[lormak]

Hi

I was not sure which forum to post this q so apologies if its in the wrong place.

I am trying to get to grips with reaction mechanisms for the GAMSAT its been years since I have studied chemistry can someone explain how I would go about determining the elementary reaction steps in a reaction. I appreciate there is a rate of reaction but I am referring to the individual inter reaction molecules which form and then break down to form the final products. I just have difficulty trying to determine which molecules would form and then break down

For example I have pasted this reaction from another site

1.  The reaction between H2 and I2 to form hydrogen iodide was originally postulated as a simple one step reaction.

H2 + I2 = 2Hl

Rate = k[H2][I2]

But, the formation of Hl has been explained on the basis of the following mechanism:

(I don't understand how to determine these steps)

            I2  →  2I     (fast)         . (i)

      H2 + I  →  H2I   (fast)          . (ii)

 H2I + I  →  2HI        (slow)        . (iii)

__________________________      

    Overall  H2+I2  →  2HI


 

Cheers

[Compaq]

These reactions are chemical equilibrium. Take, for example,

I₂ ⇌ 2I

This means, that in a given temperature, some I₂ will go to become 2I, and at the same rate, that 2I will become I₂. That was poorly explained, but I hope you understand. I₂ is broken down to 2I at the same rate as 2I forms I₂.

To apply that to your situation, lets add the correct arrows first:

I₂  ⇌  2I
H₂ + I  ⇌  H₂I
H₂I + I  ⇌  2HI

Now, when working with chemical equilibriums, one can add the three reactions to form the net reaction. (We can also multiply the equilibrium constants, and get the K for the net rxn)

Let's imagine we cross out those compounds that are present on both sides of the ⇌:

I₂ is alone on the left side, so we don't scratch that one. The two I atoms can be erased because there's the 2I molecule on the right side. H₂I is also present on both sides, and so we remove them. Now, we're left with the net reaction:

H₂ + I₂ ⇌ 2HI


I hope that helped a little bit. It might not be the best way to try to explain it, but I tried

[Schrödinger]

@Compaq : I think lormak's doubt lies in determining the sequence of steps. I think he wants to know how one can actually determine what the intermediates are.

@lormak : It is not possible to predict the intermediate always. It takes some amount of intuition, and even that doesn't help at times where the intermediates can be predicted only after experimental results are available.

[lormak]

Apologies for late reply. Have been away

Thankyou both for your help.

Compaq your advice is useful as it helps to develop my understanding

As Schrödinger wrote my problem lies with determining steps.

To anyone,

From your experiences are students generally not  asked to determine elementary steps in exams? Although one may still be required to calculate rate of reaction

Thanks