Question 18

I understand that the reactivity decreases as you go down group 7. In displacement reactions the more reactive halide replaces the least reactive halide, but in this question i do not understand the use of hexane in determining their reactivity series. I also do not understand why 5 test tubes give a purple layer as iodine should not react with sodium bromide.

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    $\begingroup$ For future reference, please crop your screen shot to just the question of interest or use LaTeX tables. $\endgroup$ – Zhe Jun 2 '17 at 17:37
  • $\begingroup$ Halogens belong to group 17 not 7 $\endgroup$ – chail10 Jun 2 '17 at 19:55

Hexane is an organic solvent. It has two important properties for the purposes of this question: it is less dense than water, and it will dissolve the molecular halogens.

I think you've already correctly identified that $\ce{Br2}$ and $\ce{Cl2}$ will react with $\ce{NaI(aq)}$ to produce $\ce{I2}$. The part you didn't consider is that $\ce{I2}$ is present in every mixture in the bottom row of that table because no displacement will occur. Therefore, the hexane solution will be purple.

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