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A-Level Chemistry- Rate Determining Step

Can someone help explain this question to me?
I know how to determine the rate constant fine, I'm just struggling to understand the rate-determining step part. Even after looking at the mark scheme I'm still confused how you get the Irons.
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Original post by Meggggggggggg
Can someone help explain this question to me?
I know how to determine the rate constant fine, I'm just struggling to understand the rate-determining step part. Even after looking at the mark scheme I'm still confused how you get the Irons.
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rate would be k(Fe3 )(I-)2 as when you double the conc of (Fe3 ) the rate doubles, if you double the conc of (I-), the rate quadruples. Rearranging gives us k= rate/(Fe3 )(I-)2 so you can use experiment 1 to do this, 8.10 x 10-4/ (4.00 x 10-2) x (3.00 x 10-2)2 k= 22.5 and don't forget units would be mol-2dm6s-1 in this case. I get this part but what I can't do is the mechanism? I had the idea of doing Fe3 2I- > FeI2 Fe and then did Fe3 Fe > 2Fe2 but this doesn't work I don't think. Anyone know how to approach this second part?
actually perhaps the two step mechanism could be:
Fe3+ +I2 > FeI2 + Fe+
Fe+ +FeI2 > 2Fe2+ +I2

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