Lab 2 Worksheet PDF

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CHEM 105B Lab Work...

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Chem 105b Lab 2

Integrated Rate Law – the relationship between concentration and time Order

Integrated Rate Law

Units for rate constant (k)

zero

Differential Rate Law k[A]0 = k

[ A] t  kt [ A] 0

first

k[A]1

ln[ A]t  kt  ln[ A]0

second

k[A]2

1 1  kt  [ A]t  A 0

amount M , such as time s 1 1 , such as time s 1 1 , such as ( amount )( time ) ( M )( s )

In this lab you will be using data from the reaction of bleach with blue dye to determine the order for the reaction in terms of both. Typically, when we use the integrated rate equations, we have set up an experiment so that we force the reaction to depend on the concentration of only one of the reactants, if there is more than one. In this case, the concentration of the bleach is much higher than the concentration of the blue dye. The initial concentration of the blue dye is 1.00 x 10-5 M and the initial concentration of the bleach is 0.20 M. Even if all of the blue dye is used up, the final concentration of the bleach would still be measure as 0.20 M. The concentration and time data collected will be used to determine the order of the reaction with respect to the blue dye. The experiment is repeated two more times at different concentration of bleach and that will allow us to determine the order of the reaction with respect to the bleach.

We are measuring the absorbance of the blue dye over time at a specific wavelength. Because of the reaction with the bleach the absorbance will decrease as time progresses. Here is the initial information that is helpful for answering the questions below. The absorbance versus time data can be downloaded from the Blackboard site. You will need to work with this data in EXCEL. Initial Concentration Blue Dye: 1.00 x 10-5 M Initial Concentration of Bleach (OCl-): 0.20 M Wavelength of Maximum Absorbance: 629 nm A calibration curve is found on the next page for the absorbance of the Blue dye versus concentration.

Blue Dye Calibration Curve

y = (1.02 x 105)x - 0.01 R² = 1.00

1.2

Absorbance at 629 nm

1

0.8

0.6

0.4

0.2

0 0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

Concentration of Blue Dye (M)

Complete the following steps to analyze the provided data: 1. Download the file: Blue Dye Time Data from Blackboard. This file contains three different sheets which contain absorbance of blue dye versus time data collected with the addition of three different amounts of bleach: 1.0 mL, 2.0 mL, and 4.0 mL. 2. In the sheet labeled 1mL Bleach there are a few columns that are already populated, four have time, which is in seconds, the other is absorbance at 629 nm. There are three other columns that are labeled, where you will calculate the values listed in the labels. 3. For the Concentration column, use the molar absorptivity from the calibration curve above to convert the absorbance in column B to concentration. The concentration will be equal to the absorbance divided by the molar absorptivity. Rather than having to use your calculator for each of the 900 different times, we can have Excel do the calculations for us. If you type = and an operation, Excel will do the calculation. The operation that we want to do is to take the absorbances in column B and divide by the molar absorptivity. You can reference a specific cell by typing it’s location, column letter followed by the number of the row. For example the cell with the first absorbance is B2. Let’s say you had a molar absorptivity of 150 (not the actual value), if you type =B2/150 into cell E2, the resulting number will be the concentration of the blue dye. You can copy the operation down the whole column by clicking on the little square on the bottom right corner of cell E2 and scrolling down. 4. For the ln(concentration) column, we can also have Excel do the calculations for us. In cell H2 type =ln(E2) the number that is returned will be the natural log of the concentration determined

using the molar absorptivity in step 3. You can copy the operation down the whole column by clicking on the little square on the bottom right corner of cell H2 and scrolling down. 5. For the 1/concentration column, we can also have Excel do the calculations for us. In cell K2 type =1/(E2) the number that is returned will be the natural log of the concentration determined using the molar absorptivity in step 3. You can copy the operation down the whole column by clicking on the little square on the bottom right corner of cell K2 and scrolling down. 6. Repeat steps 3 – 5 for the 2mL Bleach and 4mL Bleach sheets. 7. Now create each of the following plots: Sheet 1mL Bleach 1mL Bleach 1mL Bleach 2mL Bleach 2mL Bleach 2mL Bleach 4mL Bleach 4mL Bleach 4mL Bleach

Plot concentration vs time ln(concentration) vs time 1/concentration vs time concentration vs time ln(concentration) vs time 1/concentration vs time concentration vs time ln(concentration) vs time 1/concentration vs time

x-axis time time time time time time time time time

y-axis concentration ln(concentration) 1/concentration concentration ln(concentration) 1/concentration concentration ln(concentration) 1/concentration

8. On each plot add a trend line, including the equation for the line and an R2 value. 9. For each of the sets of data determine which line gives the best fit to a straight line: 1mL Bleach 2mL Bleach 4mL Bleach 10. Identify the order of the reaction with respect to Blue dye.

11. Indicate the slopes of the best fit straight lines for the identified order of the reaction with respect to Blue dye: 1mL Bleach 2mL Bleach 4mL Bleach 12. How do the slopes of the lines change as the amount of bleach changes?

13. What does this tell you about the order of the reaction with respect to the bleach?

14. What is the general form for the differential rate law of the reaction of Blue dye with bleach?

Lab Report: 1. Upload a file that contains the plots made in this lab period. Be sure to include clear labels, including on the axes, equations for trendlines, R2 values, and to adjust the axes as needed to eliminate blank space, highlighting the data. 2. What is the general form of the differential rate law for this reaction? 3. How certain are you in the orders of the reactants that you determined? 4. What further experiments would you perform to learn more about the kinetics of this reaction? 5. What challenges would there be if both reactants had initial concentrations of the same magnitude?...