with the iodine formed initially. Starch indicator is added to form a blue color

Question

 with the iodine formed initially. Starch indicator is added to form a blue color when all the thiosulfate has been used up and the iodine 
concentration begins to rise. The following  sets of mixtures are used.
                                   Initial concentrations in reaction mixtures
Run number                [I-]                      [BrO3-]              [H+]
1                                 0.002              0.008                0.02
2                                 0.002              0.016                0.02
3                                 0.004              0.008                0.02
4                                 0.002              0.008                0.04
When the experiment is repeated at 31 oC, the rate constant is found to be 2.0 times larger. 
              2-
[S2 O3^    ] is equal to 0.0001M in each reaction mixture.
The rate law is :
                    a                b           c
Rate = k [ I-]^   [ BrO3 -]^    [H+]^ 


Reaction time in run number 1    30
Reaction time in run number 2    30
Reaction time in run number 3    30
Reaction time in run number 4    15

Calculate the following
a in the rate law    0
b in the rate law    0
c in the rate law    1
k in the rate law =? 

Explanation / Answer

given that:

rate = k([I-]^a)([BrO3-]^b)([H+]^c]

Also, we have: a = 0, b = 0, c = 1

=> substituting the above values in the rate law gives: rate = k([I-]^0)([BrO3-]^0)([H+]^1]

=> rate = k[H+]

In run number 1: [H+] = 0.02M and run time is 30 => rate = 0.02/30

So,

0.02/30 = k x 0.02

=> k = 1/30 s^-1

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