your QT cup. ) Allow the flask and test tube to remain in the QT cup for ~ minutes. Keep monitoring the temperature to keep it close to your assigned value. ) Now repeat steps 6 for your sample. Of course, it goes without saying, you have recorded your data in a well organized data table! 6

4 Your tables may look something like this: (You will work on a lap top here, and generate these data table for Day # by using Excel. Have the computer do the calculations to fill in columns #,, 6, & 8.) Day # Data Table for varying [Crystal Violet] (From this table, graph ln[cryviolet] vs. ln(/t) Trial Time /t ln(/t) [CryViolet] ln[cryviolet] [NaOH] ln[naoh] 6 7 Data Table for Varying [NaOH] From this table, graph ln[naoh] vs. ln(/t) Share data with your classmates, and pick points where the [CryViolet] is equal on your classmates data and yours. Then graph those points. Trial Time /t ln(/t) [CryViolet] ln[cryviolet] [NaOH] ln[naoh] 6 (You will work on a lap top here, and generate these data table for Day # by using Excel. Have the computer do the calculations to fill in columns #,, 6, & 7.) Day # From this table graph ln(/t) vs. /T Share data with your classmates, and pick points where the [CryViolet] is equal on your classmates data and yours. Then graph those points. Trial Time Rate (/t) ln(/t) Temp. ( o C) Temp. (K) /Temp. (/K) 7

5 QUESTIONS & CALCULATIONS: ) Pick one row on each of the data tables and show how you calculated the values on that row. Remember, state what you are calculating, the equation you are using, show what data along with its units you are substituting into the equation, and your correctly rounded answer. ) Create a hard copy of the graphs for each part of the lab. You need the slope of the best fit lines and the y-intercept for each graph. Each lab partner needs a copy of the graphs. It would really be cool if you generated these graphs using LoggerPro so you get printable hard copies. If you use your hand-held calculator, make sure you place a good sketch of the graph in you lab notebook with the required slopes and intercepts. ) Write: A) Molecular equation for the reaction. B) The general rate statement for this reaction. C) The rate statement for the reaction including the order of the reaction for crystal violet and the OH - species. (RT will walk you through the necessary calculations for this problem during the post-lab session.) ) Use the Arrhenius equation and determine the value for E act and A. ) Discuss logically what would happen to the rate of reaction if the value for E act was larger than what you found. 6) What would happen to the rate of reaction if the value for A was smaller than your measured value? 7) What would happen to the value for E act if your thermometer consistently read the temperature low? Sources: ) History of Chemistry hand out pp ) CHEM NEWS /Oct. 00 ) 8