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 oxides arranged in order of increasing acidity are: \(\mathrm{CaO} < \mathrm{Al}_{2} \mathrm{O}_{3} < \mathrm{SiO}_2 < \mathrm{CO}_2 < \mathrm{P}_{4} \mathrm{O}_{10} < \mathrm{SO}_3\) (Calcium oxide) \(\mathrm{CaO} < \mathrm{Al}_{2} \mathrm{O}_{3}\) (Aluminum oxide) \(< \mathrm{SiO}_2\) (Silicon dioxide) \(< \mathrm{CO}_2\) (Carbon dioxide) \(< \mathrm{P}_{4} \mathrm{O}_{10}\) (Phosphorus pentoxide) \(< \mathrm{SO}_3\) (Sulfur trioxide).

Step-by-step solution

Classify the Oxides based on Metals and Non-Metals

In the given set of oxides, we need to first identify the metal and non-metal based oxides. Metal oxides are typically basic in nature, whereas non-metal oxides are acidic. \(\mathrm{CO}_{2}\) - Non-metal oxide (Carbon dioxide) \(\mathrm{CaO}\) - Metal oxide (Calcium oxide) \(\mathrm{Al}_{2} \mathrm{O}_{3}\) - Metal oxide (Aluminum oxide) \(\mathrm{SO}_{3}\) - Non-metal oxide (Sulfur trioxide) \(\mathrm{SiO}_{2}\) - Non-metal oxide (Silicon dioxide) \(\mathrm{P}_{4} \mathrm{O}_{10}\) - Non-metal oxide (Phosphorus pentoxide)

Determine the Acidic Character of Non-metal Oxides

Non-metal oxides are acidic in nature, and their acidity increases with increasing electronegativity of the non-metal. Hence, we can compare the electronegativity values of non-metals to determine their acidic nature. Here are the electronegativity values of the non-metals in the given oxides: Carbon: 2.55 Sulfur: 2.58 Silicon: 1.90 Phosphorus: 2.19 Based on these electronegativity values, we can deduce the order of increasing acidity for non-metal oxides as: \(\mathrm{SiO}_2 < \mathrm{CO}_2 < \mathrm{P}_{4} \mathrm{O}_{10} < \mathrm{SO}_3\)

Determine the Acidic Character of Metal Oxides

Metal oxides are basic in nature, and their basicity decreases with increasing electronegativity of the metal. Some metal oxides, like aluminum oxide, exhibit amphoteric behavior, which means that they can act as both an acid and a base. In this step, we can compare the electronegativity values of the metals to determine their acidic nature. Here are the electronegativity values of the metals in the given oxides: Calcium: 1.00 Aluminum: 1.61 Based on these electronegativity values, we can deduce the order of increasing acidity for metal oxides as: \(\mathrm{CaO} < \mathrm{Al}_{2} \mathrm{O}_{3}\)

Combine the order of Acidic Character for Non-metal Oxides and Metal Oxides

With the order of increasing acidity determined in Steps 2 and 3, we can now combine the two orders to form a comprehensive order of increasing acidity for all the given oxides: \(\mathrm{CaO} < \mathrm{Al}_{2} \mathrm{O}_{3} < \mathrm{SiO}_2 < \mathrm{CO}_2 < \mathrm{P}_{4} \mathrm{O}_{10} < \mathrm{SO}_3\) Thus, the oxides arranged in order of increasing acidity are: (Calcium oxide) \(\mathrm{CaO} < \mathrm{Al}_{2} \mathrm{O}_{3}\) (Aluminum oxide) \(< \mathrm{SiO}_2\) (Silicon dioxide) \(< \mathrm{CO}_2\) (Carbon dioxide) \(< \mathrm{P}_{4} \mathrm{O}_{10}\) (Phosphorus pentoxide) \(< \mathrm{SO}_3\) (Sulfur trioxide).

Key concepts

Electronegativity

Electronegativity is a key concept in understanding the behavior of atoms in compounds. It is the measure of an atom’s ability to attract and bind with electrons. The higher the electronegativity, the stronger the atom attracts electrons.
This property is crucial in determining the acidity or basicity of oxides. Typically, non-metal oxides display acidic characteristics, while metal oxides are basic. This is because non-metals, with high electronegativity, attract electrons more strongly, allowing them to form acidic solutions when dissolved in water.

For instance, in determining the acidity of non-metal oxides like \(\mathrm{CO}_{2}, \, \mathrm{SO}_{3}, \, \mathrm{SiO}_{2},\) and \(\mathrm{P}_{4} \mathrm{O}_{10},\) the electronegativity values play a pivotal role:

• Carbon: 2.55
• Sulfur: 2.58
• Silicon: 1.90
• Phosphorus: 2.19

Using these values, the order of increasing acidity follows the order of their electronegativity: \(\mathrm{SiO}_2 < \mathrm{CO}_2 < \mathrm{P}_{4} \mathrm{O}_{10} < \mathrm{SO}_3.\) Thus, electronegativity not only helps in understanding how oxides interact with water but also aids in predicting their chemical behavior.

Metal oxides

Metal oxides, as their name suggests, are compounds formed between metals and oxygen. Most of these oxides are basic due to the lower electronegativity of the metals involved. The nature of these oxides can determine their basicity when reacting with water or acids.
Basic metal oxides, such as \(\mathrm{CaO}\) (Calcium oxide), typically dissolve in water forming alkaline solutions like calcium hydroxide, \(\mathrm{Ca(OH)_2}.\)
Some metal oxides like aluminum oxide, \(\mathrm{Al}_{2} \mathrm{O}_{3},\) can exhibit both acidic and basic properties, making them amphoteric. This dual nature allows them to react with both acids and bases.

To determine their basicity or potential to act as acids, we consider the electronegativity of the metal:

• Calcium: 1.00 (lower electronegativity, thus more basic)
• Aluminum: 1.61 (higher electronegativity, making it partially acidic)

Using the electronegativity values, we arrange their increasing acidity as \(\mathrm{CaO} < \mathrm{Al}_{2} \mathrm{O}_{3}.\)
Understanding the properties of metal oxides helps in various applications, from industrial uses to environmental chemistry, making their study crucial.

Non-metal oxides

Non-metal oxides are usually acidic due to the high electronegativity of non-metals. These oxides, when dissolved in water, form acids. For example, carbon dioxide \((\mathrm{CO}_2)\) forms carbonic acid \((\mathrm{H_2CO_3})\) in water.
Non-metal oxides illustrate a clear picture of how electronegativity affects acidity;
as mentioned earlier, the higher the electronegativity of the non-metal, the stronger its acidic nature.

Taking some examples like sulfur trioxide \((\mathrm{SO}_3)\) and phosphorus pentoxide \((\mathrm{P}_4 \mathrm{O}_{10}),\) we observe that:

• Sulfur, with an electronegativity of 2.58, forms sulfuric acid, which is strong and widely used.
• Phosphorus, with an electronegativity of 2.19, forms phosphoric acid upon hydration, commonly used in food and industry.

Other non-metal oxides such as \(\mathrm{SiO}_{2},\) due to their relatively lower electronegativity (1.90 for silicon), have weaker acidic characteristics. These properties are essential in contexts ranging from environmental chemistry to industrial applications, highlighting the importance of accurately understanding the role of non-metal oxides.