Question

Arrange the oxides in each of the following groups in order of increasing basicity: (a) \(\mathrm{K}_{2} \mathrm{O}, \mathrm{Al}_{2} \mathrm{O}_{3}, \mathrm{BaO},\) (b) \(\mathrm{CrO}_{3}, \mathrm{CrO}, \mathrm{Cr}_{2} \mathrm{O}_{3}\)

Short answer

(a) \( \mathrm{Al}_{2} \mathrm{O}_{3} < \mathrm{BaO} < \mathrm{K}_{2} \mathrm{O} \); (b) \( \mathrm{CrO}_{3} < \mathrm{Cr}_{2} \mathrm{O}_{3} < \mathrm{CrO} \).

Step-by-step solution

Understand Basicity of Metal Oxides

Metal oxides can be classified as basic, amphoteric, or acidic depending on the metal's group and the oxidation state. Basicity generally increases down a group and decreases across a period in the periodic table.

Order Oxides in Group (a)

Consider the oxides \( \mathrm{K}_{2} \mathrm{O} \), \( \mathrm{Al}_{2} \mathrm{O}_{3} \), and \( \mathrm{BaO} \):- \( \mathrm{K}_{2} \mathrm{O} \) (potassium oxide) and \( \mathrm{BaO} \) (barium oxide) are from groups 1 and 2, respectively, and are strong basic oxides. \( \mathrm{K}_{2} \mathrm{O} \) is a stronger base than \( \mathrm{BaO} \).- \( \mathrm{Al}_{2} \mathrm{O}_{3} \) (aluminum oxide) is an amphoteric oxide being less basic than both.Thus, the order is \( \mathrm{Al}_{2} \mathrm{O}_{3} < \mathrm{BaO} < \mathrm{K}_{2} \mathrm{O} \).

Order Oxides in Group (b)

Consider the oxides \( \mathrm{CrO}_{3} \), \( \mathrm{CrO} \), and \( \mathrm{Cr}_{2} \mathrm{O}_{3} \):- \( \mathrm{CrO} \) has a low oxidation state of chromium (+2), making it basic.- \( \mathrm{Cr}_{2} \mathrm{O}_{3} \) is amphoteric due to the +3 oxidation state.- \( \mathrm{CrO}_{3} \) has the highest oxidation state (+6) and is acidic.Thus, the order is \( \mathrm{CrO}_{3} < \mathrm{Cr}_{2} \mathrm{O}_{3} < \mathrm{CrO} \).

Compile the Results

Using the analysis in Steps 2 and 3, the ordered lists of oxides are:- (a) \( \mathrm{Al}_{2} \mathrm{O}_{3} < \mathrm{BaO} < \mathrm{K}_{2} \mathrm{O} \)- (b) \( \mathrm{CrO}_{3} < \mathrm{Cr}_{2} \mathrm{O}_{3} < \mathrm{CrO} \)

Key concepts

Periodic Table Trends

The periodic table is organized in such a way that it reveals many chemical properties, including the basicity of metal oxides. As you move down a group in the periodic table, the basicity of metal oxides generally increases. This occurs because elements further down a group have larger atomic sizes and more loosely held electrons, facilitating their release to form oxides that can readily react with acids. On the other hand, as you move across a period (from left to right), the basicity decreases. This is due to the increase in electronegativity and oxidation states, which hold onto the electrons more tightly, reducing the tendency to interact with acids and donate electrons. In simple terms, where an element is placed in the periodic table can often predict how its oxide will behave: metals on the left tend to form basic oxides, while non-metals on the right are more likely to form acidic oxides.

Metal Oxides Classification

Metal oxides can be classified into three main categories based on their chemical behavior: basic, acidic, and amphoteric. These classifications are determined by the type of metal and its position in the periodic table, as well as its oxidation state.

• **Basic Oxides**: These are typically formed by metals, especially those from groups 1 and 2. Examples include potassium oxide (\( \mathrm{K}_{2} \mathrm{O} \)) and barium oxide (\( \mathrm{BaO} \)). They can react with acids to form salts and water.
• **Acidic Oxides**: Commonly formed by non-metals, these oxides, such as chromium trioxide (\( \mathrm{CrO}_{3} \)), react with bases to produce salts.
• **Amphoteric Oxides**: These oxides can behave either as an acid or a base, depending on the reacting partner. Aluminum oxide (\( \mathrm{Al}_{2} \mathrm{O}_{3} \)) and chromium(III) oxide (\( \mathrm{Cr}_{2} \mathrm{O}_{3} \)) are examples of amphoteric oxides.

Acidity and Basicity

Acidity and basicity refer to the ability of a substance to donate or accept protons (or hydrogen ions), respectively. In terms of metal oxides, basicity is correlated with the tendency to donate electrons to an acid, while acidity is linked to the ability to accept electrons from a base. A high basicity in metal oxides generally arises from a low oxidation state and a position in the periodic table where elements have a propensity to lose electrons more easily. For instance, potassium oxide (\( \mathrm{K}_{2} \mathrm{O} \)) is a strong base due to its position in group 1, where metals readily lose electrons. Conversely, oxides like chromium trioxide (\( \mathrm{CrO}_{3} \)) are acidic because the high oxidation state of chromium (+6) increases its ability to accept electrons, coupling with its decreased tendency to release electrons due to tighter hold on them.

Oxidation States

An oxidation state denotes the degree of oxidation or reduction of an element in a compound. It provides insight into the electron distribution between the bonded atoms in a substance. In the context of metal oxides, the oxidation state of the metal significantly influences whether the oxide behaves as basic, acidic, or amphoteric. For instance:

• **Low Oxidation States**: These often lead to basic behavior in metal oxides, such as chromium(II) oxide (\( \mathrm{CrO} \)) where chromium is in a low oxidation state of +2.
• **Intermediate Oxidation States**: Can result in amphoteric properties, as seen with chromium(III) oxide (\( \mathrm{Cr}_{2} \mathrm{O}_{3} \)), which can both donate and accept protons.
• **High Oxidation States**: High oxidation states correlate with acidic oxides. An example is chromium(VI) oxide (\( \mathrm{CrO}_{3} \)), where the +6 oxidation state leads to strong acidic properties.

Understanding oxidation states helps anticipate how a metal oxide will react in chemical interactions, guiding predictions about its acidic or basic nature.