Questions & explanations
1. The anomalous behaviour of fluorine compared to other halogens is primarily due to:
- Its high electronegativity and small atomic size.
- Presence of d-orbitals in its valence shell.
- Its ability to form multiple bonds.
- Its low ionisation enthalpy.
Answer: Its high electronegativity and small atomic size.
Fluorine is the smallest and most electronegative halogen. Its small size and high electronegativity lead to anomalous properties: low F-F bond enthalpy, absence of positive oxidation states, formation of only one unstable oxoacid (HOF), and strong oxidizing power. Option B is wrong because fluorine has no d-orbitals. Option C is not a primary reason; fluorine typically forms only single bonds. Option D is wrong because fluorine has a high ionisation enthalpy.
2. The F-F bond enthalpy is anomalously low compared to Cl-Cl bond enthalpy. Which factor best explains this?
- Small size of fluorine leads to high interelectronic repulsion in the F₂ molecule.
- Fluorine has high electronegativity.
- Fluorine atom is highly reactive.
- F₂ molecule is paramagnetic.
Answer: Small size of fluorine leads to high interelectronic repulsion in the F₂ molecule.
In F₂, the small internuclear distance causes strong repulsion between non-bonding electron pairs on the two fluorine atoms, weakening the bond. This reduces bond dissociation energy compared to Cl₂. Option B is true but not the direct cause; electronegativity contributes to polarity but not the bond energy anomaly. Option C is a consequence, not an explanation. Option D is false; F₂ is diamagnetic (all electrons paired).
3. Which of the following orders correctly represents the acidic strength of hydrogen halides in aqueous solution?
- HF > HCl > HBr > HI
- HCl > HBr > HI > HF
- HF < HCl < HBr < HI
- HI < HBr < HCl < HF
Answer: HF < HCl < HBr < HI
Acidic strength of hydrogen halides increases down the group: HF is a weak acid due to the strong H-F bond and high electronegativity of F; HCl is stronger, HBr even stronger, HI is the strongest because bond dissociation enthalpy decreases and size of anion increases, stabilising the conjugate base. Option A reverses the order. Option B incorrectly places HF as stronger than HCl. Option D also reverses the order.
4. Which of the following statements about the oxidation states of halogens is correct?
- Fluorine exhibits variable oxidation states from -1 to +7.
- Chlorine can exhibit oxidation states of -1, +1, +3, +5, and +7.
- All halogens show only -1 oxidation state.
- Iodine does not show positive oxidation states.
Answer: Chlorine can exhibit oxidation states of -1, +1, +3, +5, and +7.
Fluorine is the most electronegative and always shows -1; it does not show positive states. Other halogens (Cl, Br, I) can show positive states due to vacant d-orbitals. Chlorine shows -1, +1 (hypochlorous), +3 (chlorous), +5 (chloric), +7 (perchloric). Option A is false because fluorine has no positive states. Option C is false because others show positive states. Option D is false: iodine shows +5, +7 etc.
5. The oxidising power of halogens decreases down the group. Which of the following orders correctly represents this trend?
- F₂ > Cl₂ > Br₂ > I₂
- I₂ > Br₂ > Cl₂ > F₂
- Cl₂ > F₂ > Br₂ > I₂
- Br₂ > I₂ > Cl₂ > F₂
Answer: F₂ > Cl₂ > Br₂ > I₂
Oxidising power is the ability to gain electrons. Fluorine is the strongest oxidising agent because of its high electronegativity and low bond dissociation enthalpy (though bond enthalpy is low, the high reduction potential dominates). Down the group, the tendency to gain electrons decreases, so oxidising power decreases: F₂ > Cl₂ > Br₂ > I₂. The other orders are incorrect.
6. Which of the following correctly describes the trend among halogens (F, Cl, Br, I) down the group?
- Atomic size increases, ionisation enthalpy increases, electronegativity decreases.
- Atomic size increases, ionisation enthalpy decreases, electronegativity decreases.
- Atomic size decreases, ionisation enthalpy decreases, electronegativity increases.
- Atomic size decreases, ionisation enthalpy increases, electronegativity increases.
Answer: Atomic size increases, ionisation enthalpy decreases, electronegativity decreases.
Down the group, atomic radius increases due to additional shells, ionisation enthalpy decreases because outer electrons are farther from the nucleus, and electronegativity decreases due to increased shielding and atomic size. Other options reverse one or more trends.
7. In the laboratory, chlorine gas is commonly prepared by reacting:
- NaCl with concentrated H₂SO₄.
- MnO₂ with concentrated HCl.
- KCl with water.
- NaCl with dilute HNO₃.
Answer: MnO₂ with concentrated HCl.
Chlorine is prepared in the lab by oxidation of HCl with MnO₂: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O. Option A gives HCl gas, not chlorine. Option C gives KCl solution, no chlorine. Option D does not produce chlorine. This is a standard preparation method.
8. The correct order of acidic strength of oxoacids of chlorine is:
- HClO < HClO2 < HClO3 < HClO4
- HClO4 < HClO3 < HClO2 < HClO
- HClO2 < HClO < HClO3 < HClO4
- HClO3 < HClO2 < HClO < HClO4
Answer: HClO < HClO2 < HClO3 < HClO4
Acidic strength increases with increasing oxidation number of chlorine. HClO (Cl⁺¹) is the weakest, HClO₂ (Cl⁺³) is stronger, HClO₃ (Cl⁺⁵) is stronger still, and HClO₄ (Cl⁺⁷) is the strongest oxoacid of chlorine.
9. Which of the following electronic configurations is characteristic of the halogens (Group 17 elements)?
- ns²np³
- ns²np⁴
- ns²np⁵
- ns²np⁶
Answer: ns²np⁵
Halogens have five electrons in the p-subshell of the valence shell, i.e., ns²np⁵ configuration. Other options correspond to other groups: ns²np³ (Group 15), ns²np⁴ (Group 16), ns²np⁶ (Group 18).
10. Which of the following interhalogen compounds has a square pyramidal structure?
- ClF3
- BrF5
- IF7
- ICl
Answer: BrF5
BrF5 has five bond pairs and one lone pair on the central bromine atom, resulting in a square pyramidal geometry. ClF3 is T-shaped, IF7 is pentagonal bipyramidal, and ICl is linear.
11. Bleaching powder is produced by passing chlorine gas over dry slaked lime. The main product of this reaction is:
- CaCl2
- Ca(OH)2
- Ca(OCl)2
- Ca(OCl)Cl
Answer: Ca(OCl)Cl
The reaction Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O yields calcium hypochlorite-chloride, commonly known as bleaching powder. It is not simply CaCl₂, Ca(OH)₂, or pure Ca(OCl)₂.