Problem 64 - Chemistry: The Central Science

Question

Write balanced chemical equations that correspond to the steps in the following brief account of the metallurgy of molybdenum: Molybdenum occurs primarily as the sulfide, \(\mathrm{MoS}_{2}\). On boiling with concentrated nitric acid, a white residue of \(\mathrm{MoO}_{3}\) is obtained. This is an acidic oxide; when it is dissolved in excess hot concentrated ammonia, ammonium molybdate crystallizes on cooling. On heating ammonium molybdate, white \(\mathrm{MoO}_{3}\) is obtained. On further heating to \(1200^{\circ} \mathrm{C}\) in hydrogen, a gray powder of metallic molybdenum is obtained.

Step-by-Step Solution

Answer

The balanced chemical equations for the metallurgy steps of molybdenum are: 1. \( \mathrm{MoS}_{2} + 6 \, \mathrm{HNO}_{3} \rightarrow \mathrm{MoO}_{3} + 2 \, \mathrm{H}_{2}\mathrm{SO}_{4} + 6 \, \mathrm{NO} \) 2. \( \mathrm{MoO}_{3} + 2 \, \mathrm{NH}_{4}\mathrm{OH} \rightarrow ( \mathrm{NH}_{4} )_{2}\mathrm{MoO}_{4} + \mathrm{H}_{2}\mathrm{O} \) 3. \( ( \mathrm{NH}_{4} )_{2}\mathrm{MoO}_{4} \rightarrow \mathrm{MoO}_{3} + 2 \, \mathrm{NH}_{3} + \mathrm{H}_{2}\mathrm{O} \) 4. \( \mathrm{MoO}_{3} + 3 \, \mathrm{H}_{2} \rightarrow \mathrm{Mo} + 3 \, \mathrm{H}_{2}\mathrm{O} \)

Step 1: 1. Boiling MoS₂ with concentrated nitric acid

First, molybdenum sulfide (MoS₂) is boiled with concentrated nitric acid (HNO₃). The reaction produces a white residue of molybdenum trioxide (MoO₃). The balanced chemical equation for this reaction is: \[ \mathrm{MoS}_{2} + 6 \, \mathrm{HNO}_{3} \rightarrow \mathrm{MoO}_{3} + 2 \, \mathrm{H}_{2}\mathrm{SO}_{4} + 6 \, \mathrm{NO} \]

Step 2: 2. Dissolving MoO₃ in excess hot concentrated ammonia

Next, the molybdenum trioxide (MoO₃) obtained in the previous step is dissolved in excess hot concentrated ammonia (NH₄OH) solution. On cooling, ammonium molybdate ((NH₄)₂MoO₄) crystallizes. The balanced chemical equation for this reaction is: \[ \mathrm{MoO}_{3} + 2 \, \mathrm{NH}_{4}\mathrm{OH} \rightarrow ( \mathrm{NH}_{4} )_{2}\mathrm{MoO}_{4} + \mathrm{H}_{2}\mathrm{O} \]

Step 3: 3. Heating ammonium molybdate

Ammonium molybdate ((NH₄)₂MoO₄) is then heated, which results in white molybdenum trioxide (MoO₃) being obtained. The balanced chemical equation for this reaction is: \[ ( \mathrm{NH}_{4} )_{2}\mathrm{MoO}_{4} \rightarrow \mathrm{MoO}_{3} + 2 \, \mathrm{NH}_{3} + \mathrm{H}_{2}\mathrm{O} \]

Step 4: 4. Heating MoO₃ in hydrogen

Finally, molybdenum trioxide (MoO₃) is heated to 1200°C in the presence of hydrogen (H₂). The product is a gray powder of metallic molybdenum (Mo) and water (H₂O). The balanced chemical equation for this reaction is: \[ \mathrm{MoO}_{3} + 3 \, \mathrm{H}_{2} \rightarrow \mathrm{Mo} + 3 \, \mathrm{H}_{2}\mathrm{O} \]

