Problem 33 - Chemistry in 30 days

Question

For metal oxides, when \(\Delta G^{\circ}\) is plotted against temperature, there is a point where the graph crosses \(\Delta G=\) zero line. Above this temperature (a) the oxide is stable (b) the oxide is unstable (c) the oxide melts (d) the oxide is reduced

Step-by-Step Solution

Answer

Above this temperature, the oxide is unstable (option b).

Step 1: Understanding Gibbs Free Energy

The change in Gibbs Free Energy (9G) determines the spontaneity of a reaction. If 9G is negative, the reaction is spontaneous, while if 9G is positive, the reaction is non-spontaneous. At 9G = 0, the system is at equilibrium.

Step 2: Graph Analysis for Metal Oxides

In the given problem, the graph of 9G^{6B} for a metal oxide against temperature is considered. The point where the graph crosses the 9G = 0 line indicates the temperature at which the formation of the oxide is in equilibrium.

Step 3: Temperature Implications

At temperatures below the crossover point (9G < 0), the formation of the oxide is spontaneous, making the oxide stable. Above this temperature (9G > 0), the reaction becomes non-spontaneous, meaning the oxide is not stable.

Step 4: Selecting the Correct Option

Given the options, the correct interpretation is that above the temperature where 9G = 0, the oxide cannot form spontaneously and is therefore unstable. Hence, option (b) is the correct answer.

Key Concepts

Metal Oxides

Metal oxides are compounds formed when metals react with oxygen. They are typically ionic compounds and are found in various forms such as

• Iron oxide (rust)
• Aluminum oxide (found in bauxite ore)
• Copper oxide

Metal oxides play significant roles in different applications including catalysts, pigments, and refractory materials. Yet, their stability often depends on various factors, with temperature and reaction spontaneity being key. Studying Gibbs Free Energy changes helps predict these properties. By analyzing a plot of Gibbs Free Energy against temperature for metal oxides, one can understand their behavior at different temperature levels.

Spontaneity of Reactions

In chemistry, a reaction's spontaneity indicates whether it will occur without additional energy. This concept is evaluated using Gibbs Free Energy, denoted as \[ \Delta G \quad (\text{Gibbs Free Energy change}) \]When

• \(\Delta G < 0\), the reaction is spontaneous.
• \(\Delta G = 0\), the system is at equilibrium (neither spontaneous nor non-spontaneous).
• \(\Delta G > 0\), the reaction is non-spontaneous.

With metal oxides, spontaneity helps determine if they can form or decompose under given conditions. Assessing this through Gibbs Free Energy is crucial, especially in industrial applications where energy efficiency is paramount.

Stability of Oxides

The stability of metal oxides is largely dependent on whether the reaction forming them is spontaneous. Stability indicates a compound's resistance to change under specific conditions.When the Gibbs Free Energy graph crosses the zero line, it marks a critical point:

• Below the line, \(\Delta G < 0\), indicating spontaneous formation, hence stable.
• Above the line, \(\Delta G > 0\), indicating non-spontaneous formation, hence unstable.

The oxide maintains stability as long as it remains in favorable conditions, often dictated by a combination of temperature, pressure, and chemical environment.Understanding the stability factor is essential for practical purposes, including storage and handling of these compounds.

Temperature and Gibbs Free Energy

Temperature significantly influences Gibbs Free Energy and, consequently, the stability and spontaneity of metal oxide formation. As temperature fluctuates, the Gibbs Free Energy change \(\Delta G\) varies:

• Below a certain temperature, \(\Delta G\) is negative, which means spontaneous oxide formation.
• Above this temperature, \(\Delta G\) becomes positive, leading to non-spontaneous or unstable conditions for the oxide.

Since temperature alters the thermodynamic properties, it is a crucial factor in manipulating reactions and stability in industrial processes. High temperatures, for example, may be used to decompose oxides, reverse reactions, or prevent unwanted formation. Evaluating the relationship between temperature and Gibbs Free Energy is key for chemists and engineers alike.