Problem 18 - Chemistry in 30 days

Question

The heat of formation of \(\mathrm{MgO}, \mathrm{Al}_{2} \mathrm{O}_{3}\) and \(\mathrm{SiO}_{2}\) are \(-602,-1676\) and, \(-911 \mathrm{~kJ} \mathrm{~mol}^{-1}\), respectively, Most stable oxide is (a) \(\mathrm{MgO}\) (b) \(\mathrm{Al}_{2} \mathrm{O}_{3}\) \(\begin{array}{ll}\text { (c) } \mathrm{SiO}_{2} & \text { (d) Cannot be predicted } \\ \text { enfoge }\end{array}\)

Step-by-Step Solution

Answer

The most stable oxide is \( \mathrm{Al}_{2} \mathrm{O}_{3} \).

Step 1: Understanding Heat of Formation

The heat of formation is the change in enthalpy that occurs when one mole of a substance is formed from its elements in their standard states. The more negative the heat of formation, the more stable the compound is since it indicates that more energy is released during formation.

Step 2: Analyzing Given Data

We are given the heats of formation for three oxides: - \( \mathrm{MgO} = -602 \text{ kJ/mol} \) - \( \mathrm{Al}_{2} \mathrm{O}_{3} = -1676 \text{ kJ/mol} \) - \( \mathrm{SiO}_{2} = -911 \text{ kJ/mol} \).A more negative value signifies greater stability.

Step 3: Comparing Heat of Formation Values

Compare the given heat of formation values of each oxide. - \( -602 \text{ kJ/mol} \) for \( \mathrm{MgO} \) - \( -1676 \text{ kJ/mol} \) for \( \mathrm{Al}_{2} \mathrm{O}_{3} \) - \( -911 \text{ kJ/mol} \) for \( \mathrm{SiO}_{2} \). The value for \( \mathrm{Al}_{2} \mathrm{O}_{3} \) is the most negative.

Step 4: Conclude the Most Stable Oxide

Since \( \mathrm{Al}_{2} \mathrm{O}_{3} \) has the most negative heat of formation value, \(-1676 \text{ kJ/mol} \), it is the most stable oxide among the listed options.

Key Concepts

Understanding Enthalpy Change

Enthalpy change is a crucial concept in chemistry, representing the heat absorbed or evolved during a chemical reaction at constant pressure. It is often symbolized as \( \Delta H \). When we talk about the heat of formation, we're referring to a specific type of enthalpy change. This refers to the enthalpy change when one mole of a compound is formed from its elements in their standard states.
An important aspect of enthalpy change is its sign. A negative \( \Delta H \) indicates an exothermic reaction, meaning that energy is released as heat, which often leads to increased stability of the compound formed. In the context of oxides like \( \mathrm{MgO}, \mathrm{Al}_{2} \mathrm{O}_{3} \), and \( \mathrm{SiO}_{2} \), the more negative the enthalpy change, the more stable the compound is considered to be.

• Exothermic reactions release heat and generally form stable products.
• Endothermic reactions absorb heat and might result in unstable products.

Understanding the concept of enthalpy change is essential for predicting the stability and reactivity of various chemical compounds.

Exploring Oxide Stability

Oxide stability is a measure of how likely an oxide is to maintain its chemical structure without decomposing or reacting further. It is directly influenced by the heat of formation: the greater the magnitude of the negative value, the more stable the oxide is.
Among the oxides given - \( \mathrm{MgO} \), \( \mathrm{Al}_{2} \mathrm{O}_{3} \), and \( \mathrm{SiO}_{2} \) - the oxide with the most negative heat of formation is \( \mathrm{Al}_{2} \mathrm{O}_{3} \). This means \( \mathrm{Al}_{2} \mathrm{O}_{3} \) is the most stable oxide.
When considering stability:

• Stable oxides like \( \mathrm{Al}_{2} \mathrm{O}_{3} \) do not easily decompose.
• They are less reactive under various environmental conditions.
• Materials with highly stable oxides are often used in applications requiring durability.

So, oxide stability is a key factor in determining the potential uses and longevity of materials in various industrial and practical applications.

Thermodynamics in Chemistry

Thermodynamics plays a pivotal role in understanding the flow of energy during chemical reactions. It deals with concepts such as enthalpy, entropy, and free energy, which help predict whether a reaction will occur spontaneously and how much energy it will consume or release.
The principles of thermodynamics are vital for analyzing the energy changes in reactions and the stability of compounds. Specifically, the first law of thermodynamics, which is the conservation of energy principle, states that energy cannot be created or destroyed. This principle helps understand how chemical reactions like the formation of oxides are energetically feasible.
In the case of oxide formation:

• The measurement of enthalpy change gives insight into the energy exchange.
• Negative enthalpy changes indicate reactions releasing energy, thus favoring product stability.
• Energy considerations also dictate reaction pathways and mechanisms.

Through thermodynamic concepts, chemists can better predict the behavior of substances in various environments, optimizing reactions for industrial processes, energy production, and material science.