Problem 155 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{H}_{2} \mathrm{~S}\) is passed through \(\mathrm{Hg}_{2}{\underline{\phantom{xx}}}^{2+}\), we get (a) \(\mathrm{Hg}_{2} \mathrm{~S}\) (b) \(\mathrm{HgS}\) (c) \(\mathrm{HgS}+\mathrm{Hg}_{2} \mathrm{~S}\) (d) \(\mathrm{HgS}+\mathrm{Hg}\)

Step-by-Step Solution

Answer

The products are \(\mathrm{HgS} + \mathrm{Hg}\); hence, option (d) is correct.

Step 1: Understanding the Reaction

When hydrogen sulfide (\(\mathrm{H_2S}\)) is passed through a solution containing mercury (I) ions (\(\mathrm{Hg_2^{2+}}\)), a precipitation reaction occurs in which the sulfur from \(\mathrm{H_2S}\) bonds with mercury.

Step 2: Identifying the Products

Since \(\mathrm{Hg_2^{2+}}\) is present, it can form mercury (II) sulfide (\(\mathrm{HgS}\)), which is black and insoluble, rather than \(\mathrm{Hg_2S}\) or a mixture. The reaction also typically results in pure mercury (\(\mathrm{Hg}\)) being deposited.

Step 3: Writing the Balanced Equation

The reaction can be written as: \[ \mathrm{Hg_2^{2+} + H_2S \rightarrow HgS + Hg + 2H^+} \]. This shows \(\mathrm{HgS}\) as a product along with mercury.

Step 4: Selecting the Correct Option

From the products identified, the correct option based on the reaction is \((d) \mathrm{HgS} + \mathrm{Hg}\).

Key Concepts

Mercury Compounds

Mercury compounds are quite interesting because they exhibit unique chemical behaviors due to mercury's distinct properties. One of the most fascinating aspects of mercury is that it can exist in different oxidation states, most commonly +1 and +2. For example, mercury in the +1 oxidation state forms the dimeric ion  Hg_2^{2+} , which is rather unusual. When two mercury atoms bond together in this form, they create a stable ion that behaves similarly to a single ion.

• Mercury (I) sulfide is represented as  Hg_2S , but it is not as commonly encountered as mercury (II) sulfide.
• Mercury (II) sulfide, often seen as  HgS , presents itself in black form known as cinnabar in nature.

This black form of mercury sulfide is a result of mercury bonding with sulfur, showcasing mercury's affinity to form compounds with chalcogens like sulfur. Understanding these mercury compounds' formation is crucial in fields like chemistry and environmental science, where mercury compounds play significant roles.

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking of bonds and the formation of new ones. When considering mercury compounds, a precipitation reaction is a common type that occurs when certain ions in solutions react to form an insoluble solid, or precipitate. Let's break down the reaction step by step with an example involving  H_2S and  Hg_2^{2+} ions.

• As  H_2S is introduced into the solution, sulfur atoms are available to react with mercury ions.
• This reaction results in the formation of mercury (II) sulfide ( HgS ) as a solid, which precipitates out of the solution due to its low solubility in water.

The equation for the reaction is \[\mathrm{Hg_2^{2+} + H_2S \rightarrow HgS + Hg + 2H^+}\]. It shows the balanced process where initial reactants transform into  HgS and metallic mercury ( Hg ). Precipitation reactions like this one are crucial in various industrial and laboratory processes as they provide a means of separating and identifying substances through simple means.

Reaction Stoichiometry

Stoichiometry is the science of measuring and calculating quantities in chemical reactions. It's like setting the perfect recipe to ensure every ingredient is used properly without leftovers. In stoichiometry, you use the balanced chemical equations to understand and quantify the changes taking place during a reaction.

• A balanced equation like \[\mathrm{Hg_2^{2+} + H_2S \rightarrow HgS + Hg + 2H^+}\] is essential as it shows the correct proportions of reactants and products.
• Notice the stoichiometric coefficients indicating molecule ratios necessary for the reaction to complete: 1 molecule of  Hg_2^{2+} reacts with 1 molecule of  H_2S to form 1 molecule of  HgS , 1 molecule of  Hg , and 2 hydrogen ions.

Stoichiometry ensures that every aspect of the reaction is accounted for, which is essential in chemical manufacturing, where exact quantities are crucial for efficiency and safety. Understanding these principles allows chemists to scale reactions up from a lab setting to industrial production without any unwanted surprises.