Problem 46 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

The more acidic members in each pair are: (a) Mn2O7, (b) SnO2, (c) SO3, (d) SO2, (e) Ga2O3, and (f) SO2.

Step 1: (a) Mn2O7 and MnO2 comparison

Mn2O7 and MnO2 are oxides of manganese. In Mn2O7, the oxidation state of manganese is +7, whereas in MnO2, it is +4. Manganese has a higher oxidation state in Mn2O7, and therefore it will form a stronger bond with oxygen, making it a more acidic compound. So, Mn2O7 is more acidic than MnO2.

Step 2: (b) SnO and SnO2 comparison

SnO and SnO2 are oxides of tin. In SnO, the oxidation state of tin is +2, while in SnO2, it is +4. Tin has a higher oxidation state in SnO2, which means it will form a stronger bond with oxygen, making it a more acidic compound. So, SnO2 is more acidic than SnO.

Step 3: (c) SO2 and SO3 comparison

SO2 and SO3 are oxides of sulfur. In SO2, the oxidation state of sulfur is +4, whereas, in SO3, it is +6. Sulfur has a higher oxidation state in SO3, and thus forms a stronger bond with oxygen, making it a more acidic compound. So, SO3 is more acidic than SO2.

Step 4: (d) SiO2 and SO2 comparison

SiO2 is an oxide of silicon, and SO2 is an oxide of sulfur. As we move across a period in the periodic table, the acidity of oxides increases. Since sulfur is to the right of silicon in the periodic table, its oxide will be more acidic. So, SO2 is more acidic than SiO2.

Step 5: (e) Ga2O3 and In2O3 comparison

Ga2O3 and In2O3 are oxides of gallium and indium, respectively. As we move down a group in the periodic table, the acidity of oxides decreases. Gallium is above indium in the periodic table, meaning its oxide will be more acidic. So, Ga2O3 is more acidic than In2O3.

Step 6: (f) SO2 and SeO2 comparison

SO2 and SeO2 are oxides of sulfur and selenium, respectively. As we move down a group in the periodic table, the acidity of oxides decreases. Sulfur is above selenium in the periodic table, meaning its oxide will be more acidic. So, SO2 is more acidic than SeO2.

Key Concepts

Oxidation State

When discussing acidic oxides like manganese oxides, understanding the concept of oxidation state is crucial. The oxidation state, or oxidation number, represents how many electrons an atom gains, loses, or shares when it forms chemical bonds. In the context of oxides, a higher oxidation state usually translates to higher acidity. This is because a metal or non-metal atom with a higher oxidation state in an oxide forms a stronger bond with oxygen, thus displaying greater ability to donate protons when the oxide is dissolved in water.

Consider manganese oxides such as \(\mathrm{Mn}_{2} \mathrm{O}_{7}\) and \(\mathrm{MnO}_{2}\). In \(\mathrm{Mn}_{2} \mathrm{O}_{7}\), manganese has an oxidation state of +7, compared to +4 in \(\mathrm{MnO}_{2}\). This higher oxidation state in \(\mathrm{Mn}_{2} \mathrm{O}_{7}\) results in stronger acidic properties. As a general rule:

• Higher oxidation states often lead to stronger acidic behavior in oxides.
• This is due to increased polarity of the metal-oxygen bond.
• The ability to donate protons (H\(^{+}\)) is enhanced.

Periodic Trends

Periodic trends significantly influence the acidity of oxides across the periodic table. These trends help us understand how acidity varies not only with oxidation states but also with the position of the elements in the periodic table.

As you move from left to right across a period, the acidity of oxides generally increases. This is because elements on the right side of the periodic table have higher electronegativity and form oxides that are better proton donors.

• Silicon and sulfur, for instance: Moving from \(\mathrm{SiO}_{2}\) to \(\mathrm{SO}_{2}\), sulfur's position further to the right makes \(\mathrm{SO}_{2}\) more acidic.

When moving down a group, the acidity of oxides tends to decrease. This decrease is due to the increasing size of the atoms, which leads to weaker bonds with oxygen and thus lower acidity. For example, the acidity comparison between \(\mathrm{Ga}_{2} \mathrm{O}_{3}\) and \(\mathrm{In}_{2} \mathrm{O}_{3}\) shows that gallium's higher position makes its oxide more acidic than indium's.

Chemical Bonding

Chemical bonding is at the heart of the acidity of oxides. The type and strength of bonds an elemental oxide forms can determine how readily it behaves as an acid.

For oxides, the more polar the bond between the oxygen and the element, the stronger the acid. Polarization increases with higher oxidation states, as seen in the sulfur oxides example: \(\mathrm{SO}_{2}\) and \(\mathrm{SO}_{3}\). Here, \(\mathrm{SO}_{3}\)'s sulfur-oxygen bonds are stronger and more polar than those in \(\mathrm{SO}_{2}\), explaining why \(\mathrm{SO}_{3}\) is the more acidic of the two.

• Polarity of bonds affects the distribution of electron cloud.
• Stronger, more polarized bonds facilitate greater acidity.

Manganese Oxides

Manganese forms several oxides, each showing different properties based on their oxidation state. Understanding manganese oxides provides insights into how variable oxidation states impact acidity.

Two common manganese oxides are \(\mathrm{Mn}_{2} \mathrm{O}_{7}\) and \(\mathrm{MnO}_{2}\). With manganese in \(\mathrm{Mn}_{2} \mathrm{O}_{7}\) at an oxidation state of +7, this compound is highly acidic. In contrast, \(\mathrm{MnO}_{2}\) features manganese in a +4 state, resulting in less acidic behavior.

• Higher oxidation states lead to stronger acidic properties.
• This makes \(\mathrm{Mn}_{2} \mathrm{O}_{7}\) a more powerful acid than \(\mathrm{MnO}_{2}\).

Such variability in behavior underscores the importance of understanding both oxidation states and chemical bonding in predicting and explaining the properties of manganese oxides.