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

Both (b) zinc oxide can be reduced by carbon and (a) zinc can be oxidized by CO are analyzed, but only (b) is correct.

Step 1: Understanding the Concepts

The exercise requires us to analyze the given reactions and their Gibbs free energy changes (\(\Delta G^{\circ}\)) to determine the thermodynamically feasible reactions.

Step 2: Gibbs Free Energy and Reaction Feasibility

In any reaction, a negative \(\Delta G^{\circ}\) indicates a spontaneous reaction under standard conditions. Here, both reactions (I and II) have negative \(\Delta G^{\circ}\), suggesting both are spontaneous.

Step 3: Analyze Reaction (I)

Reaction (I) is \(2 \text{C} + \text{O}_2 \rightarrow 2 \text{CO}\) with \(\Delta G^{\circ} = -460 \text{ kJ/mol}\). This means carbon can reduce other compounds to form CO, as it strongly favors this transformation by releasing energy.

Step 4: Analyze Reaction (II)

Reaction (II) is \(2 \text{Zn} + \text{O}_2 \rightarrow 2 \text{ZnO}\) with \(\Delta G^{\circ} = -360 \text{ kJ/mol}\). Zinc can be oxidized by other substances capable of providing \(\text{O}_2\).

Step 5: Determining if Zinc Oxide can be Reduced by Carbon

For zinc oxide to be reduced by carbon, carbon must be able to donate \(\text{O}_2\) to ZnO forming CO. The reaction would be \(\text{2C} + \text{2ZnO} \rightarrow \text{2CO} + \text{2Zn}\). This reaction must have a negative \(\Delta G^{\circ}\) calculated as sum of given reactions, \(\Delta G^{\circ} = -460 + 360 = -100 \text{ kJ/mol},\) indicating it is feasible.

Step 6: Evaluating Each Option

(a) Zinc can be oxidized by \(\text{CO}\). The standard potential direction is from \(\text{O}_2\) to CO, but CO as a reducing agent won't oxidize zinc directly without additional energy. (b) Zinc oxide can be reduced by carbon as shown in step 5. Hence, option (b) is correct.

Key Concepts

Reaction Feasibility

In chemistry, understanding whether a reaction is likely to occur involves analyzing its feasibility. This is typically indicated by the Gibbs free energy change (\(\Delta G^{\circ}\)) for the reaction.

• A negative \(\Delta G^{\circ}\) signifies that the reaction is spontaneous under standard conditions, which means it is thermodynamically favorable.
• A spontaneous reaction indicates that once started, the process can continue without needing an external energy input.

Considering the exercise's reactions, both have negative \(\Delta G^{\circ}\) values, implying these reactions are feasible at the specified temperature.
Determining reaction feasibility is crucial in chemical processes, as this understanding can guide how reactions are performed in an industrial or laboratory setting.

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, play a fundamental role in chemistry. In these reactions, the transfer of electrons occurs from one substance to another, involving oxidation and reduction processes.

• Oxidation involves the loss of electrons from a substance, which becomes more positive as a result.
• Reduction involves the gain of electrons, where the substance becomes more negative.

In the given exercise, the two reactions involve the transfer of oxygen:
- For Reaction (I), carbon is oxidized to carbon monoxide.
- For Reaction (II), zinc is oxidized to zinc oxide.
Understanding the redox processes in these reactions helps in determining which substances could act as reducing or oxidizing agents. In practical applications, identifying agents that can facilitate electron transfer is essential for designing efficient chemical systems.

Thermodynamics in Chemistry

Thermodynamics is a branch of physical science that uncovers how different forms of energy are interconverted in chemical processes. Central to this is the Gibbs free energy (\(\Delta G\)) concept, which predicts the spontaneity and feasibility of reactions.

• \(\Delta G = \Delta H - T \Delta S\)

Here:

• \(\Delta H\) represents the enthalpy change, indicating heat absorbed or released.
• \(\Delta S\) is the entropy change, representing disorder or randomness increase.
• \(T\) is the absolute temperature, in Kelvin

Analyzing the exercise with these principles clarifies why the reactions are feasible. A negative \(\Delta G^{\circ}\) suggests both reactions are thermodynamically favored, highlighting the importance of energy distribution and entropy in predicting chemical behavior.
Thermodynamics provides a framework not only for predicting reaction feasibility but also for understanding energy transformations that affect reaction rates and equilibria.