Q4. Find [I−]0 in flask E for 25 mL of A + 25 mL of D.
25 mL of D holds 7.5 mL of 0.1 M KI = 7.5 × 10−4 mol, in 50 mL: 1.5 × 10−2 M (volumes from Lab Manual page 13; reasoned).
Q5. A student finds blue after 72 s with B, 37 s with C and 25 s with D. Is the reaction first order in iodide?
t × [I−]0: 72 × 0.0050 = 0.36, 37 × 0.0100 = 0.37, 25 × 0.0150 = 0.375, nearly constant, so the rate is proportional to [I−]: consistent with first order in iodide within experimental uncertainty (reasoned).
Q6. Why is the watch started when half of the solution from flask B has been poured in?
Mixing takes a few seconds; starting at the halfway point takes the middle of the pouring as time zero, which defines a reproducible approximate time origin; it does not exactly cancel mixing error. Pour and mix consistently and rapidly compared with the clock time (Lab Manual page 13; reasoned).
Q7. What would happen to the time for the blue colour if more thiosulphate were added to every flask?
It would be longer: more iodine must be made before the thiosulphate runs out (Wikipedia, “Iodine clock reaction”).
Q8. Why does the blue appear only after the thiosulphate is used up?
Until then thiosulphate reduces the iodine back to iodide as fast as it forms, so free iodine cannot build up to give the blue complex with starch (Lab Manual page 12).
Q9. Assertion: the time for the blue colour gives the initial rate of the reaction. Reason: only a small fraction of the hydrogen peroxide reacts before the blue appears, and the iodide is regenerated. Are both true, and does the reason explain the assertion?
The assertion is approximately true, and the reason explains the approximation: about 2.3% of the peroxide reacts, so the major concentrations change little. The directly measured quantity is still an average rate; mixing and endpoint delays must also be small (reasoned; see Example 4).
Q10. Suggest how this experiment could be changed to study the effect of temperature on the rate.
Repeat one mixture, say A + B, with the flasks kept in water baths at different temperatures, as the NCERT Laboratory Manual’s Experiment 2.1 does for the thiosulphate–acid reaction (Lab Manual pages 9–10). A shorter time at a higher temperature means a faster reaction. NCERT notes that the rate constant nearly doubles for a 10° rise (NCERT page 78).
Q11. Why does iodine give a blue colour with starch?
Iodine equilibrates with iodide to form triiodide and other polyiodide species. Association of iodine/polyiodide chains with amylose in starch gives the intense blue colour; it is not adequately described as a single isolated I3− molecule simply turning blue (Lab Manual page 12; Rutgers Chemistry demonstration).
Q12. Could another oxidant replace H2O2?
Yes. Peroxodisulphate ions, S2O82−, also oxidise iodide to iodine and are timed the same way (Wikipedia, “Iodine clock reaction”).