Problem 177 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following reaction at \(1100^{\circ} \mathrm{C}\) [2002] (I) \(2 \mathrm{C}+\mathrm{O}_{2} \longrightarrow 2 \mathrm{CO} \Delta \mathrm{G}^{\circ}=-460 \mathrm{~kJ} \mathrm{~mol}^{-1}\) (II) \(2 \mathrm{Zn}+\mathrm{O}_{2} \longrightarrow 2 \mathrm{ZnO} \Delta \mathrm{G}^{\circ}=-360 \mathrm{~kJ} \mathrm{~mol}^{-1}\) based on these, select correct alternate (a) zinc can be oxidized by \(\mathrm{CO}\) (b) zinc oxide can be reduced by carbon (c) both are correct (d) none is correct

Step-by-Step Solution

Answer

Zinc oxide can be reduced by carbon (b).

Step 1: Understand Reaction I

Reaction I is given as \( 2\, \text{C} + \text{O}_2 \rightarrow 2\, \text{CO} \) with \( \Delta \text{G}^{\circ} = -460\, \text{kJ/mol} \). This indicates that carbon is oxidized to carbon monoxide and the reaction is thermodynamically favorable due to the negative change in Gibbs free energy.

Step 2: Understand Reaction II

Reaction II is given as \( 2\, \text{Zn} + \text{O}_2 \rightarrow 2\, \text{ZnO} \) with \( \Delta \text{G}^{\circ} = -360\, \text{kJ/mol} \). This implies zinc is oxidized to zinc oxide. The negative Gibbs free energy indicates that this reaction is also spontaneous under standard conditions.

Step 3: Analyze Zinc and Carbon Reactions

Since reaction I produces \( \text{CO} \) and reaction II produces \( \text{ZnO} \), we analyze alternate possibilities: whether \( \text{CO} \) can oxidize zinc (first alternate) and if zinc oxide can be reduced by carbon (second alternate). We compare changes in Gibbs free energy to determine possible reactions.

Step 4: Evaluate Alternate (a)

The alternate (a) suggests \( \text{CO} \) can oxidize zinc. However, for zinc to be oxidized by carbon monoxide, the combined reaction should have a negative \( \Delta G^{\circ} \). Calculate net \( \Delta G^{\circ} \) for \( 2\, \text{Zn} + 2\, \text{CO} \rightarrow 2\, \text{ZnO} + 2\, \text{C} \). The input \( \Delta G^{\circ} \) is negative, suggesting this reaction isn't feasible.

Step 5: Evaluate Alternate (b)

The alternate (b) suggests that zinc oxide can be reduced using carbon. For this, calculate the net \( \Delta G^{\circ} \) for the reaction \( 2\, \text{C} + 2\, \text{ZnO} \rightarrow 2\, \text{Zn} + 2\, \text{CO} \). This results in \( \Delta G^{\circ} = -460 + 360 = -100 \text{kJ/mol} \), making the reaction favorable. Thus, carbon can reduce zinc oxide.

Step 6: Conclusion

Based on the evaluated alternatives, alternate (b) is correct as the zinc oxide reduction by carbon leads to a favorable change in Gibbs free energy. Therefore, only (b) is the correct answer.

Key Concepts

Thermodynamics

Thermodynamics is a branch of physics that deals with heat, work, temperature, and energy. It's essential for understanding chemical reactions because it helps predict whether a reaction will occur spontaneously. A key concept in thermodynamics is Gibbs free energy, denoted as \( \Delta G \). This value indicates the amount of free energy capable of doing work during a chemical process.When \( \Delta G \) is negative, as seen in both Reaction I and II from the exercise, the reactions are spontaneous. This means they can occur on their own without external energy input. Let's break down why:

• Reaction I: \( 2 \text{C} + \text{O}_2 \rightarrow 2 \text{CO} \), with \( \Delta G^\circ = -460 \, \text{kJ/mol} \). This indicates carbon is oxidized to carbon monoxide spontaneously.


• Reaction II: \( 2 \text{Zn} + \text{O}_2 \rightarrow 2 \text{ZnO} \), with \( \Delta G^\circ = -360 \, \text{kJ/mol} \). Similarly, zinc spontaneously becomes zinc oxide.

Understanding these values helps us predict which reactions favorably proceed under given conditions. Thus, thermodynamics provides a powerful tool for analyzing and understanding chemical reaction tendencies.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, commonly referred to as redox reactions, involve the transfer of electrons between substances. These reactions are critical because they are responsible for many key biological and chemical processes. In any redox process, one substance is oxidized (loses electrons) while another is reduced (gains electrons). In the exercise provided, both reactions are examples of oxidation:

• In Reaction I, carbon is oxidized to carbon monoxide, releasing electrons.


• In Reaction II, zinc is oxidized to zinc oxide, similarly involving electron loss.

The concept of redox reactions is important because it explains how energy is transferred in reactions. Electrons are small, but they are powerful; their movement is what facilitates these transformations. Redox reactions are vital outside of the chemical lab as well. For example, in biological systems, redox reactions are essential for processes like respiration and photosynthesis, where energy transfer is crucial for life. Understanding these reactions helps in designing batteries, treating pollution, and even in metallurgy, such as the reduction of metal ores, indicating the wide application of these processes.

Chemical Reactions Analysis

Analyzing chemical reactions involves understanding how reactants transform into products, the conditions under which these reactions occur, and the feasibility of the reactions. The analysis of the exercise involves careful examination of Gibbs free energy for determining which reactions are spontaneous and the viability of alternative reactions proposed.In the step-by-step solution, the reaction analysis for zinc and carbon was efficiently tackled:

• Alternative (a): The possibility of zinc being oxidized by carbon monoxide was assessed but required a positive \( \Delta G \), which was not feasible.


• Alternative (b): The reduction of zinc oxide by carbon, leading to zinc and carbon monoxide, was favorable with a negative \( \Delta G \) of \(-100 \text{kJ/mol}\).

Chemical analysis also considers the balance of reactants and products, the energy transformation involved, and the resulting efficiency of the reaction. It's about piecing together each tiny change in the reaction, predicting outcomes, and making decisions that align well with scientific principles.