Question

6. In Experiment 13, the class studied the aquation of [Co(NH3)5Cl]2+. Below are sets of absorbance data taken during this study using 1.2 x 10-2 M [Co(NH3)5Cl]2+ in 0.1 and 0.3 M HNO3.

0.1 M HNO3

t (min) A @ 550 nm

15 0.346

30 0.314

40 0.300

60 0.280

75 0.264

0.3 M HNO3

t (min) A @ 550 nm

15 0.492

30 0.446

40 0.420

60 0.393

75 0.372

The first-order rate constants can be determined using the following equation ln(A-A) = -kt + ln(A0-A) where A0 is the initial absorbance at 550 nm before aquation began to occur.

A. (10 POINTS) What is the value of A? (show work)

B. (20 POINTS) Plot ln(A-A) vs. t and give the values of k and A0 for both sets of data above (attach both plots and show all work).

C. (5 POINTS) Based upon the above rate constants, by which mechanism to you believe that the aquation is occurring? Explain.

Answer 1

A) A_infinity is the absorbance of the solution at infinite time. Experimentall this can be determined by taking an aborbance of the solution after an extended amount of time after completing the experiment. SInce this data has not been taken we can assume that the last data point on this experimental data set as A_infinity. So for the experiment where 1.2 x 10^-2 M [Co(NH₃)₅Cl]²⁺ in 0.1 M HNO₃. A_infinity value will be 0.264

and for the experiment where 1.2 x 10^-2 M [Co(NH₃)₅Cl]²⁺ in 0.3 M HNO₃. A_infinity value will be 0.372.

B) Attached are the data and the plot with the equation of the straight line.

From this data we get the slope which is -k

and from the intercept we get the ln(Ao) (intercept is the point that the straight line meets the y axis which is the initial absorbance Ao)

data with 0.1 M HNO₃

slope = -k = -0.0363

so k the rate constant = 0.0363 min^-1

ln(Ao-A_infinity) is -1.9228

(Ao-A_infinity) = 0.14619

Ao = 0.14619+0.264

Ao = 0.410

Data with 0.3 M HNO₃

slope = -k = -0.0391

so k the rate constant = 0.0391 min^-1

ln(Ao-A_infinity) is -1.4878

(Ao-A_infinity) = 0.2258

Ao = 0.2258+0.372

Ao = 0.598

C) Based upon the rate constant the ligand exchange is happening via a dissociative mechanism. This can be directly understood by the fact that the rates of both the reactions are very similar 0.0363 min^-1 and 0.391 min^-1. In case the ligand exchange was happening via a associative mechanism there would have been a substantial difference in the rates between these two reactions. Because in a dissociative mechanism the rate limiting step in the dissociation of the ligand and so the concentration of the incoming species does not matter.