Question

Which is more basic? (a) \(\mathrm{CrO}\) or \(\mathrm{Cr}_{2} \mathrm{O}\) (b) \(\mathrm{SnO}_{2}\) or \(\mathrm{SnO}\) (c) \(\mathrm{As}_{2} \mathrm{O}_{3}\) or \(\mathrm{As}_{2} \mathrm{O}_{5}\)

Short answer

CrO, SnO, and As2O3 are more basic.

Step-by-step solution

Understand the Nature of Oxides

Oxides of metals tend to be basic, and their basicity decreases as the oxidation number of the metal increases. This is because higher oxidation states typically mean stronger pull on the oxide ion, making it less able to act as a base and release the oxide ion.

Analyze the Oxides in Part A

For (a), we compare \(\mathrm{CrO}\) and \(\mathrm{Cr}_{2}\mathrm{O}_3\). In \(\mathrm{CrO}\), the chromium has an oxidation state of +2. In \(\mathrm{Cr}_{2}\mathrm{O}_3\), each chromium is +3. The lower the oxidation state, the more basic the oxide is, so \(\mathrm{CrO}\) is more basic than \(\mathrm{Cr}_{2}\mathrm{O}_3\).

Analyze the Oxides in Part B

For (b), we compare \(\mathrm{SnO}_{2}\) and \(\mathrm{SnO}\). \(\mathrm{SnO}\) has tin in the +2 oxidation state, while \(\mathrm{SnO}_{2}\) has tin in the +4 oxidation state. Again, the lower oxidation state corresponds to higher basicity, so \(\mathrm{SnO}\) is more basic than \(\mathrm{SnO}_{2}\).

Analyze the Oxides in Part C

For (c), we compare \(\mathrm{As}_{2}\mathrm{O}_{3}\) and \(\mathrm{As}_{2}\mathrm{O}_{5}\). \(\mathrm{As}_{2}\mathrm{O}_{3}\) has arsenic in the +3 oxidation state, while \(\mathrm{As}_{2}\mathrm{O}_{5}\) has arsenic in the +5 oxidation state. The oxide with the lower oxidation state of the element is more basic, so \(\mathrm{As}_{2}\mathrm{O}_{3}\) is more basic than \(\mathrm{As}_{2}\mathrm{O}_{5}\).

Key concepts

Oxidation States

The oxidation state of an element in a compound represents the degree of oxidation, essentially the number of electrons it can lose, gain, or share when it forms chemical compounds. Understanding oxidation states is crucial in predicting the behavior of oxides. A lower oxidation state usually indicates higher basicity for metal oxides. This is because metals in lower oxidation states have a weaker attraction for oxygen, allowing them to donate the oxygen more easily as an oxide ion in reactions. Conversely, metals with higher oxidation states exert a stronger pull on the oxygen, reducing the basicity of the oxide.

CrO vs Cr2O3

When comparing the basicity of chromium oxides

• In chromium(II) oxide (\(CrO\)), chromium is in a +2 oxidation state.
• In chromium(III) oxide (\(Cr_2O_3\)), chromium is in a +3 oxidation state.

Lower oxidation states, as seen in \(CrO\), mean less attraction for the oxide ion, making it more basic. Therefore, \(CrO\) is more basic than \(Cr_2O_3\). This makes sense since \(CrO\) being more basic reacts more easily with acids.

SnO vs SnO2

Tin oxides provide another perfect example of how oxidation states influence basicity.

• In \(SnO\), tin is in the +2 oxidation state.
• In \(SnO_2\), tin is in the +4 oxidation state.

The higher oxidation state in \(SnO_2\) leads to a larger pull on the oxide ion. This makes it less capable of behaving as a base compared to \(SnO\). Thus, \(SnO\) is more reactive with acids, demonstrating greater basicity.

As2O3 vs As2O5

Arsenic oxides demonstrate a clear trend influenced by oxidation states:

• \(As_2O_3\) features an oxidation state of +3 for arsenic.
• \(As_2O_5\), on the other hand, has arsenic in a +5 oxidation state.

Lower oxidation states, like that of \(As_2O_3\), usually point to greater basicity. This means it will more readily react with acids compared to \(As_2O_5\). Because \(As_2O_3\) has a smaller pull on the oxide ion, it serves to be more basic than its higher oxidation counterpart, \(As_2O_5\), which is more acidic.