Consider the data for the following reaction at 25 o C 2 HgCl 2 (aq) + C 2 O 4 2
ID: 876274 • Letter: C
Question
Consider the data for the following reaction at 25 oC
2 HgCl2(aq) + C2O42-(aq) --------2 Cl-(aq) + 2 CO2 (g) + Hg2Cl2(s) Where Rate = [C2O42-]/t
Determine the 'orders for each reactant.
Determine the rate constant for this reaction.
Determine the instantaneous rate of change for HgCl2 when [HgCl2] = O.l22 M and [C2O42- ] = 0.50 M.
Experiment
[HgCl2] (M)
[C2O42-] (M)
Rate (M/s)
1
0.164
0.15
3.2 x 10-5
2
0.164
0.45
2.9 x 10-5
3
0.082
0.45
1.4 x 10-5
4
0.246
0.15
4.8 x 10-5
Experiment
[HgCl2] (M)
[C2O42-] (M)
Rate (M/s)
1
0.164
0.15
3.2 x 10-5
2
0.164
0.45
2.9 x 10-5
3
0.082
0.45
1.4 x 10-5
4
0.246
0.15
4.8 x 10-5
Explanation / Answer
suppose the rate law expression is ,r = k [HgCl2]^a [C2O42-]^b
where a is the order of reaction with respect to HgCl2 and b is order of reaction with respect to C2O4(2-)
From experiments 1 and 2 we can see that when concentration of HgCl2 is same i.e. in both the experiments concentration of HgCl2 is 0.164 M ....and concentration of C2O4(2-) has been tripled from 0.15 M to 0.45 M, the rate of reaction remained unchanged, that means with respect to C2O4(2-) order of reaction is 0..
Also, from experiments 2 and 3 we can see that when concentraton of C2O4(2-) is kept same (0.45 M),and concentration of HgCl2 is halved (0.164/0.082 = 2) rate of reaction is reduced to half (2.9 x 10-5/1.4 x 10-5 = 2.07 almost 2) that means with respect to HgCl2 order of reaction is 1...
thus combining both these facts .. we can say that
rate law,expression ,r = k [HgCl2]
thus overall order of reaction is 1+0 = 1
now to find the rate constant k ...choose any one experiment..suppose experiment 1..
rate = 3.2 x 10-5 M/s
[HgCl2] = 0.164 M
[C2O42-] = 0.15 M
putting these values ..in rate law expression..
3.2 x 10-5 = k X 0.164
Solving for k,
k = 1.95 x 10^-4 s-1
Thus,
The orders with respect to [HgCl2] = 1, and with respect to [C2O4(2-)] = 0
The rate constant for this reaction k = 1.95 x 10^-4 s-1
The instantaneous rate of change for HgCl2 when [HgCl2] = 0.l22 M will be,
Rate = k[HgCl2]
= 1.95 x 10^-4 x 0.122
= 2.38 x 10^-5 M/s
Thus, the instantaneous rate will be 2.38 x 10^-5 M/s