Question

Among \(\mathrm{A} 1_{2} \mathrm{O}_{3}, \mathrm{SiO}_{2}, \mathrm{P}_{2} \mathrm{O}_{3}\) and \(\mathrm{SO}_{2}\) the correct order of acidic strength is: (a) \(\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{SiO}_{2}<\mathrm{P}_{2} \mathrm{O}_{3}<\mathrm{SO}_{2}\) (b) \(\mathrm{SO}_{2}^{2}<\mathrm{P}_{2} \mathrm{O}_{3}<\mathrm{SiO}_{2}<\mathrm{Al}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{SiO}_{2}<\mathrm{SO}_{2}^{2}<\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{P}_{2} \mathrm{O}_{3}^{\prime}\) (d) \(\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{SiO}_{2}<\mathrm{SO}_{2}<\mathrm{P}_{2} \mathrm{O}_{3}\)

Short answer

The correct order of acidic strength is (a): \(\mathrm{Al}_{2}\mathrm{O}_{3} < \mathrm{SiO}_{2} < \mathrm{P}_{2}\mathrm{O}_{3} < \mathrm{SO}_{2}\).

Step-by-step solution

Understand the concept of acidic strength

The acidic strength of oxides generally corresponds to the position of the element in the periodic table. Elements that are more non-metallic tend to form acidic oxides, and the acidity increases across a period and decreases down a group.

Analyze the given oxides

The oxides in the given question are \(\mathrm{Al}_{2}\mathrm{O}_{3}\), \(\mathrm{SiO}_{2}\), \(\mathrm{P}_{2}\mathrm{O}_{3}\), and \(\mathrm{SO}_{2}\). Aluminum is a metal, while silicon, phosphorus, and sulfur are non-metals. This implies \(\mathrm{Al}_{2}\mathrm{O}_{3}\) should be the least acidic, while the others are more acidic with varying degrees.

Arrange oxides based on non-metallic character

Among the non-metals: sulfur is more non-metallic than phosphorus, which is more non-metallic than silicon. Thus, \(\mathrm{SO}_{2}\) will have the highest acidity, followed by \(\mathrm{P}_{2}\mathrm{O}_{3}\), and \(\mathrm{SiO}_{2}\). \(\mathrm{Al}_{2}\mathrm{O}_{3}\) being amphoteric is the least acidic.

Select the correct order

Based on the above analysis, the correct acidic strength order from least to most acidic is \(\mathrm{Al}_{2}\mathrm{O}_{3} < \mathrm{SiO}_{2} < \mathrm{P}_{2}\mathrm{O}_{3} < \mathrm{SO}_{2}\). This corresponds to option (a).

Key concepts

Periodic Table Trends

When examining the periodic table, several trends become apparent that help in understanding the properties of elements, including their behavior as oxides. As we move across a period from left to right, the non-metallic character of the elements generally increases. This means that elements on the right side of a period, such as phosphorus and sulfur, are more non-metallic compared to elements like aluminum which is a metal. Additionally, as we progress across a period, the acidity of oxides also tends to increase.
Moving down a group, however, the trend is the opposite. The metallic character increases, and therefore, the acidic nature of the oxides formed decreases. For example, sulfur down a group to selenium will show a decrease in the acidic behavior of their oxides. These trends are essential for predicting the acidic strength of oxides and understanding chemical behaviors without direct experimentation.

Acidic Oxides

Acidic oxides are formed when non-metals react with oxygen. These oxides typically dissolve in water to form acids. For example, sulfur dioxide ( SO_2 ) combines with water to form sulfurous acid ( H_2SO_3 ). The acidic nature of these oxides is linked to the electronegativity and non-metallic character of the parent element. The more non-metallic an element, the more likely it will form an acidic oxide.
We can observe this through the progression from aluminum oxide ( Al_2O_3 ), which is amphoteric and has both acidic and basic properties, to sulfur dioxide ( SO_2 ), which is entirely acidic. Understanding the type of oxide formed by an element helps predict its behavior in different chemical environments. Acidic oxides play a crucial role in environmental chemistry, notably in phenomena like acid rain.

Non-Metallic Character

The non-metallic character of an element is a measure of how much an element exhibits properties typical of non-metals. These properties include high electronegativity, the ability to gain electrons easily, and forming acidic oxides which dissolve to form acids. Key characteristics distinguish non-metals from metals and predict how they will react with other elements and compounds.
Sulfur, a highly non-metallic element, will form stronger acidic compounds compared to less non-metallic elements like silicon. The non-metallic character is often evaluated in terms of the position of the element within the periodic table. The further right and up the table an element is, the more non-metallic it is. Sulfur, positioned above phosphorus and silicon, has the strongest non-metallic character among them, hence it forms the most acidic oxide ( SO_2 ) among the components under consideration. By understanding this concept, students can easily deduce the nature and behavior of elemental oxides.