Determining Order Of A Reaction Using Labquest and Colorimeter

Order of a reaction


The objective of this experiment is to determine the order of a reaction. For this we will look at the redox reaction between Fe3+ ions and I ions. The equation for the reaction is represented as follows.

· 2Fe(NO3)3 + 2KI ® 2Fe(NO3)2 + 2KNO3 + I2

· 2Fe3+(aq) + 2I(aq) ® 2Fe2+(aq) + I2

The I2 produced in this reaction imparts a color to this solution; hence we can study the rate of formation of Iodine using a colorimeter and measure the change in absorbance using the Beer Lamberts law.

A = elc

‘A’ represents absorbance, which we will be measuring. ‘e’ is molar absorptivity. ‘I0is the intensity of light at zero absorbance or the blank. ‘I’ represents the absorbance of the colored solution or intensity of light passing through the sample solution. If I < I0 some light has been absorbed by the solution. ‘l’ represents the length of the light path through the solution and ‘c’ represents the concentration of the solution.

Using Lab-Quest and Colorimeter.

The chemical reaction

· 2Fe(NO3)3 + 2KI ® 2Fe(NO3)2 + 2KNO3 + I2

· 2Fe3+(aq) + 2I(aq) ® 2Fe2+(aq) + I2

Setting up the equipment’s

· Turn on the lab-quest and connect the colorimeter – Leave the colorimeter turned on at least 5 minutes before you start data collection.

· Lab quest set up.

o Sensors

o Data Collection

o Time Based

o Rate = 1

o Interval = 1

o Length 200 s

o OK


· Colorimeter

o Select the wavelength 470 / 575 nm using < or >

o Prepare a blank; in this case it is the Fe3+ ion solution, before mixing with the KI solution. Fill the cuvette 3/4th with the blank

o Place it in the colorimeter, the clear side of the cuvette should face the white triangle

o Press calibrate. It is a one button calibration.

o When the red light stops blinking and the absorbance on the Lab quest is 000 the instrument is calibrated.


Reagents required:

· Fe3+ 0.02 M in a 100 mL

· 0.02 M KI in a 100 mL

· Distilled water

· Mix the volumes needed according to the table provided (at least 3 trials; 1,2 &3)

NOTE: Do not mix the two solutions until you are ready to record the readings.




FeCl3 (mL)

KI (mL)

H2O (mL)














This table now gives you values which can be used to solve for the order of the reaction using initial rate method.





Initial rate (s–1)



Value of ‘m’ from Labquest













Procedure for data collection:

Note: There should be consistency for the time taken between mixing the two solutions and data collection for all the three trials.

Labquest Setup for data collection:

· Select sensor > Data Collection> Mode> Time Based>

· Rate = 1 sample/ second

· Interval = 1 sample / second

· Duration = 200 seconds


Trial 1

· Pipette 10 mL of Fe3+ (Iron solution) into a 50 mL clean dry beaker followed by 10 mL of I (Potassium Iodide) solution into the same beaker. It would be a good idea to start a stop watch to maintain consistency in time between mixing the reagents and start of data collection. After mixing the solutions properly using a glass rod, transfer the mixture to a clean cuvette using an eye dropper or a beral pipette

· Place the solution in the colorimeter that has been calibrated

· Press the play button to start the data collection

· Wait for 200 seconds for the collection to be complete

Trial 2

· Dispose of the solutions used appropriately or as instructed

· Prepare the solutions for trial 2 using the data table provided.

· After mixing the solution properly transfer it to the colorimeter.

· Place the cuvette in the colorimeter and click the file cabinet icon, now you can collect trial two data in the same file.

· Repeat all these steps for trial three.

Data Analysis on the Labquest:

· Open Run 1

· Select a segment of the graph for 25 or 30 seconds

· Select analyze

· Choose curve fit

· Absorbance

· Linear

· Now write down the value of ‘m’ or slope of line in the data table

· Next select OK

· Choose Run 2 and repeat the process until you find the slopes for all the three runs. Record the data.

· Once you have the values you can calculate the order of the reaction


Note: When the slope is determined after collection of data for all three runs, you get the slope for the same time interval.

Extra care should be taken such that the start time for recording absorbance for all experiments should be the same from the time of mixing of the solutions

Teacher Note the order of the reaction is 1 with respect the iodide ions [I] and zero with respect to Fe3+


The order can be determined using initial rate method calculation

Should give you the order of iodide ions

Solve for ‘x’

‘x’ is the order of Iodide ions

Similarly solve for ‘y’

Should give you the order of Fe3+ ions

Solve for ‘y’

‘y’ is the order of Fe3+ ions

Rate = k[Fe3+]y [I]x

Repeat calculation for the different trials performed to complete the lab.