Chapter 13: Halogens

The given ionic equation is Br, + 21 → 2Br + 12 Explanation is a This redox (oxidation-reduction) specifically a displacement reaction, reaction. • Bromine (Brz) is more reactive than iodine, so it oxidizes iodide ions (I) to form iodine (12). • At the same time, bromine is reduced to bromide ions (Br). Oxidation and Reduction Iodide ion (1) loses electrons → oxidation 21 → 12 + 2e Bromine (Brz) gains electrons → reduction Br2 + 2e → 2Br Conclusion reaction demonstrates the relative This reactivity of halogens. Since Br is more reactive than Iz, it displaces iodide ions from solution. Br, +21 → Br + 12 Species that is oxidized Iodide ion (I) is oxidized. Why? • Oxidation is defined as the loss of electrons. • Each iodide ion (IF) loses one electron to become iodine (12): • Therefore, I is oxidized because it loses electrons in the reaction. 21, →1, + 2e Conclusion Iodide ion (IF) is the species that is oxidized in this redox reaction, as it gives up electrons and is converted into elemental iodine (12).

London dispersion forces play a key role in the trend of volatility of halogens as you go down Group 17 (from fluorine to iodine). Role of London dispersion forces • London dispersion forces are weak intermolecular forces that exist between all molecules, especially nonpolar ones like halogens (F2, Cl2, Brz, 12). • These forces increase with molecular size and mass because larger atoms have more electrons, resulting in stronger temporary dipoles. Trend of volatility • Volatility means how easily a substance turns into gas. • As you go down the group, the size and mass of halogen molecules increase gives stronger London dispersion forces gives more energy needed to break these forces responsible for lower volatility. Conclusion As the London dispersion forces increase down the group, the volatility of halogens decreases. Fluorine is the most volatile, and iodine is the least volatile halogen.

See Q.6 from theory.

The reactivity of halogens with hydrogen decreases down the group (from fluorine to iodine). Trend: F2 > Cl > Br2>I2 Explanation: 1. Bond strength of hydrogen halides (H- X) • As you go down the group, the H-X bond becomes weaker due to the increasing atomic radius of the halogen. • Weaker H-X bonds form less easily, so reactivity decreases. 2. Oxidizing power of halogens • Fluorine is the strongest oxidizing agent, so it reacts most with vigorously hydrogen. • Iodine is the weakest, so it reacts least readily. Examples • Fluorine reacts explosively with hydrogen even in cold and dark. • Chlorine reacts vigorously with hydrogen in light or with spark. • Bromine reacts slowly with hydrogen on heating. reacts • Iodine very slowly and incompletely. Conclusion The reactivity of halogens with hydrogen decreases from F2 to I2 due to weaker oxidizing power and larger atomic size down the group.

Iodine (1z) is the halogen used as an antiseptic. How it works • Iodine acts as a disinfectant by killing or inactivating bacteria, viruses, and fungi. • It works by oxidizing vital proteins and enzymes in microbial cells. • This disrupts the structure and function of the cell, leading to cell death. Common Use • Often used in the form of tincture of iodine (iodine dissolved in alcohol). • Applied to cuts, wounds, and skin before or injections to prevent surgeries infection. Conclusion Iodine is effective as an antiseptic due to its that destroys strong oxidizing action microorganisms.

See Q.7 from theory.

When chlorine (Cl2) bromine (Brz) from a solution of a bromide salt (e.g., NaBr), a colour change is observed: Observation • The solution changes from colourless to orange-brown. Explanation • Chlorine is more reactive than bromine and displaces it: Cl, + 3Br → 2Ce + Br • Bromine (Brz), which is formed, has an orange-brown colour in solution. Conclusion: Colourless → Orange-brown due to the formation of bromine.

Halogen acids (hydrogen halides) such as HF, Hcl, HBr, and HI ionize in water by dissociating into hydrogen ions (Ht) and halide ions (X). General Ionization Equation HX (aq)H (aq) +X (aq) Examples Hcl in water HCl (aq) - HI in water HI (ag) H(ag + (aq) • HC!, HBr, and HI are strong acids - they completely ionize in water. • HF is a weak acid - it partially ionizes due to strong hydrogen bonding and high bond strength. Conclusion Halogen acids ionize in water by releasing Ht ions, contributing to acidity, with strength increasing from HE < HI < HBr < HI.

Hydrofluoric acid (HF) is a weaker acid than hydrochloric acid (Hcl) because of the following reasons: 1. Stronger H-F Bond • The H-F bond is much stronger (bond dissociation energy ~ 569 kJ/mol) than the H-Cl bond (= 431 kJ/mol). • It takes more energy to break the H-F bond, so fewer Ht ions are released in solution. • This makes HF less ionized in water compared to Hcl. 2. Extensive Hydrogen Bonding • HF forms strong hydrogen bonds with water molecules. • These bonds stabilize the HF molecule, further resisting ionization. 3. Lower Degree of Dissociation • Because of the above factors, HF partially ionizes, making it a weak acid • In contrast, Hcl completely ionizes, making it a strong acid. Conclusion HF is a weaker acid than Hcl because its strong bond and hydrogen bonding prevent easy release of Ht ions in water.

