Question

Consider the following double-displacement reactions. For each, identify the driving force that causes the reaction to go to completion. (a) \(\mathrm{Hg}\left(\mathrm{NO}_{2}\right)_{2}(\mathrm{ay})+\mathrm{H}_{2} \mathrm{~S}(\mathrm{a} q) \longrightarrow\) \(\mathrm{HgS}(s)+2 \mathrm{HNO}_{2}(a q)\) (b) \(\mathrm{MnS}(s)+2 \mathrm{HCl}(a q) \longrightarrow \mathrm{MnCl}_{3}(a q)+\mathrm{H}_{2} \mathrm{~S}(g)\) (c) \(\left.\mathrm{Ba}(\mathrm{OH})_{2}(a q)+\mathrm{H}_{2} \mathrm{SO}_{\text {faq }}\right) \longrightarrow\) \(\mathrm{BaSO}_{4}(x)+2 \mathrm{H}_{2} \mathrm{O}(l)\)

Short answer

The driving force for reaction (a) is the formation of the precipitate \( \mathrm{HgS}(s) \), for reaction (b) is the release of gas \( \mathrm{H}_{2}\mathrm{~S}(g) \), and for reaction (c) it's the formation of the precipitate \( \mathrm{BaSO}_{4}(s) \) and water \( \mathrm{H}_{2}\mathrm{O}(l) \).

Step-by-step solution

Identify the driving force for reaction (a)

Reaction (a) is \( \mathrm{Hg}\left(\mathrm{NO}_{2}\right)_{2}(\mathrm{ay})+\mathrm{H}_{2}\mathrm{~S}(\mathrm{a} q) \longrightarrow \mathrm{HgS}(s)+2\mathrm{HNO}_{2}(a q) \). This reaction proceeds to completion due to the formation of a precipitate, \( \mathrm{HgS}(s) \).

Identify the driving force for reaction (b)

Reaction (b) is \( \mathrm{MnS}(s)+2 \mathrm{HCl}(a q) \longrightarrow \mathrm{MnCl}_{3}(a q)+\mathrm{H}_{2} \mathrm{~S}(g) \). This reaction goes to completion because of the release of a gas, \( \mathrm{H}_{2}\mathrm{~S}(g) \).

Identify the driving force for reaction (c)

The reaction (c) is \( \mathrm{Ba}(\mathrm{OH})_{2}(a q)+\mathrm{H}_{2} \mathrm{SO}_{4}(a q) \longrightarrow \mathrm{BaSO}_{4}(s)+2 \mathrm{H}_{2}\mathrm{O}(l) \). The reaction is driven to completion due to two different driving forces: the formation of a precipitate, \( \mathrm{BaSO}_{4}(s) \), and the formation of water, \( \mathrm{H}_{2}\mathrm{O}(l) \).

Key concepts

Chemical Reaction Completion

Understanding when and why a chemical reaction is completed is a fundamental concept in chemistry. A reaction is considered complete when it has gone to the fullest extent possible under the given conditions, meaning the reactants have been converted to the products as much as possible. This completion is often driven by the formation of a product that removes reactants from the equilibrium mixture, which can be a solid precipitate, a gas, or a weakly ionized substance like water. In the reactions provided, completion is signaled by either the formation of a precipitate, as seen with mercury(II) sulfide (HgS) in reaction (a) and barium sulfate (BaSO4) in reaction (c), or by the eviction of a gas as in reaction (b) where hydrogen sulfide (H2S) is released. These products do not remain in solution and can't reverse into reactants, which drives the reaction towards completion.

Formation of a Precipitate

The formation of a precipitate plays a key role in reactions and is one of the telltale signs that a double-displacement reaction has occurred. A precipitate is a solid that forms when two aqueous solutions react and their ions come together to form an insoluble compound. This is what happens in reactions (a) and (c), where insoluble mercury(II) sulfide and barium sulfate are formed respectively. Precipitation is a critical concept because it not only signifies the progression of a reaction but is also used in various applications such as purifying water, isolating desired products, or detecting the presence of certain ions in a solution.

Release of a Gas

In addition to the formation of a precipitate, the release of a gas is another strong indicator that a reaction has reached completion. When a gas forms in a chemical reaction, it escapes the reaction mixture as bubbles, effectively driving the reaction forward as there's a decrease in the concentration of reactants. In reaction (b), hydrogen sulfide gas (H2S) forms and is released. This effervescence is a clear sign that the reaction is moving towards completion. The release of a gas is a common observation in acid-base reactions, decomposition reactions, and other chemical processes.

Reaction Driving Forces

The driving forces behind chemical reactions are the reasons why reactions proceed. In double-displacement reactions, such as the ones in the exercise, these include the aforementioned formation of precipitates and release of gases. Another fundamental driving force is the formation of a weak electrolyte, like water, that does not substantially ionize in the solution. These driving forces are essentially the factors that push the chemical reaction to completion and dictate the direction in which the reaction moves. Understanding these forces helps chemists to predict the outcome of reactions and to design processes that yield the desired products efficiently.