Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the correct relationship between the pHs of isomolar solutions of sodium oxide \(\left(\mathrm{pH}_{1}\right)\), sodium sulphide \(\left(\mathrm{pH}_{2}\right)\), sodium selenide \(\left(\mathrm{pH}_{3}\right)\) and sodium telluride \(\left(\mathrm{pH}_{4}\right)\) ? (a) \(\mathrm{pH}_{1}>\mathrm{pH}_{2} \approx \mathrm{pH}_{3}>\mathrm{pH}_{4}\) (b) \(\mathrm{pH}_{1}<\mathrm{pH}_{2}<\mathrm{pH}_{3}<\mathrm{pH}_{4}\) (c) \(\mathrm{pH}_{1}<\mathrm{pH}_{2}<\mathrm{pH}_{3} \approx \mathrm{pH}_{4}\) (d) \(\mathrm{pH}_{1}>\mathrm{pH}_{2}>\mathrm{pH}_{3}>\mathrm{pH}_{4}\)

Step-by-Step Solution

Answer

Option (d): \(\mathrm{pH}_{1} > \mathrm{pH}_{2} > \mathrm{pH}_{3} > \mathrm{pH}_{4}\).

Step 1: Understand the Compounds

We need to consider the dissociation and nature of the compounds: sodium oxide \((\text{Na}_2\text{O})\), sodium sulphide \((\text{Na}_2\text{S})\), sodium selenide \((\text{Na}_2\text{Se})\), and sodium telluride \((\text{Na}_2\text{Te})\). These compounds, when dissolved in water, form basic solutions due to the respective oxide, sulphide, selenide, and telluride ions forming hydroxide ions \((\text{OH}^-)\).

Step 2: Evaluate the Strength of Basicity

The basic strength of these ions depends on their ability to attract protons. As we go down the chalcogen group (O, S, Se, Te), the bond strength decreases, making them less effective in pulling protons. Therefore, the basicity should decrease in order: \(\text{O}^{2-} > \text{S}^{2-} > \text{Se}^{2-} > \text{Te}^{2-}\).

Step 3: Relate Basicity and pH

Since pH is a measure of basicity, a more basic solution corresponds to a higher pH. Therefore, a solution containing \(\text{Na}_2\text{O}\) should have a higher pH than \(\text{Na}_2\text{S}\), \(\text{Na}_2\text{Se}\), and \(\text{Na}_2\text{Te}\), in decreasing order of pH.

Step 4: Analyze the Provided Options

Given our order \(\text{pH}_1 > \text{pH}_2 > \text{pH}_3 > \text{pH}_4\), we compare with the options: (d) \(\mathrm{pH}_{1} > \mathrm{pH}_{2} > \mathrm{pH}_{3} > \mathrm{pH}_{4}\) is the only option that matches our analysis.

Key Concepts

Basicity of Oxides and Chalcogenides

Understanding the basicity of oxides and chalcogenides is important for predicting the behavior of their solutions. Basicity refers to a compound's ability to attract and accept protons. This ability is directly related to the compound's anionic structure. For oxides and their counterparts in the chalcogen group (sulphides, selenides, and tellurides), their basicity comes from the oxo, sulfide, selenide, and telluride ions respectively.
Sodium oxide \((\text{Na}_2\text{O})\) produces strong basic solutions due to the presence of \({\text{O}}^{2-}\) ions, which have a robust attraction for protons, making them highly basic.
As we progress down the group from oxygen to tellurium in the periodic table, each element's capacity to attract protons decreases. This is because the effective size of the anion increases, and its charge density becomes lower. Therefore, the order of decreasing basicity is: \(\text{O}^{2-} > \text{S}^{2-} > \text{Se}^{2-} > \text{Te}^{2-}\).

pH Measurement

pH is a scale used to determine the acidity or basicity of a solution. It stands for "potential of hydrogen" or "power of hydrogen." The pH scale ranges from 0 to 14, with 7 being neutral. Values below 7 denote acidity, and those above 7 indicate basicity.
In terms of measuring pH, various methods can be utilized. The most common ones include:

• pH meters: These devices provide a digital reading of a solution's pH.
• Litmus paper or pH strips: These are sensitive to the hydrogen ion concentration and change color to indicate pH.

The ability of a compound to form hydroxide ions (OH⁻) when dissolved in water influences the pH of its solution. More hydroxide ions correspond to a higher pH, meaning a more basic solution. In the context of oxides and chalcogenides from this exercise, the sodium oxide solution will have a higher pH than the others due to its stronger basic nature compared to sodium sulphide, selenide, and telluride.

Chemical Dissociation and pH

When compounds like sodium oxides, sulphides, selenides, and tellurides dissolve in water, they undergo chemical dissociation. Dissociation is the process by which compounds split into smaller units, usually ions, when they mix with a solvent like water.
For the compounds in question, this dissociation results in the formation of ions that significantly affect the pH level of the solutions. As an example, \(\text{Na}_2\text{O}\) dissociates to produce \(2\text{Na}^+\) and \(\text{O}^{2-}\) ions. These \(\text{O}^{2-}\) ions are highly basic and effectively react with water to generate hydroxide ions (OH⁻), increasing the solution's pH.
In contrast, while the same dissociation occurs for \(\text{Na}_2\text{S}\), \(\text{Na}_2\text{Se}\), and \(\text{Na}_2\text{Te}\), the relative weakness in \(\text{S}^{2-}, \text{Se}^{2-},\) and \(\text{Te}^{2-}\) ions to attract protons results in lower production of hydroxide ions compared to \(\text{O}^{2-}\). Hence, their solutions have successively lower pH values, reflecting their reduced basicity.