Problem 171 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following orders presents the correct sequence of the increasing basic nature of the given oxides? (a) \(\mathrm{Na}_{2} \mathrm{O}<\mathrm{K}_{2} \mathrm{O}<\mathrm{MgO}<\mathrm{Al}_{2} \mathrm{O}_{3}\) (b) \(\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{MgO}<\mathrm{Na}_{2} \mathrm{O}<\mathrm{K}_{2} \mathrm{O}\) (c) \(\mathrm{MgO}<\mathrm{K}_{2} \mathrm{O}<\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{Na}_{2} \mathrm{O}\) (d) \(\mathrm{K}_{2} \mathrm{O}<\mathrm{Na}_{2} \mathrm{O}<\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{MgO}\)

Step-by-Step Solution

Answer

(b) \(Al_2O_3 < MgO < Na_2O < K_2O\)

Step 1: Understanding Basic Nature of Oxides

Metals generally form basic oxides. Basicity increases as we move down a group in the periodic table and decreases along the period from left to right.

Step 2: Analyze Oxides of Group 1 Elements

Oxides of Group 1 elements (such as Na and K) are highly basic and their basicity increases as we move down the group. Thus, \(K_2O\) is more basic than \(Na_2O\).

Step 3: Compare Group 1 and Group 2 Oxides

\(MgO\) is a Group 2 oxide and it is less basic compared to Group 1 oxides like \(Na_2O\) and \(K_2O\). Hence, \(MgO < Na_2O < K_2O\) in terms of basicity.

Step 4: Understanding \(Al_2O_3\)

\(Al_2O_3\) is amphoteric, meaning it can behave like both acidic and basic, but is less basic compared to both Group 1 and Group 2 oxides.

Step 5: Determine Correct Order of Increasing Basic Nature

Based on the analysis, \(Al_2O_3\) is less basic than \(MgO\), which is less basic than \(Na_2O\), and \(Na_2O\) is less basic than \(K_2O\). Therefore, the correct order is \(Al_2O_3 < MgO < Na_2O < K_2O\).

Key Concepts

Periodic Table Trends

The periodic table is an essential tool for chemists as it reveals trends that help predict the properties of elements and their compounds. One such trend is the behavior of oxides formed by elements in different groups and periods.

• As you move across a period from left to right, elements become less metallic. This trend results in oxides that shift from being basic to amphoteric and ultimately to acidic.
• Conversely, as you move down a group, elements become more metallic. Their oxides tend to become more basic. This is because metals typically form oxides that easily react with water to produce hydroxide ions, giving them basic characteristics.

Understanding these trends helps us predict how oxides behave under different conditions, guiding us in classifying them as either basic, acidic, or amphoteric.

Oxides of Metals

Metal oxides are compounds formed when metals react with oxygen. These oxides generally display basic properties, but the degree of basicity can vary widely depending on their position in the periodic table.

• Group 1 metals, such as sodium (Na) and potassium (K), form oxides like \(Na_2O\) and \(K_2O\), which are highly basic.
• Group 2 metals form slightly less basic oxides, like magnesium oxide \(MgO\).

The basicity of these metal oxides increases as you move down the group since the atomic size increases, which makes it easier for the oxide to release hydroxide ions in solution, enhancing its basic behavior. Recognizing these patterns is crucial in predicting the reactivity and chemical behavior of metal oxides.

Amphoteric Oxides

Amphoteric oxides are unique because they can exhibit both acidic and basic properties depending on the reacting substances.
Aluminum oxide \(Al_2O_3\) is a prime example of an amphoteric oxide. It reacts with acids to form aluminum salts and with bases to form aluminates. This dual behavior is a result of the oxide's intermediate nature between a metallic and non-metallic character.

• An amphoteric oxide can accept protons in acidic conditions, behaving as a base.
• In contrast, it can donate electrons in basic conditions, showing acidic properties.

Amphoteric behavior is particularly interesting because it allows certain oxides to play versatile roles in various chemical reactions and industrial processes.

Basicity and Group Trends

The concept of basicity involves the ability of a substance to accept protons or donate electron pairs during chemical reactions. In the context of oxides, basicity is influenced strongly by their position in the periodic table.

• For Group 1 oxides, basicity increases as you move down the group. This is attributed to the decrease in ionization energy, which makes it easier for these elements to shed electrons and participate in reactions that form hydroxide ions.
• In contrast, as you move across a period from left to right, basicity generally decreases. This is due to the increasing electronegativity and non-metallic character of the elements, which leads to the formation of oxides that are more acidic.

Understanding these trends is vital for predicting chemical behaviors and reactivity, which is why the periodic table is such a powerful tool in the study of chemistry.