Question

Classify each of the following reactions as a precipitation, acidbase neutralization, or oxidation-reduction: (a) \(\mathrm{Hg}\left(\mathrm{NO}_{3}\right)_{2}(a q)+2 \mathrm{NaI}(a q) \longrightarrow 2 \mathrm{NaNO}_{3}(a q)+\mathrm{HgI}_{2}(s)\) (b) \(2 \mathrm{HgO}(s) \stackrel{\text { heat }}{\longrightarrow} 2 \mathrm{Hg}(l)+\mathrm{O}_{2}(g)\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{4}(a q)+3 \mathrm{KOH}(a q) \longrightarrow \mathrm{K}_{3} \mathrm{PO}_{4}(a q)+3 \mathrm{H}_{2} \mathrm{O}(l)\)

Short answer

(a) Precipitation, (b) Oxidation-reduction, (c) Acid-base neutralization.

Step-by-step solution

Identify Precipitation Reaction

A precipitation reaction occurs when two aqueous solutions combine to form an insoluble solid, known as a precipitate. In reaction (a), \( \mathrm{Hg}(\mathrm{NO}_{3})_{2}(aq) \) and \( \mathrm{NaI}(aq) \) react to form \( \mathrm{NaNO}_{3}(aq) \) and \( \mathrm{HgI}_{2}(s) \). Since \( \mathrm{HgI}_{2} \) is a solid precipitate, reaction (a) is classified as a precipitation reaction.

Identify Oxidation-Reduction Reaction

In an oxidation-reduction (redox) reaction, electrons are transferred between substances, resulting in a change in oxidation states. For reaction (b), \( 2\mathrm{HgO}(s) \) decomposes into \( 2\mathrm{Hg}(l) \) and \( \mathrm{O}_{2}(g) \). Here, \( \mathrm{Hg} \) goes from a +2 oxidation state in \( \mathrm{HgO} \) to a 0 oxidation state. Since there is a change in oxidation states, reaction (b) is an oxidation-reduction reaction.

Identify Acid-Base Neutralization Reaction

An acid-base neutralization reaction involves an acid and a base reacting to form a salt and water. In reaction (c), \( \mathrm{H}_{3}\mathrm{PO}_{4}(aq) \) (an acid) reacts with \( \mathrm{KOH}(aq) \) (a base) to form \( \mathrm{K}_{3}\mathrm{PO}_{4}(aq) \) (a salt) and \( \mathrm{H}_{2}\mathrm{O}(l) \). This is characteristic of an acid-base neutralization reaction.

Key concepts

Precipitation Reaction

A precipitation reaction occurs when two solutions in aqueous form react to create an insoluble substance, often capturing attention because of a visible change such as the formation of a solid known as a precipitate. This precipitate settles out of the solution, appearing as a solid substance. In chemical equations, this is sometimes indicated by an (s), denoting a solid.

For instance, let's examine how this happens in reaction (a): \( \mathrm{Hg}(\mathrm{NO}_{3})_{2}(aq) + 2 \mathrm{NaI}(aq) \longrightarrow 2 \mathrm{NaNO}_{3}(aq) + \mathrm{HgI}_{2}(s) \). Here, the mercury(II) iodide \( \mathrm{HgI}_{2} \) forms as a solid precipitate from the original aqueous solutions.

• The "(aq)" symbol next to substances like \( \mathrm{Hg}(\mathrm{NO}_{3})_{2} \) and \( \mathrm{NaI} \) shows they are dissolved in water.
• When mixed, they rearrange to form two new substances: \( \mathrm{NaNO}_{3}(aq) \) and \( \mathrm{HgI}_{2}(s) \).
• \( \mathrm{HgI}_{2} \) doesn't dissolve in water, thus "precipitates" as a solid.

Observing this process helps students to see, quite clearly, how new compounds with different properties emerge from a simple chemical reaction.

Oxidation-Reduction Reaction

In oxidation-reduction reactions, commonly known as redox reactions, electrons are transferred between substances, leading to a change in the oxidation states of the involved elements. This kind of reaction is crucial because it underpins numerous processes, from industrial applications to biological systems.

Consider reaction (b): \( 2 \mathrm{HgO}(s) \stackrel{\text { heat }}{\longrightarrow} 2 \mathrm{Hg}(l) + \mathrm{O}_{2}(g) \). Here, mercury (Hg) undergoes a change in oxidation state.

• In \( \mathrm{HgO} \), mercury is in an oxidation state of +2.
• In its liquid state as \( \mathrm{Hg}(l) \), mercury enters an oxidation state of 0.
• This change from +2 to 0 indicates the gain of electrons, classifying this as a reduction for mercury.

Meanwhile, the decomposition of \( \mathrm{HgO} \) contributes to an increase in free oxygen as \( \mathrm{O}_{2}(g) \), highlighting the oxidation process. Being able to identify and understand these changes is central to grasping how energy and matter interact chemically.

Acid-Base Neutralization

An acid-base neutralization reaction takes place when an acid interacts with a base to yield a salt and water. These reactions are pivotal in both laboratory and daily life, as they help adjust pH levels, creating balanced conditions for various applications.

In reaction (c): \( \mathrm{H}_{3}\mathrm{PO}_{4}(aq) + 3 \mathrm{KOH}(aq) \longrightarrow \mathrm{K}_{3}\mathrm{PO}_{4}(aq) + 3 \mathrm{H}_{2}\mathrm{O}(l) \), a classic neutralization reaction unfolds.

• \( \mathrm{H}_{3}\mathrm{PO}_{4} \) is a triprotic acid, meaning it can donate three hydrogen ions (H\(^+\)).
• \( \mathrm{KOH} \) acts as a strong base, able to accept hydrogen ions.
• The reaction forms \( \mathrm{K}_{3}\mathrm{PO}_{4} \), a salt, and water through the combination of H\( ^+ \) from the acid and OH\( ^- \) from the base.

The process highlights the power of opposites attracting, yielding new compounds from these fundamental chemical building blocks. This understanding is fundamental when exploring how substances interact to neutralize and stabilize each other. This insight is especially significant in understanding biological systems and industrial processes.