Problem 120 - A Complete Resource Book in Chemistry for JEE Main

Question

120\. Match the following: List I 1\. thermal stability 2\. acidic nature 3\. boiling points 4\. \(\angle \mathrm{MMH}\) bond angle List II (i) \(\mathrm{H}_{2} \mathrm{~S}<\mathrm{H}_{2} \mathrm{Se}<\mathrm{H}_{2} \mathrm{Te}<\mathrm{H}_{2} \mathrm{PO}\) (ii) \(\mathrm{H}_{2} \mathrm{O}>\mathrm{H}_{2} \mathrm{~S}>\mathrm{H}_{2} \mathrm{Se}>\mathrm{H}_{2} \mathrm{Te}\) (iii) \(\mathrm{H}_{2} \mathrm{~S}<\mathrm{H}_{2} \mathrm{Se}<\mathrm{H}_{2} \mathrm{Te}<\mathrm{H}_{2} \mathrm{O}\) (iv) \(\mathrm{H}_{2} \mathrm{~S}>\mathrm{H}_{2} \mathrm{Se}>\mathrm{H}_{2} \mathrm{Te}=\mathrm{H}_{2} \mathrm{PO}\) The correct matching is 2 (a) (iii) (ii) (i) (iv) (b) (iv) (i) (ii) (iii) (c) (ii) (i) (iii) (iv) (d) (i) (ii) (iii) (iv) 121\. Which of the following order is incorrect? (1) \(\mathrm{PH}_{3}<\mathrm{AsH}_{3}<\mathrm{SbH}_{3}<\mathrm{NH}_{3}\) (Melting point) (2) \(\mathrm{PH}_{3}<\mathrm{AsH}_{3}<\mathrm{NH}_{3}<\mathrm{SbH}_{3}\) (Boiling point) (3) \(\mathrm{NH}_{3}>\mathrm{PH}_{3}>\mathrm{AsH}_{3}>\mathrm{SbH}_{3}>\mathrm{BiH}_{3}\) (Dipole moment) (a) only (b) only 2 (c) both (1) and (2) (d) none is incorrect here 122\. Match the following: List I List II 1\. most electronegative (i) I 2\. most electron affinity (ii) At 3\. liquid halogen (iii) \(\mathrm{F}_{2}\) 4\. radio active halogen (iv) \(\mathrm{Cl}_{2}\) (v) \(\mathrm{Br}_{2}\)

Step-by-Step Solution

Answer

The correct match for 120 is option (d). For 121, options (1) and (2) are incorrect orders. For 122, matches are: 1-(iii), 2-(iv), 3-(v), 4-(ii).

Step 1: Analyze List I for Question 120

We need to match the properties given in List I (thermal stability, acidic nature, boiling points, bond angle \(\angle \mathrm{MMH}\)) with the corresponding orders in List II. Let's consider the chemical concepts: thermal stability refers to how long a compound remains stable at elevated temperatures, acidic nature refers to the tendency to donate protons, boiling points depend on intermolecular forces like hydrogen bonding, and bond angles involve molecular geometry.

Step 2: Determine Matches for List I

1. **Thermal Stability:** Generally decreases down the group due to weaker bond strength. Order (iv): \(\mathrm{H}_{2} \mathrm{S} > \mathrm{H}_{2} \mathrm{Se} > \mathrm{H}_{2} \mathrm{Te} = \mathrm{H}_{2} \mathrm{PO}\) is a correct match for thermal stability.2. **Acidic Nature:** Increases down the group. Order (i): \(\mathrm{H}_{2} \mathrm{S} < \mathrm{H}_{2} \mathrm{Se} < \mathrm{H}_{2} \mathrm{Te} < \mathrm{H}_{2} \mathrm{PO}\).3. **Boiling Points:** Down the group, heavier molecules usually have higher boiling points, but hydrogen bonds in water complicate this. Order (iii): \(\mathrm{H}_{2} \mathrm{S} < \mathrm{H}_{2} \mathrm{Se} < \mathrm{H}_{2} \mathrm{Te} < \mathrm{H}_{2} \mathrm{O}\).4. **Bond Angle:** Decreases down the group as central atom size increases and lone pair repulsion effects change. Order (ii): \(\mathrm{H}_{2} \mathrm{O} > \mathrm{H}_{2} \mathrm{S} > \mathrm{H}_{2} \mathrm{Se} > \mathrm{H}_{2} \mathrm{Te}\).

