Question

What effect does acid have on the rate of the reaction between acetone and iodine? Hint: what is its function?

Answer 1

Acetone Iodine reaction:

Acetone reacts with iodine in two steps. The first step is rate determining, it is a slow enolization. Acetone reacts with protons from the acid to form an enol.

The second is a fast reaction that complexes molecular iodine to the enol. Therefore, the first step in this reaction is the rate determining step. Acids can be used as catalysts for this reaction (sulfuric acid was used in this case).

To determine the rate constant of the Acetone Iodine reaction at different temperatures and concentrations of reagents.

             The rate constant is a quantity that represents the speed of the chemical reaction. A small value for the rate constant corresponds to a slow reaction and a large rate constant corresponds to a fast, more efficent reaction. The rate of a reaction is affected by temperature.

The prediction is that for this Acetone Iodine reaction the higher the temperature is, the faster the reaction will go resulting in larger values for the rate constant. The rate of this reaction is also dependant on the concentration of the acid and acetone.

A higher concentration of acid should increase the rate of the reaction while a low concentration of acid will slow the reaction. A high concentration of acetone should decrease the rate of reaction and a low concentration of acetone should reduce the reaction time.

The rate of the reaction can be achieved by monitoring the concentration of one species as it changes in time throughout the reaction.

Iodine was monitored because it absorbs in the visible range (470nm). If we assume that iodine reacts quickly with the enol created in step one, the concentration of Iodine can be observed to monitor the progress of the first step of the reaction (the rate determining step).

                The reaction is observed at 470 nm because it is the isobestic point for the I2 I3- system. In the reaction, free iodine reacts with Iodide ions in solution. This forms equilibrium:

          Therefore, the absorption is observed at this isobestic point because at this point I2 and I3- have the same absorption coefficient at this wavelength (470nm).

The absorbance at this point will reflect the total amount of Iodine in the system whether it is free or bound. The absorbance can be monitored using a RedTide spectrophotometer .