What is the balanced equation for the reaction in Core Practical 13a?
CH₃COCH₃(aq) + I₂(aq) → CH₃COCH₂I(aq) + H⁺(aq) + I⁻(aq)
Which reaction is used for the titration to determine remaining iodine?
2 S₂O₃²⁻(aq) + I₂(aq) → 2 I⁻(aq) + S₄O₆²⁻(aq)
Why are propanone and acid used in large excess in this experiment?
So their concentrations remain effectively constant during the reaction and the rate depends only on iodine concentration.
What do you plot on the graph to deduce order with respect to iodine?
Plot titre (or iodine concentration) vs time; the shape or gradient shows the order.
If the graph of concentration vs time is a straight line, what is the order with respect to iodine?
Zero-order with respect to iodine (rate independent of [I₂]).
What is the rate expression if the reaction is zero order in iodine and first order in propanone and hydrogen ions?
Rate = k [CH₃COCH₃][H⁺]
What is the purpose of adding sodium hydrogencarbonate to the withdrawn sample?
To stop (quench) the reaction immediately and fix the concentration of iodine at that time.
What equipment and concentrations are used in the procedure?
25 cm³ of 1.0 mol dm⁻³ propanone + 25 cm³ of 1.0 mol dm⁻³ sulfuric acid, 50 cm³ of 0.02 mol dm⁻³ iodine solution.
At what interval is a 10 cm³ sample withdrawn from the reaction mixture?
Every ~3 minutes (or suitable regular interval) after the start of the reaction.
Why must the stopwatch/clock be kept running and not stopped during sampling?
Because the exact time when sodium hydrogencarbonate is added (i.e., when the sample is quenched) is essential for accurate time-concentration data.
What is one major safety hazard associated with this practical?
The product iodopropanone is a lachrymator (strong eye irritant); propanone is highly flammable; sulfuric acid is corrosive.
What is meant by a continuous monitoring method in this context?
Taking periodic samples of the reaction mixture, quenching them, and titrating to get data points of concentration over time.
How do you use the titration data to obtain iodine concentration?
Titrate the sample with sodium thiosulphate to find moles of I₂ remaining; convert to concentration using sample volume.
If the graph of titre vs time shows a curve that flattens as time progresses, what might this indicate?
That the reaction is not zero-order in iodine or that other assumptions (excess reagents) are invalid.
Why is it important to mix the reaction mixture well when iodine is added?
To ensure uniform concentration and that the reaction will proceed uniformly from the moment timing starts.
Why is the initial concentration of iodine low (0.02 mol dm⁻³) compared to propanone and H₂SO₄?
Because iodine is the reactant being monitored; keeping it lower allows measurable changes in concentration over time while others remain effectively constant.
What should you note in your log when you withdraw a sample?
The exact time you add sodium hydrogencarbonate (quench time) and any delay in measurement; record sample volume, titration reading, etc.
After plotting the concentration-time graph, what is one way to check the reaction order other than the straight-line test?
Use the half-life method (for first order half-life constant) or plot rate vs concentration and see if it gives a straight line.