Key Concepts

Chemical Equations

Chemical equations serve as the language of chemistry, detailing the reactants and products involved in a chemical reaction. These equations are not just simple mathematical symbols; they represent the conservation of mass, where the number of atoms for each element in the reactants equals the number in the products. For example, in the reaction between molybdenum sulfide (\(\mathrm{MoS}_{2}\)) and nitric acid (\(\mathrm{HNO}_{3}\)), the balanced equation is written as: \[ \mathrm{MoS}_{2} + 6 \, \mathrm{HNO}_{3} \rightarrow \mathrm{MoO}_{3} + 2 \, \mathrm{H}_{2}\mathrm{SO}_{4} + 6 \, \mathrm{NO} \].
The coefficients in front of each formula ensure that the number of each type of atom on both sides is the same. Understanding how to balance these equations is crucial as it lays the foundation for solving problems in chemical reactions, stoichiometry, and even understanding the broader principles of chemistry.

Molybdenum Trioxide

Molybdenum trioxide (\(\mathrm{MoO}_{3}\)) is an important chemical compound in the metallurgy of molybdenum. It serves as a crucial intermediate during the extraction and purification processes of the metal.
Molybdenum trioxide is characterized by its white crystalline appearance and forms when molybdenum sulfide is reacted with concentrated nitric acid:

• The reaction yields molybdenum trioxide, showcasing its role as a key product in extracting molybdenum from its natural sulfide form.
• In subsequent processes, molybdenum trioxide can further react with ammonia and other reagents to advance the extraction steps, eventually forming metallic molybdenum.
• Its ability to transform into pure molybdenum under specific conditions makes it indispensable in various industrial applications.

Recognizing the properties and reactions of molybdenum trioxide is essential for any metallurgy enthusiast.

Hydrogen Reduction

Hydrogen reduction is a pivotal step in transforming molybdenum trioxide into metallic molybdenum. This process involves heating molybdenum trioxide (\(\mathrm{MoO}_{3}\)) with hydrogen gas (\(\mathrm{H}_{2}\)) at high temperatures (about 1200°C):
The chemical equation representing this transformation is:\[ \mathrm{MoO}_{3} + 3 \, \mathrm{H}_{2} \rightarrow \mathrm{Mo} + 3 \, \mathrm{H}_{2}\mathrm{O} \].
This reaction shows the conversion of the oxide to its metallic state, releasing water as a byproduct:

• The high temperature aids in breaking the bonds in molybdenum trioxide, facilitating the formation of metallic molybdenum.
• Hydrogen acts as a reducing agent, removing oxygen from the molybdenum oxide.
• This step is essential for obtaining high-purity molybdenum metal, widely used in steel alloys and other industrial applications.

Understanding hydrogen reduction is critical for those exploring metallurgical processes.

Ammonium Molybdate

Ammonium molybdate ((\(\mathrm{NH}_{4})_{2}\mathrm{MoO}_{4}\)) forms when molybdenum trioxide (\(\mathrm{MoO}_{3}\)) is dissolved in hot concentrated ammonia. As the solution cools, ammonium molybdate crystallizes, purifying the compound and allowing separation:

• The balanced equation for its formation is: \[ \mathrm{MoO}_{3} + 2 \, \mathrm{NH}_{4}\mathrm{OH} \rightarrow ( \mathrm{NH}_{4} )_{2}\mathrm{MoO}_{4} + \mathrm{H}_{2}\mathrm{O} \].
• This compound provides a stable form for molybdenum that can easily be reprocessed into molybdenum trioxide upon heating.
• Its crystalline structure makes it useful for industrial and chemical manufacturing, serving as a precursor in the production of various molybdate compounds and as a corrosion inhibitor.

Recognizing the role of ammonium molybdate in the metallurgy process enhances our ability to process and utilize molybdenum efficiently.