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JAMB Chemistry 2019 Paper

Question : 14

Total: 80

chlorine attacks the starch to give the blue-black color
chlorine attacks the iodide ion to give the blue-black color
chlorine oxidizes the iodide ion to produce iodine which attacks the starch to give the blue-black color
iodine attacks the starch to give the blue-black color

Solution:

Concept:

Chlorine oxidizes iodide ions to iodine, which reacts with starch to produce a blue-black color.

Explanation:

Starch iodide paper contains iodide ions,

\mathrm{I}^-

Chlorine gas,

\mathrm{Cl}_2

, is a stronger oxidizing agent than iodine.

So chlorine oxidizes iodide ions to iodine:

\mathrm{Cl}_2 + 2\mathrm{I}^- \rightarrow 2\mathrm{Cl}^- + \mathrm{I}_2

The iodine produced forms a blue-black complex with starch.

Therefore, the key processes are oxidation of iodide to iodine, then reaction of iodine with starch.

Answer:

C. chlorine oxidizes the iodide ion to produce iodine which attacks the starch to give the blue-black color

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