Problem 19 - Chemistry in 30 days

Question

On passing \(\mathrm{H}_{2} \mathrm{~S}\) through acidified \(\mathrm{FeCl}_{3}\) solution, \(\mathrm{FeCl}_{3}\) is converted into (a) \(\mathrm{FeCl}_{2}\) (b) \(\mathrm{Fe}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) (c) \(\mathrm{Fe} \mathrm{S}\) (d) \(\mathrm{FeSO}_{4}\)

Step-by-Step Solution

Answer

The product is \(\mathrm{FeCl}_{2}\).

Step 1: Identify the Reaction Components

In this reaction, we are dealing with hydrogen sulfide (\(\mathrm{H}_{2}\mathrm{S}\)) and ferric chloride (\(\mathrm{FeCl}_{3}\)), which is an iron(III) chloride salt.

Step 2: Understand the Reaction Environment

The reaction occurs in an acidified solution. Acidification often facilitates redox reactions, where an exchange of electrons takes place, especially in reactions involving transition metals like iron.

Step 3: Determine Possible Reaction

\(\mathrm{H}_{2}\mathrm{S}\) is a reducing agent and can reduce \(\mathrm{FeCl}_{3}\), which is in the +3 oxidation state, to a lower oxidation state. This often results in the transition of \(\mathrm{Fe^{3+}}\) (from \(\mathrm{FeCl}_{3}\)) to \(\mathrm{Fe^{2+}}\).

Step 4: Predict the Reaction Product

With the reduction of \(\mathrm{FeCl}_{3}\) to \(\mathrm{Fe^{2+}}\), we need to identify the compound formed. \(\mathrm{Fe^{2+}}\) in the presence of chloride ions would form \(\mathrm{FeCl}_{2}\).

Step 5: Analyze Other Options

Other potential products suggested by the options, like \(\mathrm{Fe}_{2}\left(\mathrm{SO}_{4}\right)_{3}\) or \(\mathrm{FeSO}_{4}\), involve sulfate ions, which are not present in significant amounts due to the introduction of \(\mathrm{H}_{2}\mathrm{S}\) alone. \(\mathrm{FeS}\) would imply a direct reaction with \(\mathrm{S^{2-}}\) ions which primarily precipitates in absence of excess chloride ions.

Key Concepts

Hydrogen Sulfide (H₂S)

Hydrogen sulfide, with the chemical formula \(\mathrm{H}_2\mathrm{S}\), is a colorless gas known for its distinctive rotten egg smell. It is a naturally occurring compound produced during the decomposition of organic material. Aside from its natural occurrence, H₂S plays an important role in various chemical reactions.

In chemical reactions, hydrogen sulfide is often used as a reducing agent. A reducing agent is a substance that donates electrons to another substance, thereby reducing the oxidation state of that substance while itself getting oxidized. In the context of the reaction from the exercise, \(\mathrm{H}_2\mathrm{S}\) reduces \(\mathrm{FeCl}_3\) by donating electrons.

• It is commonly found in volcanic gases and hot springs.
• Being a weak acid, it is also soluble in water, forming weakly acidic solutions.

Handling H₂S requires care because it is toxic and flammable at high concentrations.

Ferric Chloride (FeCl₃)

Ferric chloride, or iron(III) chloride, is a chemical compound with the formula \(\mathrm{FeCl}_3\). It appears as a brownish-yellow anhydrous solid.

As a transition metal compound, \(\mathrm{FeCl}_3\) displays common characteristics of transition metals, such as variable oxidation states. In this compound, iron is in the +3 oxidation state, commonly referred to as the "ferric" state.

• It is widely used in water treatment processes, etching printed circuit boards, and as a catalyst in organic synthesis.
• In an aqueous solution, \(\mathrm{FeCl}_3\) undergoes hydrolysis, producing \(\mathrm{HCl}\) and resulting in an acidic medium.

During the redox reaction mentioned in the exercise, \(\mathrm{FeCl}_3\) undergoes reduction by \(\mathrm{H}_2\mathrm{S}\), moving to a lower oxidation state.

Transition Metals

Transition metals are a group of elements located in the central block of the periodic table, specifically groups 3 through 12. These metals include well-known elements like iron (\(\mathrm{Fe}\)), copper (\(\mathrm{Cu}\)), and silver (\(\mathrm{Ag}\)).

Some key characteristics of transition metals include:

• They have multiple oxidation states, allowing them to participate in various types of reactions, including redox reactions.
• Many have the ability to form colored compounds due to d-d electron transitions.
• They often act as catalysts in industrial and biological processes.

Due to their extensive range of oxidation states, transition metals like iron can exist in forms such as iron(II) chloride and iron(III) chloride, with differing reactivity based on their oxidation state.

Reducing Agents

Reducing agents are substances that cause reduction by donating electrons to another compound, thereby becoming oxidized themselves. They are crucial in many chemical reactions, often in conjunction with oxidizing agents.

In our exercise, hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)) acts as the reducing agent. When \(\mathrm{H}_2\mathrm{S}\) reacts with \(\mathrm{FeCl}_3\), it donates electrons to \(\mathrm{FeCl}_3\), reducing iron from its +3 oxidation state (ferric) to a +2 oxidation state (ferrous).

• Reducing agents are essential in processes like metallurgy, where metals are extracted from their ores.
• They play a vital role in redox reactions, which are essential for energy transfer in biological systems.

Understanding the role of reducing agents helps in predicting the outcomes of chemical reactions, such as the formation of \(\mathrm{FeCl}_2\) in the presence of \(\mathrm{H}_2\mathrm{S}\).