Problem 45 - Chemistry The Molecular Science

Question

Galena, \(\mathrm{PbS},\) is the principal lead ore. To produce lead metal, galena is first reacted with oxygen; the oxide is then reacted with carbon. (a) Write balanced chemical equations for these two reactions. (b) Is either of these reactions a redox reaction? If so, explain your answer by identifying the oxidizing and reducing agent in each case.

Step-by-Step Solution

Answer

(a) 1st: \(2\text{PbS} + 3\text{O}_2 \rightarrow 2\text{PbO} + 2\text{SO}_2\), 2nd: \(\text{PbO} + \text{C} \rightarrow \text{Pb} + \text{CO}_2\). (b) Both are redox reactions; 1st: O\(_2\) is oxidizing, PbS is reducing. 2nd: PbO is oxidizing, C is reducing.

Step 1: Identify the First Reaction

The first reaction involves galena (PbS) reacting with oxygen to form lead(II) oxide (PbO) and sulfur dioxide (SO\(_2\)). Write the unbalanced chemical equation for this reaction: \[ \text{PbS} + \text{O}_2 \rightarrow \text{PbO} + \text{SO}_2 \]

Step 2: Balance the First Reaction

Balance the chemical equation by ensuring that the number of each type of atom on the reactant side equals the number on the product side. The balanced equation is \[ 2\text{PbS} + 3\text{O}_2 \rightarrow 2\text{PbO} + 2\text{SO}_2 \].

Step 3: Identify the Second Reaction

The second reaction involves the reaction of lead(II) oxide (PbO) with carbon to produce lead (Pb) and carbon dioxide (CO\(_2\)). Write the unbalanced equation: \[ \text{PbO} + \text{C} \rightarrow \text{Pb} + \text{CO}_2 \].

Step 4: Balance the Second Reaction

Balance the equation by matching the number of each type of atom on both sides. The balanced equation is \[ \text{PbO} + \text{C} \rightarrow \text{Pb} + \text{CO}_2 \].

Step 5: Determine if the First Reaction is a Redox Reaction

In the first reaction, PbS is oxidized to PbO, and O\(_2\) is reduced to SO\(_2\). Lead in PbS is being oxidized since its oxidation state changes from +2 to +4. Sulfur is also oxidized from -2 to +4. Oxygen is reduced from 0 to -2 in SO\(_2\). Thus, this is a redox reaction.

Step 6: Determine if the Second Reaction is a Redox Reaction

In the second reaction, PbO is reduced to Pb, as the oxidation state of lead changes from +2 to 0. Carbon is oxidized from 0 to +4 as it becomes CO\(_2\). Therefore, the second reaction is also a redox reaction. PbO acts as the oxidizing agent, and carbon acts as the reducing agent.

Key Concepts

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, involve changes in the oxidation state of atoms. These reactions are fundamental in chemistry because they involve the transfer of electrons between substances. In a redox reaction, one substance loses electrons, becoming oxidized, while another gains electrons, becoming reduced.
The terms oxidation and reduction were historically associated with the gain or loss of oxygen in reactions. For example, when iron rusts, it oxidizes as it gains oxygen. However, a broader definition involves changes in electrons. Oxidation is the loss of electrons, while reduction is the gain of electrons. Remember this with the mnemonic "OIL RIG" - Oxidation Is Loss, Reduction Is Gain.

Balancing Chemical Equations

Balancing chemical equations is crucial in chemistry to accurately represent the conservation of mass. An unbalanced equation does not correctly reflect the quantities of reactants and products in a chemical reaction. To balance an equation, every atom must appear the same number of times on both sides of the equation.

• Identify each type of atom in the reaction.
• Adjust coefficients to ensure that each atom is balanced on both sides.
• Start by balancing atoms of elements that appear in only one reactant and product, if possible.

For the reaction of galena with oxygen: 2PbS + 3O₂ → 2PbO + 2SO₂ Each type of atom (Pb, S, and O) appears equally on both sides, showing proper balance. This ensures that the reaction follows the law of conservation of mass.

Oxidizing Agents

An oxidizing agent, or oxidant, causes another substance to lose electrons, itself getting reduced in the process. In redox reactions, the oxidizing agent plays a crucial role by accepting electrons from the reducing agent.
In the first reaction of galena with oxygen, oxygen ( O₂ ) acts as the oxidizing agent. It accepts electrons from lead sulfide ( PbS ), thereby reducing itself to sulfur dioxide ( SO₂ ). This transformation is coupled with the oxidation of lead and sulfur, indicating a classic redox reaction.
The ability of an oxidizing agent to drive oxidation is a key concept in chemistry, defining its effectiveness in gaining electrons.

Reducing Agents

A reducing agent, or reductant, does the opposite of an oxidizing agent; it donates electrons to another substance, itself becoming oxidized. In redox processes, the reducing agent loses electrons, becoming more positive.
In the step where lead(II) oxide ( PbO ) reacts with carbon, the carbon acts as the reducing agent. It donates electrons to lead(II) oxide, reducing it to metallic lead ( Pb ). Carbon, in the form of carbon monoxide ( CO₂ ), becomes oxidized.
The classification of a substance as a reducing agent is pivotal when considering electron transfers in these reactions.

Lead Processing

Lead processing involves several steps that allow the extraction and refinement of lead metal from its ore. The first step is commonly the ore's reaction with oxygen to form lead oxide and sulfur dioxide. This is followed by treating the lead oxide with carbon to produce pure lead metal.

• The initial reaction: 2PbS + 3O₂ → 2PbO + 2SO₂ transforming galena into lead oxide and releasing sulfur dioxide.
• The next step involves reducing lead oxide with carbon: PbO + C → Pb + CO₂. Here, carbon serves to remove oxygen from the lead oxide, freeing up the lead in metallic form.

Understanding this process is key in metallurgy, aiding the industrial production of lead, widely used in batteries, construction, and more.