Question

For kinetic study of the reaction of iodide ion with \(H _2 O _2\) at room temperature : 
(A) Always use freshly prepared starch solution 
(B) Always keep the concentration of sodium thiosulphate solution less than that of \(KI\) solution 
(C) Record the time immediately after the appearance of blue colour 
(D) Record the time immediately before the appearance of blue colour 
(E) Always keep the concentration of sodium thiosulphate solution more than that of \(KI\) solution Choose the correct answer from the options given below :

• A. (A), (B), (C) only
• B. (A), (D), (E) only
• C. (D), (E) only
• D. (A), (B), (E) only

Answer 1

The Correct Option is A

To determine the correct answer for the given problem, let's break down each statement provided in the context of a kinetic study involving the reaction between iodide ion and hydrogen peroxide (\(H_2O_2\)) at room temperature.

1. The first statement (A) is: "Always use freshly prepared starch solution."
   • Explanation: Iodine-starch complex forms a deep blue color, which is a classic indicator in iodometry. The starch solution should be freshly prepared because it tends to decompose or grow bacteria over time, which can interfere with the accuracy of the test. Hence, this statement is correct.
2. The second statement (B) is: "Always keep the concentration of sodium thiosulphate solution less than that of \(KI\) solution."
   • Explanation: In the iodine-clock reaction, sodium thiosulphate is used to reduce iodine back to iodide until the thiosulphate is exhausted, allowing a sudden formation of the blue color. To delay color appearance effectively, the concentration of sodium thiosulphate solution should be less than that of \(KI\) solution to ensure enough KI is present to replace the reduced iodide. Thus, this statement is correct.
3. The third statement (C) is: "Record the time immediately after the appearance of blue colour."
   • Explanation: In a kinetic study, precise time measurement when the blue color appears indicates a sudden drop in the concentration of thiosulphate and signals the end-point of the reaction being monitored. Recording time immediately after the color appears allows us to accurately measure reaction time. Thus, this statement is correct.
4. The fourth statement (D) is: "Record the time immediately before the appearance of blue colour."
   • Explanation: This contradicts the typical practice in iodometry where timing immediately after the appearance of blue color is essential. Therefore, this statement is incorrect.
5. The fifth statement (E) is: "Always keep the concentration of sodium thiosulphate solution more than that of \(KI\) solution."
   • Explanation: This is unnecessary and contrary to the typical setup of the iodine-clock experiment where sodium thiosulphate concentration is maintained to ensure it is completely consumed to yield the expected stop point. Hence, this statement is incorrect.

After considering the explanations above, the correct set of statements is (A), (B), (C) only. This aligns with the typical experimental setup for studying kinetics using the iodine clock reaction.