Rate and Order of a Reaction LabThis is a featured page

In order to determine the rate and order of a reaction of substances, an experiment between these substances must be carried out because rate and order can only be determined by experimental means. The apparatus to be utilized in this experiment is the same as that of the Beer's Law lab, which can be entered through the following link: Beer's Law Lab. The handling of the colorimeter remains the same as this experiment, though the only difference is that blue light waves will be passed through the substances, rather than red.
The reaction being investigated is between potassium iodide and iron (III) chloride, and is exemplified with this formula:
2I^- (aq) + 2Fe^3+ (aq) --> I2 (aq) + 2Fe^2+ (aq)

These substances will undergo this reaction while in the cuvette and by doing so, the rate of the reaction can be determined for later use in the Rate Law.

The concentration of the solutions of each trial is determined by the following:
Concentration (M) = #moles / Volume (L)

The first trial consisted of an equal amount of FeCl3 and KI being reacted. This is because it will allow for a basic trial to compare trials 2 and 3 with. Seeing as 0.02M solutions of FeCl3 and KI were used, their concentrations in trial 1 are the same.

0.02M = #moles / 0.005L
#moles = 0.0001
Concentration = 0.0001mol / 0.01L
Concentration = 0.01M

The amount of each aqueous solution was first converted to moles because the molarity of the original substance will be useless once it is mixed with another aqueous solution. This is because the molecules of the first substance will spread out among the water molecules of the second solution, therefore changing its concentration. For this reason, the number of moles for each solution is compared with the volume of the entire solution in order to determine the concentration.

To find the order of a reaction, each trial must be comparable with at least one other trial in order to see how the concentration of one substance will affect the rate of the reaction. For this reason, trial 2 consisted of a 0.01M FeCl3 solution mixed with a 0.005M KI solution, and trial 3 consisted of a 0.005M FeCl3 solution mixed with a 0.01M KI solution.

Data Collection:

Rate and Order of a Reaction Lab - Science Learning Resources
Figure 1. Graph illustrating the differing absorbancies during the reactions of each trial

[FeCl3] (M)
[KI] (M)
Initial rate (sec^-1)

Table 1. Data collected by means ofthe colorimeter

The initial rates of each reaction was determined using the following formula:
Initial rate = (Final absorbency - Initial absorbency) / 120 seconds

Take trial 1 for example. The final absorbency of trial 1 was 0.620 and the initial absorbency was 0.378.
Initial rate = (0.620 - 0.378)/120 seconds
Initial rate = 0.00202 sec^-1

Data Analysis:
The rate law expression can be determined from the data in table 1. The concentration of KI in trial 2 is half that of trial 1 and this caused the initial rate to decrease. It can then be concluded that the order of [KI] is approximately 1. However, when the concentration of FeCl3 was cut in half with trial 3, the initial rate barely changed, suggesting that the order of [FeCl3] is 0.

The rate constant k must also be determined in order to complete the rate law. This is accomplished by merely plugging in the data collected for each trial.
Rate = k [FeCl3]^0 [KI]^1
0.00202 = k [0.01]
k = 0.202

The complete rate law is:
Rate = 0.202 [KI]

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