Question

Consider this redox reaction: $$ \begin{array}{r} \mathrm{IO}_{4}^{-}(a q)+2 \mathrm{I}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \\ \mathrm{I}_{2}(s)+\mathrm{IO}_{3}^{-}(a q)+2 \mathrm{OH}^{-}(a q) \end{array} $$ When \(\mathrm{KIO}_{4}\) is added to a solution containing iodide ions labeled with radioactive iodine- \(128,\) all the radioactivity appears in \(\mathrm{I}_{2}\) and none in the \(\mathrm{IO}_{3}^{-}\) ion. What can you deduce about the mechanism for the redox process?

Short answer

The mechanism of the given redox process is such that iodide ions (\(\mathrm{I}^{-}\)) are oxidized to form iodine (\(\mathrm{I}_{2}\)), while the iodate (\(\mathrm{IO}_{4}^{-}\)) ion is reduced by water to form iodate (\(\mathrm{IO}_{3}^{-}\)). These two processes are individual of each other.

Step-by-step solution

Understand the Transfer of Radioactivity

As given in the problem, \(\mathrm{I}_{2}\) is radioactive but \(\mathrm{IO}_{3}^{-}\) is not. This indicates that the radioactive iodide ions are becoming \(\mathrm{I}_{2}\) but are not involved in forming \(\mathrm{IO}_{3}^{-}\).

Determine the Iodine Species Involved

Considering the balancing coefficients in the reaction, two iodide ions are consumed for each iodate ion reduced. Given our previous conclusion, we deduce that these two iodide ions combine to form \(\mathrm{I}_{2}\). Therefore, the radioactive iodide ions must play the role of the I- ions in the reaction.

Identify the Reactant in the Reduction of IO4- Ion

The radioactive iodide ions are found in \(\mathrm{I}_{2}\) but not in \(\mathrm{IO}_{3}^{-}\). This infers that they do not directly react with the \(\mathrm{IO}_{4}^{-}\) ion. The \(\mathrm{IO}_{4}^{-}\) ion must therefore be reduced by the water molecule, forming the \(\mathrm{IO}_{3}^{-}\) ion.

Deduction of the Mechanism

Thus, the overall mechanism involves two separate processes: the \(\mathrm{IO}_{4}^{-}\) ion is reduced by water to form \(\mathrm{IO}_{3}^{-}\) and the iodide ions (\(\mathrm{I}^{-}\)) react to form \(\mathrm{I}_{2}\).

Key concepts

Radioactive Tracers in Chemistry

Radioactive tracers are isotopes that emit radiation and are used to track the movement of substances in chemical reactions or in living organisms. They have the same chemical properties as their non-radioactive counterparts but can be detected because of their radioactivity.

They are invaluable tools in research and medicine, for example, in the diagnosis of health conditions using PET scans. In chemistry, tracers provide insights into reaction mechanisms, as they can pinpoint which reactant ends up in which product. The exercise illustrates the use of iodine-128 as a tracer to determine where iodide ions end up in a redox reaction. The fact that all radioactivity is observed in \(\mathrm{I}_2\) but not in \(\mathrm{IO}_3^{-}\) tells us exactly which species the radioactive iodine becomes, allowing scientists to deduce the specific pathway and intermediate steps of the redox process.

Iodine Redox Reactions

Iodine can exist in various oxidation states, making it a versatile element in redox reactions. In the provided exercise, we see iodine transitioning between different species: \(\mathrm{IO}_4^{-}\), \(\mathrm{I}^{-}\), and \(\mathrm{I}_2\). When \(\mathrm{IO}_4^{-}\) (periodate) is reduced, it becomes \(\mathrm{IO}_3^{-}\) (iodate), while \(\mathrm{I}^{-}\) (iodide) is oxidized to form \(\mathrm{I}_2\) (iodine).

The use of radioactive iodine-128 in this reaction helps identify the source of the iodine atoms in the products. It serves as a clear example of how iodine can cycle through different oxidation states and helps elucidate the exact path taken by the iodide ions in the reaction. This insight is critical because it impacts how we balance the reaction and understand the process from a molecular viewpoint.

Chemical Reaction Balancing

Balancing chemical reactions is a foundational concept in chemistry that involves equalizing the number of atoms of each element on both sides of the equation. It is guided by the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction.

In the context of redox reactions, not only must the atoms balance, but also the charges. Balancing involves determining the correct stoichiometric coefficients that will ensure that the number of electrons lost in oxidation equals the number gained in reduction. The exercise illustrates the importance of balancing in understanding complex redox mechanisms. It also highlights that identifying the fate of specific atoms (like radioactive iodine-128 in this case) can inform the balancing process by revealing the underlying stoichiometry of the reaction components.