Question

In a lab experiment: iodination of propanone. Iodine and propanone are mixed. The raection is stopped at intervals by adding a portion of sodium hydrogen carbonate solution. This is then titrated against sodium thiosulphate.

A graph was plotted of sodium thiosulphate volume VS time although sodium thiosulphate does not take part in the reaction. Why is this acceptable? Would it be more accurate to work out the iodine concentration and plot this instead?

Also, how does one ascertain the dependence of the rate upon the iodine concentration?

Answer 1

   CH₃COCH₃ + I₂ ---> CH₃COCH₂I + HI

Propanone    iodine iodopropanone hydrogen iodide

Usually of rate of a reaction depends upon the concentration of reactants.

   - d[I₂]/dt = k[ CH₃COCH₃]^a [I₂]^b

But propanone is in excess amount its concentration remains more or less constant.

So, the rate of reaction is rearranged to   - d[I₂]/dt = constant [I₂]^b

So, the order of reaction b is determined with respect to iodine only. This is determined by the titration of unreacted iodine with standard sodium thiosulphate, reducing agent at different time intervals using starch indicator.

   2S₂O₃^2- + I₂ ---> S₄O₆^-2 + 2I^-

Based on initial concentration of iodine and unreacted iodine concentration, we can determine the iodine concentration that takes part in the reaction accurately i.e change in the concentration of iodine with respect to time.