Problem 15 - A Complete Resource Book in Chemistry for JEE Main

Question

The most acidic oxide is (a) \(\mathrm{MgO}\) (b) \(\mathrm{CaO}\) (c) \(\mathrm{Na}_{2} \mathrm{O}\) (d) \(\mathrm{Al}_{2} \mathrm{O}_{3}\)

Step-by-Step Solution

Answer

(d) \( \mathrm{Al}_{2} \mathrm{O}_{3} \) is the most acidic oxide among the options.

Step 1: Understanding Acidic Oxides

Acidic oxides are typically oxides of non-metals or oxides that can react with water to form acids or react with bases to form salts. These oxides accept electrons, showing non-metallic character.

Step 2: Analyzing Each Option

- Option (a), MgO is an oxide of a metal and is a basic oxide. - Option (b), CaO is also an oxide of a metal and is basic. - Option (c), Na2O is another metal oxide and is basic. - Option (d), Al2O3, is an amphoteric oxide, meaning it can react both as a base and as an acid, but it behaves more like an acidic oxide compared to the other options.

Step 3: Comparing Acidic Character

Among the given options, the oxides of metals usually exhibit basic characteristics. Al2O3 is different from the typical metal oxides because it shows amphoteric behavior. While it's not strongly acidic, it's more acidic compared to the other metal oxides listed.

Key Concepts

Understanding Amphoteric Behavior

Amphoteric behavior refers to the ability of certain substances to react both as an acid and as a base. This behavior is primarily observed in oxides or hydroxides of certain elements, such as aluminum and zinc. The word 'amphoteric' comes from Greek, where 'amphi' means "both". This dual behavior allows amphoteric substances to balance out by reacting with acidic or basic compounds.

A classic example is aluminum oxide ( Al_2O_3 ), which can react with strong acids to form salts and waters. For instance, it reacts with hydrochloric acid to form aluminum chloride and water. On the other hand, it can also react with strong bases like sodium hydroxide, resulting in salts known as aluminates.

• Reaction with Acid: Al_2O_3 + 6HCl → 2AlCl_3 + 3H_2O


• Reaction with Base: Al_2O_3 + 2NaOH + 3H_2O → 2NaAl(OH)_4

This characteristic sets amphoteric oxides apart from oxides that are strictly acidic or basic, providing them unique flexibility in chemical reactions.

Insights into Metal Oxides

Metal oxides are compounds composed of metal elements combined with oxygen. Typically, these oxides are basic in nature. That means they can react with acids to form salts and water. This basic characteristic arises from their ability to lose electrons easily. Most metal oxides are ionic, having distinct positive and negative ions, which grant them their basic properties. For instance, magnesium oxide ( MgO ) and calcium oxide ( CaO ) are metal oxides known for their basic behavior. They both react promptly with acids, exhibiting typical metal oxide reactivity.

• Magnesium oxide reaction: MgO + 2HCl → MgCl_2 + H_2O


• Calcium oxide reaction: CaO + 2HCl → CaCl_2 + H_2O

Since metal oxides tend to form ionic bonds, their structures are solid, often appearing as crystalline substances. Moreover, their tendency to neutralize acids makes them a valuable part of various industrial processes.

Exploring Non-metallic Character

Non-metallic character typically involves elements found mostly on the right side of the periodic table, in contrast to metallic elements. These elements like sulfur, phosphorus, and oxygen tend to gain electrons due to their high electronegativity, forming covalent bonds. Non-metallic elements often form acidic oxides, reacting with water to generate acids. For instance, sulfur dioxide ( SO_2 ) dissolves in water to form sulfurous acid (H_2SO_3). This behavior is indicative of their non-metallic nature, where they often accept electrons rather than donating.

• Formation of sulfurous acid: SO_2 + H_2O → H_2SO_3

Non-metallic oxides show a variety of properties which are crucial in many biological and environmental processes. For instance, carbon dioxide ( CO_2 ) plays a vital role in the carbon cycle and photosynthesis. The complex behavior of non-metallic elements significantly impacts both natural and industrial environments, guiding many chemical processes.