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) \(\mathrm{MgO}^{3}<\mathrm{K}_{2} \mathrm{O}<\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{Na}_{2} \mathrm{O}\) (d) \(\mathrm{K}_{2} \mathrm{O}<\mathrm{Na}_{2} \mathrm{O}<\mathrm{Al}_{2} \mathrm{O}_{3}<\mathrm{MgO}\)

Step-by-Step Solution

Answer

The correct order is option (b) Al₂O₃ < MgO < Na₂O < K₂O.

Step 1: Understand Basic Nature

The basic nature of oxides generally increases as you move from left to right across the periodic table in a given period and decreases from top to bottom in a group. Therefore, oxides of alkali metals (such as Na and K) are typically more basic than those of alkaline earth metals (such as Mg) and much more basic than those of transition metals or metalloids (like Al).

Step 2: Analyze each option

- Option (a) suggests a sequence from Na₂O to Al₂O₃. Here, Na₂O and K₂O, both being alkali metal oxides, should have a higher basic nature than the oxides of Mg and Al. - Option (b) lists Al₂O₃ first, followed by MgO, Na₂O, and ending with K₂O, which aligns better with their basicity trend. - Option (c) begins with MgO, which is less basic than both Na₂O and K₂O, making it incorrect. - Option (d) starts with K₂O, suggesting an incorrect order as it is typically more basic than Na₂O.

Step 3: Verify Correct Sequence

In comparing the basicity of these oxides, alkali metal oxides (Na₂O, K₂O) are more basic than alkaline earth metal oxides (MgO) and significantly more basic than metal oxides (Al₂O₃). Among alkali metal oxides, K₂O is more basic than Na₂O, and MgO is more basic than Al₂O₃. Therefore, the order from least to most basic should be Al₂O₃ < MgO < Na₂O < K₂O.

Step 4: Choose the Correct Option

The correct order of increasing basicity is given by option (b) Al₂O₃ < MgO < Na₂O < K₂O, as this matches the typical trend based on their position in the periodic table.

Key Concepts

Alkali Metal Oxides

Alkali metal oxides are compounds formed by the reaction of alkali metals with oxygen. The alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). These metals are located in group 1 of the periodic table and are renowned for their high reactivity.
Alkali metal oxides, such as sodium oxide (\(\text{Na}_2\text{O}\)) or potassium oxide (\(\text{K}_2\text{O}\)), are characterized by their strong basic nature. When they dissolve in water, they form hydroxides and produce basic solutions:

• Sodium oxide: \( \text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH} \)
• Potassium oxide: \( \text{K}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{KOH} \)

These oxides are highly basic because they readily donate electrons and easily combine with hydrogen ions to form water and other solutions. The basicity increases as we move down the group from lithium to cesium due to the decreasing ionization energy and increasing atomic size, which facilitates the release of electrons.

Alkaline Earth Metal Oxides

Alkaline earth metal oxides are compounds made by the reaction of alkaline earth metals with oxygen. These metals are situated in group 2 of the periodic table and include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Like the alkali metals, these metals are also quite reactive but generally less so than the alkali metals.
The general form of alkaline earth metal oxides is RO, where R represents the alkaline earth metal. For instance, magnesium oxide (\(\text{MgO}\)) and calcium oxide (\(\text{CaO}\)) are common examples.
These oxides are also basic, albeit less basic than their alkali metal counterparts. When dissolved in water, they form hydroxides, for example:

• Magnesium oxide: \( \text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 \)
• Calcium oxide: \( \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \)

Compared to alkali metal oxides, alkaline earth metal oxides require more energy to donate electrons because of their higher ionization energies, thus they exhibit decreased basicity.

Periodic Table Trends

Understanding periodic table trends is crucial for predicting the properties of elements, including the basicity of their oxides. As we move across a period from left to right, the basic nature of oxides usually decreases. This is because the metallic character of elements decreases, leading to the formation of oxides that are more acidic or amphoteric rather than basic.
Moving down a group in the periodic table, we generally see an increase in the basicity of oxides. This occurs because elements become more metallic and lose electrons more easily due to increasing atomic size and decreasing ionization energy. For example, alkali metal oxides like \(\text{K}_2\text{O}\) are more basic than sodium oxide \(\text{Na}_2\text{O}\).

• Left-to-right across a period: Basicity decreases.
• Top-to-bottom down a group: Basicity increases.

These trends help to determine the order of basicity among different oxides, as seen in the sequence from the given exercise.