Questions & explanations
1. Among Group 15 elements, the +5 oxidation state becomes less stable down the group due to:
- Increase in electronegativity
- Inert pair effect
- Decrease in atomic size
- Increase in ionisation enthalpy
Answer: Inert pair effect
The inert pair effect arises from the reluctance of s-electrons to participate in bonding, making lower oxidation states (e.g., +3) more stable for heavier elements. Consequently, stability of the +5 oxidation state decreases down Group 15. Electronegativity decreases, atomic size increases, and ionisation enthalpy decreases down the group.
2. Which of the following factors is primarily responsible for the anomalous behaviour of nitrogen compared to other Group 15 elements?
- Presence of d-orbitals
- Small atomic size and high electronegativity
- Ability to show variable oxidation states
- Metallic character
Answer: Small atomic size and high electronegativity
Nitrogen's small size, high electronegativity, and lack of d-orbitals lead to unique properties: it can form strong π-bonds, has a high ionisation enthalpy, cannot expand its octet, and does not exhibit metallic character. The other Group 15 elements have d-orbitals and larger sizes, allowing different bonding patterns.
3. Dinitrogen (N₂) is commercially prepared by:
- Haber's process
- Ostwald's process
- Fractional distillation of liquid air
- Decomposition of ammonium dichromate
Answer: Fractional distillation of liquid air
N₂ is obtained on a large scale by fractional distillation of liquefied air because nitrogen has a lower boiling point (-196°C) than oxygen (-183°C). Haber's process produces ammonia, Ostwald's process produces nitric acid, and ammonium dichromate decomposition yields N₂ only in the laboratory, not commercially.
4. Which of the following allotropes of phosphorus is the most reactive and is stored under water?
- A) Red phosphorus
- B) Black phosphorus
- C) White phosphorus
- D) Violet phosphorus
Answer: C) White phosphorus
White phosphorus is the most reactive allotrope due to its tetrahedral P4 structure with highly strained bonds. It ignites spontaneously in air and is stored under water to prevent oxidation. Red phosphorus is less reactive, black phosphorus is the least reactive, and violet phosphorus is an intermediate form.
5. Considering the Group 15 elements (N, P, As, Sb, Bi), which of the following properties shows a decreasing trend down the group?
- Atomic radius
- Metallic character
- Ionisation enthalpy
- Number of valence electrons
Answer: Ionisation enthalpy
Ionisation enthalpy decreases down a group due to increasing atomic size and enhanced shielding effect, making electron removal easier. Atomic radius increases, metallic character increases, and the number of valence electrons remains constant (five) down the group.
6. Which of the following oxides of nitrogen has a bent (angular) molecular structure?
- N₂O
- NO
- NO₂
- N₂O₅
Answer: NO₂
Nitrogen dioxide (NO₂) has a bent shape due to sp² hybridisation at nitrogen and the presence of one unpaired electron; the bond angle is approximately 134°. N₂O is linear, NO is linear (though paramagnetic), and N₂O₅ has a planar structure with N–O–N bridges.
7. In the Haber process for the manufacture of ammonia, the catalyst used is:
- Vanadium pentoxide
- Platinum gauze
- Finely divided iron
- Manganese dioxide
Answer: Finely divided iron
The Haber process (N₂ + 3H₂ ⇌ 2NH₃) uses finely divided iron with promoters like K₂O and Al₂O₃ as catalyst. Vanadium pentoxide is used in the Contact process for H₂SO₄, platinum gauze in the Ostwald process, and manganese dioxide in the decomposition of KClO₃.
8. Which of the following elements belongs to Group 15 of the periodic table and has the general valence shell electronic configuration ns²np³?
- Oxygen (O)
- Nitrogen (N)
- Carbon (C)
- Fluorine (F)
Answer: Nitrogen (N)
Group 15 elements have ns²np³ configuration. Nitrogen (atomic number 7) has electronic configuration 1s²2s²2p³, which fits this pattern. Oxygen belongs to Group 16 (ns²np⁴), Carbon to Group 14 (ns²np²), and Fluorine to Group 17 (ns²np⁵).
9. Which of the following oxoacids of phosphorus contains a P–H bond and is a diprotic acid?
- A) Hypophosphorous acid (H3PO2)
- B) Phosphorous acid (H3PO3)
- C) Orthophosphoric acid (H3PO4)
- D) Pyrophosphoric acid (H4P2O7)
Answer: B) Phosphorous acid (H3PO3)
H3PO3 has two ionizable hydrogen atoms (diprotic) and one P–H bond that is not ionizable. H3PO2 has one P–H and is monoprotic, H3PO4 has no P–H and is triprotic, and H4P2O7 is a condensed acid with no P–H. Hence B is correct.
10. In the Ostwald process for the manufacture of nitric acid, ammonia is first catalytically oxidised to:
- Nitrogen dioxide
- Nitric oxide
- Nitrous oxide
- Dinitrogen pentoxide
Answer: Nitric oxide
The Ostwald process begins with catalytic oxidation of ammonia: 4NH₃ + 5O₂ → 4NO + 6H₂O (using Pt/Rh catalyst at ~800°C). The nitric oxide (NO) formed is further oxidised to NO₂, which is then absorbed in water to yield HNO₃.
11. Which of the following is a characteristic property of phosphine (PH3)?
- A) It has sp³ hybridization with bond angle 109°28'.
- B) It acts as a strong base.
- C) It is a colourless, odourless gas.
- D) It is a reducing agent and has a fishy smell.
Answer: D) It is a reducing agent and has a fishy smell.
Phosphine is a reducing agent and has a characteristic fishy odour. Its bond angle is actually about 93.5° (not 109°28'), it is a weak base (not strong), and it is not odourless. These points make D correct.
12. The complete hydrolysis of phosphorus pentachloride (PCl5) yields:
- A) Phosphorous acid and HCl
- B) Phosphoric acid and HCl
- C) Phosphine and HClO
- D) Phosphorus oxychloride and HCl
Answer: B) Phosphoric acid and HCl
PCl5 undergoes complete hydrolysis to give orthophosphoric acid (H3PO4) and 5HCl. Partial hydrolysis yields POCl3, while PCl3 hydrolysis gives H3PO3 and HCl (option A). Options C and D are incorrect.