An indicator 'X' is used for studying the effect of variation in concentration of iodide on the rate of reaction of iodide ion with H_2O_2 at room temp. The indicator 'X' forms blue coloured complex with compound 'A' present in the solution. The indicator 'X' and compound 'A' respectively are :

D and F Block Elements PYQ — JEE Main 2023 · 29 Jan · Shift 2 · Q13 with Solution

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Correct answer: D

Identify the reaction being studied

The reaction of iodide ion with hydrogen peroxide is typically: $\mathrm{H_2O_2 + 2I^- + 2H^+ \rightarrow I_2 + 2H_2O}$

So, during the reaction, iodine $\mathrm{I_2}$ is produced.
Indicator used to detect iodine

In such rate experiments, the usual indicator for iodine is starch.

Starch forms a characteristic blue coloured complex with iodine.
Identify compound 'A'

Since the blue complex is formed with the species produced in the reaction, compound 'A' must be iodine.
Check the options
- A: Methyl orange and $\mathrm{H_2O_2}$ — incorrect, methyl orange does not form blue complex with $\mathrm{H_2O_2}$ .
- B: Methyl orange and iodine — incorrect, methyl orange is not the blue-complex indicator for iodine.
- C: Starch and $\mathrm{H_2O_2}$ — incorrect, starch does not form blue complex with $\mathrm{H_2O_2}$ .
- D: Starch and iodine — correct.
Final answer

Therefore, the indicator $X$ is starch and compound $A$ is iodine.

View written solutionFree

Correct answer: D

Identify the reaction being studied

The reaction of iodide ion with hydrogen peroxide is typically: $\mathrm{H_2O_2 + 2I^- + 2H^+ \rightarrow I_2 + 2H_2O}$

So, during the reaction, iodine $\mathrm{I_2}$ is produced.
Indicator used to detect iodine

In such rate experiments, the usual indicator for iodine is starch.

Starch forms a characteristic blue coloured complex with iodine.
Identify compound 'A'

Since the blue complex is formed with the species produced in the reaction, compound 'A' must be iodine.
Check the options
- A: Methyl orange and $\mathrm{H_2O_2}$ — incorrect, methyl orange does not form blue complex with $\mathrm{H_2O_2}$ .
- B: Methyl orange and iodine — incorrect, methyl orange is not the blue-complex indicator for iodine.
- C: Starch and $\mathrm{H_2O_2}$ — incorrect, starch does not form blue complex with $\mathrm{H_2O_2}$ .
- D: Starch and iodine — correct.
Final answer

Therefore, the indicator $X$ is starch and compound $A$ is iodine.