Step 3: Confirm Correct Matching for Option (d)

The correct matching based on the analysis is: 1 - (iv), 2 - (i), 3 - (iii), 4 - (ii). This corresponds with option (d) in the question.

Step 4: Analyze Order for Question 121

We need to evaluate if the given orders about chemical properties like melting and boiling points are correct. NH\(_3\) has strong hydrogen bonds, leading to higher melting points compared to heavier hydrides like PH\(_3\) and AsH\(_3\). Evaluate correlation with given options.

Step 5: Inspect Details for Question 122

1. **Most Electronegative:** Fluorine is the most electronegative. Match to (iii): \(\mathrm{F}_{2}\).2. **Most Electron Affinity:** Chlorine usually exhibits the highest electron affinity. Match to (iv): \(\mathrm{Cl}_{2}\).3. **Liquid Halogen:** Bromine is a liquid halogen. Match to (v): \(\mathrm{Br}_{2}\).4. **Radioactive Halogen:** Astatine is known to be radioactive. Match to (ii): \(\mathrm{At}\).These observations help with matching Lists correctly.

Key Concepts

Thermal Stability

Thermal stability highlights how robust a compound is against decomposition when exposed to higher temperatures. As we go down the periodic table group, the atoms tend to increase in size. This increase alters the strength of the chemical bonds formed between atoms in a molecule. For example, in chalcogen hydrides like \( \text{H}_2 \text{S} \), \( \text{H}_2 \text{Se} \), and \( \text{H}_2 \text{Te} \), the hydrogen chalcogen bond becomes weaker as the atomic radius increases down the group.

• Smaller atoms like sulfur form stronger bonds due to better overlap of atomic orbitals.
• This is why \( \text{H}_2 \text{S} \) is more thermally stable than \( \text{H}_2 \text{Te} \), which decomposes more easily at higher temperatures.

Understanding thermal stability is crucial in reactions and material sciences, as it determines how materials behave under temperature changes.

Acidic Nature

The acidic nature of compounds, particularly hydrides of group 16 elements, reflects their tendency to donate hydrogen ions (protons) in a chemical reaction. This property is influenced by the bond strength between hydrogen and the element it is bonded to.

• Down the group, as in the sequence \( \text{H}_2 \text{S} \) to \( \text{H}_2 \text{Te} \), the acidic nature increases because the bond between hydrogen and the element weakens.
• Weaker bonds enable easier release of protons, thereby enhancing acidic strength.
• Additionally, increasing atomic size down the group reduces effective nuclear charge, aiding proton release.

Thus, understanding acidic nature is essential to predict how compounds behave in various chemical environments.

Boiling Points

Boiling points in chemical substances are predominantly influenced by molecular weight and intermolecular forces. In hydrides, both of these factors come into play.

• Heavier molecules usually exhibit higher boiling points because they have more atoms, which leads to stronger van der Waals forces.
• However, compounds like water (\( \text{H}_2\text{O} \)) also display strong hydrogen bonds that significantly elevate boiling points despite lower molecular weight.
• This means that in a series such as \( \text{H}_2 \text{S} \), \( \text{H}_2 \text{Se} \), and \( \text{H}_2 \text{Te} \), the boiling point generally increases, but \( \text{H}_2\text{O} \) disrupts this trend because of hydrogen bonding.

Knowledge of boiling points is useful in applications like distillation and evaporations.

Bond Angles

Bond angles refer to the angles between bonds that attach atoms in a molecule. This angle is crucial for determining molecular shape and chemical reactivity.
In the case of water and its neighboring chalcogenides:

• The bond angle decreases from \( \text{H}_2 \text{O} \) where it is larger, down to \( \text{H}_2 \text{Te} \), due to increased size of the central atom and changing lone pair repulsions.
• The lone pairs occupy more space and push the bond pairs closer together.
• This decrease affects molecular structure, impacting physical properties such as dipole moment and molecular interactions.

Understanding bond angles aids in predicting molecular geometry and its influence on reactivity and interactions.