Problem 92 - Chemistry Principles and Practice

Question

Sphalerite is the naturally occurring mineral zinc sulfide, from which zinc metal is extracted. The ore is heated in oxygen to form zinc oxide and sulfur dioxide, followed by the reaction of metal oxide with elemental carbon to form CO. (a) Write a balanced equation for each of these reactions. (b) For each reaction, identify the element that is oxidized and the one that is reduced.

Step-by-Step Solution

Answer

(a) 2ZnS + 3O₂ → 2ZnO + 2SO₂; ZnO + C → Zn + CO. (b) In first reaction: S is oxidized, O is reduced. In second reaction: C is oxidized, Zn is reduced.

Step 1: Understand the Reactions

Firstly, identify the two reactions occurring in the extraction of zinc from sphalerite. The first reaction involves heating zinc sulfide (ZnS) with oxygen to yield zinc oxide (ZnO) and sulfur dioxide (SO₂). The second reaction involves reducing zinc oxide (ZnO) with carbon to produce zinc (Zn) and carbon monoxide (CO).

Step 2: Write Balanced Equations for Reactions

For the first reaction: ZnS + O₂ → ZnO + SO₂ Balance it: 2ZnS + 3O₂ → 2ZnO + 2SO₂ For the second reaction: ZnO + C → Zn + CO This equation is already balanced as written.

Step 3: Identify Oxidized and Reduced Elements in Reaction 1

In the first reaction (2ZnS + 3O₂ → 2ZnO + 2SO₂), sulfur in ZnS is oxidized as it goes from an oxidation state of -2 in ZnS to +4 in SO₂. Oxygen is reduced because it goes from an oxidation state of 0 in O₂ to -2 in ZnO and SO₂.

Step 4: Identify Oxidized and Reduced Elements in Reaction 2

In the second reaction (ZnO + C → Zn + CO), carbon is oxidized. It goes from an oxidation state of 0 in C to +2 in CO. Zinc is reduced, going from an oxidation state of +2 in ZnO to 0 in Zn.

Key Concepts

Chemical Reactions

Chemical reactions are at the heart of zinc extraction from sphalerite, which is zinc sulfide (ZnS). These processes involve breaking and forming chemical bonds, converting reactants into products with different properties.

During extraction, two main reactions occur:

• The oxidation of zinc sulfide to zinc oxide and sulfur dioxide.
• The reduction of zinc oxide to zinc metal, using carbon.

In the first reaction, when zinc sulfide is heated with oxygen, each atom rearranges. The sulfur in ZnS binds with oxygen atoms to form sulfur dioxide (SO₂), while the zinc forms zinc oxide (ZnO). This transformation exemplifies a chemical change, discernible by the formation of new products with new properties.

In the second reaction, zinc oxide (ZnO) undergoes reduction as elemental carbon facilitates this chemical process, turning ZnO into zinc metal (Zn) and carbon monoxide (CO). Each reaction step underscores the importance of chemical changes in extracting useful metals from their ores.

Oxidation-Reduction

Oxidation-reduction, or redox, is a type of chemical reaction involving the transfer of electrons between elements. This electron exchange alters the oxidation state of elements and is crucial in zinc extraction from sphalerite.

In the oxidation process, an element loses electrons, increasing its oxidation state. Meanwhile, reduction involves gaining electrons, which decreases the oxidation state.

For zinc extraction:

• In the first reaction (2ZnS + 3O₂ → 2ZnO + 2SO₂), the sulfur in zinc sulfide loses electrons, moving from an oxidation state of -2 in ZnS to +4 in SO₂. Thus, sulfur is oxidized.
• Oxygen, conversely, gains electrons as it goes from 0 in O₂ to -2 in both ZnO and SO₂, meaning it is reduced.
• In the second reaction (ZnO + C → Zn + CO), carbon is oxidized as it moves from an oxidation state of 0 in C to +2 in CO.
• In turn, zinc is reduced, moving from a +2 oxidation state in ZnO to 0 in metallic zinc.

Understanding these redox reactions sheds light on how elements change and interact during chemical transformations.

Balanced Equations

The practice of writing balanced equations is essential in chemistry as it ensures that the law of conservation of mass is followed. This fundamental principle states that matter cannot be created or destroyed in a chemical reaction.

When zinc is extracted from sphalerite:

• The equation for the first reaction is 2ZnS + 3O₂ → 2ZnO + 2SO₂. In balancing, we ensure the number of each type of atom on the reactant side is equal to that on the product side: 2 zinc atoms, 2 sulfur atoms, and 6 oxygen atoms.
• For the second reaction, the balanced equation ZnO + C → Zn + CO already has equal atoms on both sides: 1 zinc atom, 1 carbon atom, and 1 oxygen atom.

Balancing equations not only ensures accurate representation of reactions but helps with understanding stoichiometry—the quantitative relationship between reactants and products in a chemical reaction. This skill is essential for predicting the amounts of substances consumed and formed in reactions, as seen in zinc extraction processes.