Class 11 Chemistry – Unit 12: Halogens (FBISE)
This section provides complete, exam-oriented notes for Class 11 Chemistry – Unit 12: Halogens according to the Federal Board (FBISE) syllabus. The notes explain the chemistry of halogens in a clear and student-friendly manner, making it easier to understand and memorize.
Key topics covered include halogens reactivity, reactivity of halide ions, and reactions of halogens. Detailed examples, reaction mechanisms, and step-by-step explanations are included to help students prepare effectively for exams.
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1. Physical Properties and Volatility
- Elements: Fluorine ($F_{2}$), Chlorine ($Cl_{2}$), Bromine ($Br_{2}$), Iodine ($I_{2}$), and Astatine ($At$).
- Appearance:
- $F_{2}$: Pale yellow gas
- $Cl_{2}$: Greenish yellow gas
- $Br_{2}$: Reddish brown liquid
- $I_{2}$: Shiny Greyish black solid
- Volatility: Decreases down the group as atomic size and instantaneous dipole-induced dipole forces (intermolecular forces) increase.
- Bond Strength: Decreases from $Cl_{2}$ to $I_{2}$. $F_{2}$ has an anomalously low bond energy ($156 \text{ kJ/mol}$) due to inter-electronic repulsions between lone pairs on the small fluorine atoms.
2. Reactivity and Oxidizing Power
- Oxidizing Power: Halogens act as oxidizing agents ($X_{2} + 2e^{-} \rightarrow 2X^{-}$). Power decreases down the group: $F_{2} > Cl_{2} > Br_{2} > I_{2}$.
- Displacement: A halogen higher in the group can displace a halide ion lower in the group from its aqueous solution.Example: $F_{2(g)} + 2KCl_{(aq)} \rightarrow 2KF_{(aq)} + Cl_{2(g)}$
3. Hydrogen Halides ($HX$)
- Thermal Stability: Decreases down the group ($HF > HCl > HBr > HI$) as the $H-X$ bond length increases and bond energy decreases.
- Acidic Strength: Increases down the group ($HI > HBr > HCl > HF$). $HI$ is the strongest acid because the $H-I$ bond is the weakest, making it easiest to donate a proton ($H^{+}$).
4. Chemical Reactions
A. Reaction with Sulphuric Acid ($H_{2}SO_{4}$)
Tests the reducing power of halide ions (increases down the group):
- $F^{-}$ and $Cl^{-}$: Weak reducers; only acid-base reactions occur (e.g., $NaCl + H_{2}SO_{4} \rightarrow NaHSO_{4} + HCl$).
- $Br^{-}$: Reduces $H_{2}SO_{4}$ to $SO_{2}$ (choking gas).
- $I^{-}$: Strongest reducer; reduces $H_{2}SO_{4}$ to $H_{2}S$ (rotten egg smell).
B. Silver Nitrate Test
Used to identify halide ions via precipitate color:
- $Cl^{-}$: White ppt ($AgCl$)
- $Br^{-}$: Cream/Off-white ppt ($AgBr$)
- $I^{-}$: Yellow ppt ($AgI$)
C. Chlorine with NaOH (Disproportionation)
- Cold & Dilute: $Cl_{2} + 2NaOH \rightarrow NaOCl + NaCl + H_{2}O$ (Forms Sodium Hypochlorite).
- Hot & Conc: $3Cl_{2} + 6NaOH \rightarrow NaClO_{3} + 5NaCl + 3H_{2}O$ (Forms Sodium Chlorate).
Relevant Questions & Answers
Q1: Why does the boiling point of halogens increase down the group?A: As you move down the group, the size of the atoms and the number of electrons increase. This leads to stronger instantaneous dipole-induced dipole forces (Van der Waals forces), requiring more energy to overcome.
Q2: Why is $HF$ a weaker acid than $HI$?A: The $H-F$ bond is very strong due to the high electronegativity of Fluorine and small bond length. In contrast, the $H-I$ bond is much longer and weaker, allowing $HI$ to dissociate and release $H^{+}$ ions more easily in solution.
Q3: What is a disproportionation reaction? Give an example from the text.A: A disproportionation reaction is a redox reaction where the same element is simultaneously oxidized and reduced. An example is the reaction of chlorine with cold $NaOH$, where $Cl_{2}$ (oxidation state 0) is reduced to $NaCl$ (-1) and oxidized to $NaOCl$ (+1).
Q4: Why does the reducing power of halide ions increase down the group?A: Down the group, the ionic radius increases and the outermost electrons are further from the nucleus. The nuclear attraction is weaker, making it easier for the ion to lose an electron and act as a reducing agent.
Summary Table: Silver Nitrate Test & Ammonia Solubility
| Halide Ion ($X^{-}$) | Reaction with $AgNO_{3(aq)}$ | Precipitate Color | Solubility in Aqueous Ammonia ($NH_{3}$) |
|---|---|---|---|
| Chloride ($Cl^{-}$) | $Ag^{+}_{(aq)} + Cl^{-}_{(aq)} \rightarrow AgCl_{(s)}$ | White ppt | Remains un-dissolved (Note: Typically dissolves in dilute $NH_{3}$ per standard chemistry, but un-dissolved per provided text) |
| Bromide ($Br^{-}$) | $Ag^{+}_{(aq)} + Br^{-}_{(aq)} \rightarrow AgBr_{(s)}$ | Cream / Off-white ppt | Partially dissolves |
| Iodide ($I^{-}$) | $Ag^{+}_{(aq)} + I^{-}_{(aq)} \rightarrow AgI_{(s)}$ | Yellow ppt | Remains un-dissolved |