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# rate determining step watch

1. The student then investigates the reaction of hydrogen, H2, and iodine monochloride, ICl. The equation for this reaction is shown below.

H2 + 2ICl --> 2HCl + I2

The rate equation for this reaction is shown below.

rate = k[H2][ICl]

Predict a possible two step mechanism for this reaction. The first step should be the rate determining step.

I don't even know where to begin, I tried reading the worked example in the book but I don't understand it at all. Can someone please help me out?
2. (Original post by tammie123)
The student then investigates the reaction of hydrogen, H2, and iodine monochloride, ICl. The equation for this reaction is shown below.

H2 + 2ICl --> 2HCl + I2

The rate equation for this reaction is shown below.

rate = k[H2][ICl]

Predict a possible two step mechanism for this reaction. The first step should be the rate determining step.

I don't even know where to begin, I tried reading the worked example in the book but I don't understand it at all. Can someone please help me out?
The orders of the components in the rate equation give you the particles that collide in the RDS, plus any particles that form intermediates in a prior step that are needed for the RDS.

In this case you can see that the order wrt the two components is 1.

So you postulate that 1 hydrogen molecule collides with one ICl in the RDS.

BUT the sum of the mechanistic steps must give you the stoichiometric equation. So you need a subsequent step in which ICl collides.

This gives you (possibly):

H2 + ICl --> HCl + HI ........slow - RDS
HI + ICl --> HCl + I2 ......... fast

Add these steps up and you get the stoichiometric equation.
3. (Original post by charco)
The orders of the components in the rate equation give you the particles that collide in the RDS, plus any particles that form intermediates in a prior step that are needed for the RDS.

In this case you can see that the order wrt the two components is 1.

So you postulate that 1 hydrogen molecule collides with one ICl in the RDS.

BUT the sum of the mechanistic steps must give you the stoichiometric equation. So you need a subsequent step in which ICl collides.

This gives you (possibly):

H2 + ICl --> HCl + HI ........slow - RDS
HI + ICl --> HCl + I2 ......... fast

Add these steps up and you get the stoichiometric equation.
Oh, I think I understand now. So it's kind of like trial and error in order to get a reaction mechanism that fits?
4. (Original post by tammie123)
Oh, I think I understand now. So it's kind of like trial and error in order to get a reaction mechanism that fits?
Pretty much ...

The orders of the rate equation only provide evidence for the rate determining step. After that it's a process of logical determination, but there is absolutely no assurance that your final answer is actually correct.

However, providing that you have followed the procedure correctly and:

1. The molecularity of the RDS (and previous steps making intermediates needed for the RDS) fits the orders of the rate equation.
2. The sum of all postulated steps adds up to the stoichiometric equation
3. There are no steps involving more than two particle collisions.

Then your guess is as good as anyone else's.

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Updated: March 17, 2013
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