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Experimentally Determining the Rate Expression

Yezi Cho




1. Purpose

The following reaction between iodine ions and iron (III) ions in an aqueous solution was conducted experimentally to determine its rate expression.

As the reaction progresses, the solution becomes a yellow-orange color, which can be measured quantitatively using a colorimeter. By systematically changing the initial concentration of iodine ions and iron (III) ions, the rate order for each of the reactants can be determined. The effect of the initial concentrations on the rate of reaction was recorded for 120 seconds through LoggerPro.




2. Materials and Methods


  • 5cm3 pipette

  • 10cm3 pipette

  • Colorimeter and laptop with Logger Pro

  • Cuvettes

  • 0.020 mol dm–3 KI

  • 0.020 mol dm–3 FeCl3

  • Distilled water

  • Safety goggles


1) Set the wavelength of the colorimeter to 470 nm and calibrate the colorimeter using 1.5 cm3 of distilled water and 1.5 cm3 of KI solution.

2) Create the various concentration manipulations of iodine ion and iron (III) ion solutions by dilution.

3) Add specified amounts of iodine and iron (III) solutions and distilled water into the cuvette.

4) Place the cuvette in the colorimeter and start collecting data every 15 seconds for a total of 120 seconds.

5) Repeat the process above for the four remaining concentrations.

Two trials were collected for each type of solution.




3. Results


Figure1. Absorbances over time of varying concentrations of KI and FeCl3 to determine the rate expression of the reaction between potassium iodide and iron (III) chloride (%)



Absorbance (%) 











Time (s) →


15


30


45


60


75


90


105


120

1:  1.60 mL  KI, 0.80 mL H2O,  0.80 mL FeCl3

Trial 1

0.163


0.185


0.201


0.220

0.228


0.240


0.250

0.253

     

Trial 2

0.121


0.149


0.172


0.194


0.211


0.226


0.241


0.255

Ave. absorbance



0.142


0.167


0.187


0.207


0.220


0.233

0.246


0.254

2:  1.60 mL  KI, 1.20 mL H2O, 0.40 mL FeCl3

Trial 1

0.001

0.001


0.026


0.034


0.038

0.047

0.053


0.059

     

     

Trial 2

0.001

0.001

0.029

0.038

0.049

0.057

0.066


0.072


Ave. absorbance



0.001


0.001


0.028


0.036


0.044


0.052


0.060


0.066


3:  0.80 mL KI, 0.80 mL H2O, 1.60 mL FeCl3

Trial 1

0.092

0.120

0.143

0.165

0.184


0.203

0.219

0.235


     

Trial 2

0.109

0.132


0.154


0.170


0.189


0.204


0.219

0.232


Ave. absorbance



0.100


0.126


0.149


0.168


0.187


0.204


0.219


0.234


4:  0.80 mL KI, 1.60 mL H2O, 0.80 mL FeCl3

Trial 1

0.032


0.048

0.063


0.076


0.087


0.100


0.110


0.119



     

Trial 2

0.027


0.042


0.054


0.064


0.075

0.085


0.093


0.101


Ave. absorbance



0.030


0.045


0.059


0.070


0.081


0.093


0.097


0.110


5:  1.60 mL KI, 0.00 mL H2O, 1.60 mL FeCl3

Trial 1

0.252

0.303

0.337

0.374


0.406


0.440


0.468


0.495



     

Trial 2

0.265

0.324


0.370


0.409


0.443


0.473


0.498


0.523


Ave. absorbance



0.259

0.314

0.354

0.392

0.425

0.457

0.483

0.509




The average absorbances over time were calculated using the two trials.


Figure 2. Average absorbance (%) for 120 seconds for solution 1, solution 2, solution 3, solution 4, solution 5


Average Absorbance (%)





Time

Solution 1

Solution 2

Solution 3

Solution 4

Solution 5

15

0.142

0.001

0.1

0.03

0.259

30

0.167

0.001

0.126

0.045

0.314

45

0.187

0.028

0.149

0.059

0.354

60

0.207

0.036

0.168

0.07

0.392

75

0.22

0.044

0.187

0.081

0.425

90

0.233

0.052

0.204

0.093

0.457

105

0.246

0.06

0.219

0.097

0.483

120

0.254

0.066

0.234

0.11

0.509


Figure 3. Average absorbance for solution 1, solution 2, solution 3, solution 4, solution 5 over time


Figure 4. Slope of graph of absorbance over time


1

2

3

4

5

[KI]

0.01

0.01

0.005

0.005

0.01

[FeCl3]

0.005

0.0025

0.01

0.005

0.01

Slope 

0.00106

0.000659

0.00126

0.000740

0.00233

The concentration of KI and FeCl3 was calculated using the total volume including distilled water. The reaction rate over 120 seconds was determined using the slope of each graph of absorbance (%) over time.




4. Conclusion


Using the slopes as an indication of rate of reaction, the manipulations above show that the orders with respect to KI and FeCl3 are both closest to 1. Thus the rate expression would be Rate = k[KI][FeCl3].


It should be noted that the specific rate constant k cannot be calculated from the data above, as the data recorded was not the concentration of ions but the absorbance.

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