What is a clock reaction?”
A reaction that shows a sharp dramatic colour change after a certain time has elapsed, making it ideal for studying kinetics.
Why are clock reactions useful for studying reaction rates?
Because the sudden colour change provides a clear, easy-to-measure end point, allowing determination of reaction time.
What is the general colour change seen in an iodine clock reaction?
A sudden blue-black colour due to the formation of iodine-starch complex.
What is the main reaction in a common iodine clock reaction using hydrogen peroxide?
H2O2(aq) + 2 I⁻(aq) + 2 H⁺(aq) → I2(aq) + 2 H2O(l)
What is the role of sodium thiosulfate in the clock reaction?
It reacts with iodine as it forms (2S2O3²⁻ + I2 → 2 I⁻ + S4O6²⁻), delaying the appearance of the blue-black colour and acting as a timer.
Why is sodium thiosulfate added in a small controlled amount?
So it reacts with iodine until it is used up, at which point free iodine is detected by starch; this allows timing of the reaction.
Why is sulfuric acid used in excess?
To maintain acidic conditions and ensure the reaction rate is controlled by the other reagents; the amount is so large it can be measured with a measuring cylinder.
How is the reaction started in CP 13b?
By adding a measured volume (e.g., 1 cm³) of hydrogen peroxide and immediately starting a timer.
When is the timer stopped in the iodine clock reaction?
When the blue-black starch-iodine colour appears, indicating iodine is in excess.
Why is hydrogen peroxide sometimes given as ‘10 vol’ or 3% in schools?
‘10 vol’ means 1 cm³ of H2O2 produces 10 cm³ of O2 gas upon decomposition; 3% is approximately 0.979 mol dm⁻³.
What is the decomposition reaction of hydrogen peroxide?
2 H2O2(aq) → O2(g) + 2 H2O(l)
What does the sudden appearance of the blue-black colour indicate in terms of reactants?
It indicates that sodium thiosulfate has been used up and free iodine is present.
Why should solutions be measured in burettes where possible?
To ensure accurate and reproducible measurements of reactants, minimizing experimental error.
Why can sulfuric acid be measured in a measuring cylinder rather than a burette?
Because it is in large excess; small variations in its volume will not significantly affect reaction time.
What is the order of reaction with respect to hydrogen peroxide or iodide in typical clock reaction analysis?
The order can be deduced by varying the concentration of H2O2 or I⁻ and measuring the time for the colour change, but it is usually first order with respect to H2O2.
What is the purpose of using starch in the iodine clock reaction?
Starch forms a blue-black complex with iodine, giving a clear visual endpoint.
How can the rate of the reaction be calculated?
Rate = 1 / time taken for the blue-black colour to appear (since time ∝ 1 / rate).
Why is the blue-black colour change sudden rather than gradual?
Because the iodine is initially removed by thiosulfate until it is exhausted; once excess iodine appears, the starch complex forms immediately.
What practical tip ensures accurate timing in CP 13b?
Have all solutions and equipment ready, start the timer exactly when hydrogen peroxide is added, and observe carefully for the first appearance of the blue-black colour.
What is a key safety consideration when handling hydrogen peroxide and sulfuric acid?
Wear gloves and goggles; H2O2 is an irritant and strong oxidiser; H2SO4 is corrosive.
