When iodine is added to an acidic solution of acetone, its purple color disappea

Question

When iodine is added to an acidic solution of acetone, its purple color disappears indicating that the iodine has been used up. The reaction is determined to be zero order with respect to the iodine. A solution was made up in the following way: 20 mL 4 M acetone + 10 mL 1.0 M H^+ + 10 mL 0.0050 M I_2 + 10 ml H_2O It took 250 seconds for the iodine color to disappear from that solution. A second reaction mixture was prepared at the same temperature in the following way: 20 mL 4 M acetone + 10 mL 1.0 M H^+ + 20 mL 0.0050 M I_2 How long will it take for the iodine color to disappear in this reaction mixture? Justify your answer.

Explanation / Answer

As this is a zero order reaction :

[I]t - [I]o = kt

Final concentration of Iodine (It) = 0

So, rate constant = [I]o/t

Moles of Iodine added = 0.01L * 0.005 M = 5*10^-5 moles

Substitute the value in the above equation to find out the value of rate constant.

rate constant (k) = 5*10^-5 moles/250 sec = 2 *10^-7 mol/sec

In the first and second reaction , only concentration of I2 is changed . Other things remained same. So, the reaction rate will depend only on concentration of iodine

moles of Iodine used = 0.02 L * 0.005 M = 1*10^-4 moles

[I]t - [I]o = kt

or, (1*10^-4 moles) - 0 = 2 *10^-7 mol/sec * t

or, t = 500 sec

The solution will decolorize in 500sec.

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