Problem 56 - Chemistry: The Central Science

Question

(a) Suppose that tests of a municipal water system reveal the presence of bromate ion, \(\mathrm{BrO}_{3}^{-}\). What are the likely origins of this ion? (b) Is bromate ion an oxidizing or reducing agent? Write a chemical equation for the reaction of bromate ion with hyponitrite ion. Green Chemistry (Section 18.5)

Step-by-Step Solution

Answer

Bromate ions (\(\mathrm{BrO}_{3}^{-}\)) in a municipal water system can originate from reactions involving bromide ions and ozone, which are introduced during water disinfection. Bromate ion is an oxidizing agent, as its standard reduction potential is positive. The chemical equation for the reaction of bromate ion with hyponitrite ion is: \(\mathrm{BrO}_{3}^{-} + 6\mathrm{H}^{+} + 3\mathrm{N}_{2}\mathrm{O}_{2}^{2-} \rightarrow \mathrm{Br}^{-} + 3\mathrm{H}_{2}\mathrm{O} + 3\mathrm{N}_{2}\mathrm{O}\)

Step 1: Identify possible origins of bromate ion

Bromate ions can be formed by the reactions involving bromide (Br-) ions and ozone (O3). Bromide ions can be introduced into the water system through the use of seawater or through salts, such as calcium and magnesium bromide. During water disinfection, ozone is often used because it is a powerful oxidizing agent. It can react with bromide ions, leading to the formation of bromate ions.

Step 2: Determine whether bromate ion is an oxidizing or reducing agent

To determine if the bromate ion (\(\mathrm{BrO}_{3}^{-}\)) is an oxidizing or reducing agent, we need to look at its standard reduction potential. The half-reaction for the reduction of bromate ion to bromide ion is: \(\mathrm{BrO}_{3}^{-} + 6\mathrm{H}^{+} + 6\mathrm{e}^{-} \rightarrow \mathrm{Br}^{-} + 3\mathrm{H}_{2}\mathrm{O}\) This reaction has a positive standard reduction potential, which means that it is a thermodynamically favored reduction. Therefore, bromate ion acts as an oxidizing agent.

Step 3: Write the chemical equation for the reaction of bromate ion with hyponitrite ion

First, write down the half-reactions for both species involved: Bromate ion: \(\mathrm{BrO}_{3}^{-} + 6\mathrm{H}^{+} + 6\mathrm{e}^{-} \rightarrow \mathrm{Br}^{-} + 3\mathrm{H}_{2}\mathrm{O}\) Hyponitrite ion: \(\mathrm{N}_{2}\mathrm{O}_{2}^{2-} \rightarrow \mathrm{N}_{2}\mathrm{O} + 2\mathrm{e}^{-}\) Now, balance the electrons transferred in both half-reactions and combine them to form the overall reaction: \(\mathrm{BrO}_{3}^{-} + 6\mathrm{H}^{+} + 6\mathrm{e}^{-} \rightarrow \mathrm{Br}^{-} + 3\mathrm{H}_{2}\mathrm{O}\) \(3(\mathrm{N}_{2}\mathrm{O}_{2}^{2-} \rightarrow \mathrm{N}_{2}\mathrm{O} + 2\mathrm{e}^{-})\) -------------------------------------------------------------------------------------- \(\mathrm{BrO}_{3}^{-} + 6\mathrm{H}^{+} + 3\mathrm{N}_{2}\mathrm{O}_{2}^{2-} \rightarrow \mathrm{Br}^{-} + 3\mathrm{H}_{2}\mathrm{O} + 3\mathrm{N}_{2}\mathrm{O}\)

Key Concepts

Oxidizing and Reducing Agents

In chemical reactions, the role of oxidizing and reducing agents is pivotal for the transfer of electrons between species. An oxidizing agent, often referred to as an oxidant, gains electrons and is reduced during the process. Conversely, a reducing agent donates electrons and is oxidized.

To identify if a species acts as an oxidizer or reducer, one must refer to its standard reduction potentials. A positive value indicates it favors a reduction reaction, thus serving as an oxidizing agent. The bromate ion, \[\begin{equation}\mathrm{BrO}_{3}^{-}\end{equation}\], illustrates this by having a positive reduction potential, allowing it to oxidize other substances by accepting electrons. In the context of our exercise, this characteristic signifies that bromate can effectively remove electrons from hyponitrite ion, \[\begin{equation}\mathrm{N}_{2}\mathrm{O}_{2}^{2-}\end{equation}\], showcasing its role as a powerful oxidizing agent.

Chemical Reaction Equations

Chemical reactions are represented by chemical reaction equations, which provide a symbolic representation of the transformation of reactants into products. A balanced equation adheres to the law of conservation of mass, ensuring that the number of atoms for each element remains the same on both sides.

For our purpose, we observed the reaction between bromate ion and hyponitrite ion. Balancing the half-reactions for electrons helps us understand the electron transfer process. Each half-reaction is carefully matched to account for the exchanged electrons, after which the two are combined for the overall balanced equation. This guided process is essential for students to accurately present chemical changes in a manner that highlights the stoichiometry and conservation principles inherent in the reactions.

Water Disinfection Process

The water disinfection process is a crucial step in ensuring the safety and cleanliness of water for human consumption. In this process, harmful microorganisms are eliminated or deactivated to prevent the spread of waterborne diseases. Chemicals like ozone, chlorine, and chloramine are common disinfectants.

Ozone, a potent oxidant, reacts with naturally occurring bromide ions in water sources, potentially producing bromate ions as byproducts. The efficacy of ozone is due to its ability to rupture the cell walls of bacteria and viruses, rendering them inactive. However, care must be taken to manage its application because it can also react with other species, leading to unintended compounds like bromate, which we must monitor and control to safeguard public health.