Question

Arrange the following oxides in order of increasing acidity: \(\mathrm{CO}_{2}, \mathrm{CaO}, \mathrm{Al}_{2} \mathrm{O}_{3}, \mathrm{SO}_{3}, \mathrm{SiO}_{2}\), and \(\mathrm{P}_{2} \mathrm{O}_{5}\)

Short answer

The order of increasing acidity for the given oxides is: \(\mathrm{CaO} < \mathrm{Al}_{2} \mathrm{O}_{3} < \mathrm{CO}_{2} < \mathrm{SiO}_{2} < \mathrm{SO}_{3} < \mathrm{P}_{2} \mathrm{O}_{5}\)

Step-by-step solution

Identify the nature of the oxides

First, we must determine whether each oxide is basic, amphoteric or acidic. Generally, metal oxides (found on the left side of the periodic table) are basic and non-metal oxides (found on the right side of the periodic table) are acidic. However, there are exceptions, such as some elements that form amphoteric oxides. \(\mathrm{CO}_{2}\): Carbon dioxide is a non-metal oxide, so it will be considered acidic. \(\mathrm{CaO}\): Calcium oxide is a metal oxide, so it will be considered basic. \(\mathrm{Al}_{2} \mathrm{O}_{3}\): Aluminium oxide is amphoteric, meaning that it can act as both an acid and a base. \(\mathrm{SO}_{3}\): Sulfur trioxide is a non-metal oxide, so it will be considered acidic. \(\mathrm{SiO}_{2}\): Silicon dioxide is a non-metal oxide, so it will be considered acidic. \(\mathrm{P}_{2} \mathrm{O}_{5}\): Phosphorus pentoxide is a non-metal oxide, so it will be considered acidic.

Order the oxides based on their location in the periodic table

Now that we have identified the nature of each oxide, we can order them by increasing acidity by looking at their position on the periodic table. Basic oxides will be the least acidic, followed by amphoteric oxides, and finally acidic oxides. Here is the initial order based on their nature: 1. Basic oxides: \(\mathrm{CaO}\) (least acidic) 2. Amphoteric oxides: \(\mathrm{Al}_{2} \mathrm{O}_{3}\) 3. Acidic oxides: \(\mathrm{CO}_{2}, \mathrm{SO}_{3}, \mathrm{SiO}_{2}, \mathrm{P}_{2} \mathrm{O}_{5}\) Within the acidic oxides group, we can arrange them based on their position in the periodic table. The acidity of non-metal oxides increases from left to right across the periodic table and from bottom to top within a group: \(\mathrm{CO}_{2} < \mathrm{SiO}_{2} < \mathrm{SO}_{3} < \mathrm{P}_{2} \mathrm{O}_{5}\)

Combine the groups to get the overall order

Now, we can put the basic oxide, amphoteric oxide, and the ordered acidic oxides together to find the overall order: \(\mathrm{CaO} < \mathrm{Al}_{2} \mathrm{O}_{3} < \mathrm{CO}_{2} < \mathrm{SiO}_{2} < \mathrm{SO}_{3} < \mathrm{P}_{2} \mathrm{O}_{5}\)

Key concepts

Oxide Acidity

Oxides are compounds made up of oxygen and another element. The acidity or basicity of oxides largely depends on the type of element they are combined with, either a metal or a non-metal. Basic oxides are typically formed by metals, and they tend to react with acids. These are usually found on the left side of the periodic table. Non-metal oxides, found on the right side, mostly exhibit acidic behavior and react with bases. An example of a basic oxide is calcium oxide (CaO), while carbon dioxide (CO₂) is an acidic oxide.
\( \text{Acidic oxides react with bases to form water and a salt} \). Consider carbon dioxide, which reacts with sodium hydroxide (a base) to form sodium carbonate and water.

• Basic Oxides: Formed by metals, react with acids.
• Acidic Oxides: Formed by non-metals, react with bases.
• Neutralization: Acidic and basic oxides often react to form salts and water.

This pattern of acidity reflects the nature of the elements involved. How these oxides interact with water also aligns their basic or acidic character, with acidic oxides forming acids and basic oxides forming bases.

Amphoteric Oxides

Amphoteric oxides are unique as they can display both acidic and basic properties. This means they can react with both acids and bases. Aluminum oxide (Al₂O₃) is a classic example of an amphoteric oxide. When reacting with hydrochloric acid (an acid), it forms aluminum chloride and water. When reacting with sodium hydroxide (a base), it forms sodium aluminate and water.
Amphoterism is observed in elements found in certain groups of the periodic table. This dual capacity to interact suggests a balanced reactivity.

• Reaction with Acids: Forms a salt and water.
• Reaction with Bases: Also generates a form of salt and water.
• Location in Periodic Table: Typically elements with amphoteric oxides are located in transition between metals and non-metals.

This property is essential for various industrial applications, especially where precise chemical reactions are needed. It's crucial in metallurgy and the creation of specific compounds.

Periodicity in Chemistry

Periodicity refers to the recurring trends or patterns observed in the periodic table. These patterns arise due to similarities in atomic structure as we move across periods or down groups. Oxidation state, atomic radius, ionization energy, and electronegativity are some properties that are periodic.
The acidity of oxides, for example, shows periodic trends. As you move from left to right across a period, oxides become more acidic. Conversely, as you descend a group, oxides typically become less acidic.

• Across Periods: Oxide acidity increases.
• Down Groups: Oxide acidity typically decreases.
• Significance: Understanding periodicity helps in predicting element reactivities and properties.

These trends aid in predicting how different elements and their oxides will behave in chemical reactions. Being mindful of these patterns helps chemists anticipate the properties of unfamiliar compounds based on their position in the periodic table.