Question

Select the more acidic member of each of the following pairs: (a) \(\mathrm{Mn}_{2} \mathrm{O}_{7}\) and \(\mathrm{MnO}_{2},(\mathbf{b}) \mathrm{SnO}\) and \(\mathrm{SnO}_{2},(\mathbf{c}) \mathrm{SO}_{2}\) and \(\mathrm{SO}_{3},(\mathbf{d}) \mathrm{SiO}_{2}\) and \(\mathrm{SO}_{2},(\mathbf{e}) \mathrm{Ga}_{2} \mathrm{O}_{3}\) and \(\mathrm{In}_{2} \mathrm{O}_{3},(\mathbf{f}) \mathrm{SO}_{2}\) and \(\mathrm{SeO}_{2} .\)

Short answer

The more acidic members in each pair are: (a) \(Mn_2O_7\) (b) \(SnO_2\) (c) \(SO_3\) (d) \(SO_2\) (e) \(Ga_2O_3\) (f) \(SeO_2\)

Step-by-step solution

Pair (a) Comparison: Mn2O7 and MnO2

Mn2O7 has Mn in a higher oxidation state (+7) compared to MnO2, in which Mn has an oxidation state of +4. The higher the oxidation state, the more acidic the compound because of the higher attraction between the central atom and oxygen. Since Mn2O7 has the Mn ion in a higher oxidation state, it acts as a stronger acid in solution. Thus, Mn2O7 is more acidic than MnO2.

Pair (b) Comparison: SnO and SnO2

In SnO, Sn is in a +2 oxidation state, whereas in SnO2, Sn is in a +4 oxidation state. As mentioned earlier, the more the oxidation state of the central atom, the more acidic the compound. Therefore, SnO2 is more acidic than SnO.

Pair (c) Comparison: SO2 and SO3

In SO2, sulfur has a +4 oxidation state, while in SO3, sulfur is in a +6 oxidation state. Since SO3 exhibits a higher oxidation state, it tends to be more acidic than SO2. Thus, SO3 is more acidic than SO2.

Pair (d) Comparison: SiO2 and SO2

It's important to take into account the difference in electronegativity between the central atoms. SiO2 has silicon as the central atom while SO2 has sulfur. Although both compounds have the central atoms with a +4 oxidation state, sulfur is more electronegative than silicon, which leads the SO2 molecule to be more acidic than SiO2.

Pair (e) Comparison: Ga2O3 and In2O3

Both Ga2O3 and In2O3 have their central atoms in a +3 oxidation state. However, Ga (gallium) is more electronegative than In (indium), which means that the more electronegative central atom in Ga2O3 causes it to be more acidic than In2O3.

Pair (f) Comparison: SO2 and SeO2

In this pair, both SO2 and SeO2 have their central atoms in a +4 oxidation state. Sulfur (S) and selenium (Se) are in the same group of the periodic table, and as we move down the group, the acidity tends to increase due to the atoms' increasing size and decreasing electronegativity. So, SeO2 is more acidic than SO2. In summary, the more acidic member in each pair is: (a) Mn2O7 (b) SnO2 (c) SO3 (d) SO2 (e) Ga2O3 (f) SeO2

Key concepts

Oxidation States and Acidity

Understanding the relationship between oxidation states and acidity is crucial when dealing with acidic oxides in chemistry. High oxidation states usually indicate a strong ability of the central atom to attract electrons. As the oxidation number of the central atom increases, so does its electronegativity, which in turn enhances the compound's acidity. This occurs because the more positively charged an atom is, the more it pulls on the surrounding oxygen atoms, making the oxide more electron-withdrawing and thus more acidic.

For example, let's consider the oxide pair Mn₂O₇ and MnO₂. In Mn₂O₇, manganese (Mn) has an oxidation state of +7, which is higher than the +4 state found in MnO₂. This elevated state reflects a stronger pull on the electron density by manganese, making Mn₂O₇ a more acidic oxide. In general, remember that the greater the oxidation state of the central atom, the greater the acidity of the oxide.

Electronegativity and Acidity

The concept of electronegativity is pivotal when examining acidic properties in oxides. Electronegativity refers to an atom's tendency to attract shared electrons in a chemical bond. As a rule of thumb, the more electronegative the central atom in an oxide, the more acidic the compound will be. This assumes we are comparing oxides with central atoms in a similar oxidation state.

In examining the difference between SiO₂ and SO₂, we observe that both are composed of central atoms in the +4 oxidation state. However, sulfur (the central atom in SO₂) is more electronegative than silicon (the central atom in SiO₂). This higher electronegativity of sulfur leads to SO₂ being more acidic. To put it simply, the rule is that a more electronegative central atom within an oxide yields a more acidic oxide, given the oxidation states are comparable.

Comparison of Acidic Oxides

When comparing acidic oxides, it's not just about looking at individual properties like electronegativity and oxidation states, but understanding how these properties work together to influence acidity. Acidic oxides can be ranked based on these inherent characteristics. For instance, sulfur dioxide (SO₂) is more acidic than silicon dioxide (SiO₂) due to the higher electronegativity of sulfur in comparison to silicon.

The trend of increasing acidity with descending a group is also evident, as seen in the comparison of SO₂ with SeO₂, the latter being more acidic due to the trend that electronegativity decreases as you move down a group in the periodic table. Therefore, when comparing acidic oxides, consider both the oxidation state and electronegativity of the central atoms. Understanding these trends and rules, students can predict the acidity of various oxides and make insightful comparisons across different compounds.