To distinguish between aqueous solutions of potassium bromide (KBr) and potassium iodide (KI), you can use a chlorine water test followed by addition of organic solvent like hexane. Reagents Needed • Chlorine water (Cl solution) • Hexané (or cyclohexane) - optional but helps observe color clearly Procedure and Observations 1. Add chlorine water to both KBr and KI solutions separately 2. Shake with hexane (non-polar solvent) to extract displaced halogens. Observation with Chand Solution Explanation hexane Cl displaces Orange layer KBr Br: Cl2 + 2Br (Br2 forms) → 2CI + Br2 Clz displaces F: Purple/violet KI Cl2 + 21 → layer (12 forms) 2CI + ½2 Conclusion • KBr gives an orange color (Br), • KI gives a purple color (12). These color changes help distinguish between the two halide solutions

Chlorine is used in water purification because it is a strong oxidizing agent and can kill bacteria and other pathogens by oxidizing essential cellular components like proteins, lipids, and nucleic acids. Chemical Principle When chlorine (Clz) is added to water, it reacts to form two compounds: C/2(g) + H2O,) → HC (ag) + HOC (ag) • Hcl is hydrochloric acıd • HOCI is hypochlorous acid (also called chloric(I) acid), a weak acid but a powerful disinfectant. Disinfection Mechanism • Hypochlorous acid (HOCI) penetrates microbial cell walls easily due to its neutral charge. • It oxidizes proteins, enzymes, lipids, and nucleic acids (DNA/RNA), which disrupts cell function and kills the microorganisms. HOCI H* + OCT- • OCI (hypochlorite ion) is also a disinfectant but less effective than HOCI. Optimal Conditions • pH 6-7.5: More HOCI, more effective disinfection. • Contact Time: Longer exposure improves killing efficiency. • Dose: Sufficient chlorine must be added to ensure enough HOC/OCI is available.

One significant disadvantage of using chlorine in water purification is the formation of harmful by-products, particularly trihalomethanes (THMs). Explanation • When chlorine reacts with natural organic matter (like decaying leaves or plant it can form material) in water, trihalomethanes (THMs) such as chloroform (CHCI3). • These THMs are carcinogenic (cancer- causing) and can pose serious long-term health risks if consumed over extended periods.

A disproportionation reaction is a type reaction in which of redox a single substance is simultaneously oxidized and reduced, forming two different products with different oxidation states. Example Reaction of chlorine with cold aqueous sodium hydroxide C2(g) +2 NaCH (aq) - NaCh (ag) + NaCIO (ag) + HI, 0(1) • In this reaction • Chlorine (Clz) is oxidized from 0 to +1 in NaCIO (sodium hypochlorite) • Chlorine (Cl) is reduced from 0 to -1 in Nacl (sodium chloride).

Chlorine gas reacts with sodium hydroxide (NaOH) in different ways depending on the temperature of the solution, resulting in different products. 1. Reaction with cold dilute sodium hydroxide • Chlorine undergoes disproportionation to form sodium chloride (Nacl) and sodium hypochlorite (NaCIO). C2(g) + 2NaOH (a4) NaC (ag) + NaCIO (aq) + H, O() • Here, chlorine is both reduced (to Cl in Nacl) and oxidized (to CIO- in NaCIO). • Sodium hypochlorite is the active ingredient in bleach. 2. Reaction with hot concentrated sodium hydroxide • Chlorine disproportionates differently forming sodium chloride (Nacl) and sodium chlorate (NaCIO). 3C/½ (g) + 6NaOH (ag) - 5NaC(q) + NaCIO 3(a9) + 3H2O,) • In this case, chlorine is reduced to chloride (CI) and oxidized to chlorate (C103 ). Summary • Cold dilute NaOH: Chlorine → NaCl + NaCIO (hypochlorite) • Hot concentrated NaOH: Chlorine → NaCl + NaCIO3 (chlorate) This difference is important in industrial and laboratory processes.

(1) Reaction of chlorine with cold, dilute sodium hydroxide C/2(g) +2 NaOH (aq) NaC (ag) + NaCIO (a4) + H2O (,) (ii) Reaction of chlorine with hot, concentrated sodium hydroxide →→→→ 3C/½(g) + 6 NaOH «4) 5NaC (aq) + NaC1O3(ag) +3H2On

Here's the breakdown for each reaction: (i) Reaction with cold, dilute NaOH C/(g) + 2 NaOH (ag) - NaChay) + NaCIO (ag) + HO() SLO BASED SHORT QUESTION ANSWERS

Halogens have 7 valence electrons (ns? np), making them one electron short of a stable octet. This high reactivity and similar valence configuration justify their placement in Group VIIA (17).

Halogens form diatomic molecules to achieve a stable electronic configuration • Oxidation states of chlorine Oxidation state of C! Species 0 (elemental form) Cla (reactant) -1 (chloride ion) Nacl (product) NaCIO (product) +1 (hypochlorite ion) • Explanation Chlorine in Cl is simultaneously reduced from 0 to -1 in NaCl, and oxidized from 0 to +1 in NaCIO. This simultaneous oxidation and reduction of the same element in one reaction is called disproportionation. (i) Reaction with hot, concentrated NaOH 3C/ (g) + 6 NaOH (a4) SNaCLag) + NaC103(ag) +-3H, 0(1) Oxidation states of chlorine Oxidation state of CI Species 0 (elemental form) Ch (reactant) -1 (chloride ion) NaCl (product) +5 (chlorate ion) NaCIOs (product) Explanation Chlorine is reduced from 0 to -1 in NaCl and oxidized from O to +5 in NaCIO3, occurring is also This simultaneously. disproportionation reaction. via covalent bonding. For example, Clz has a CI-Cl single bond with each atom sharing one electron.

Reactivity decreases atomic size and decreasing increasing electronegativity, which weakens the ability to attract electrons. Fluorine is the most reactive halogen. the trend of

Electronegativity decreases from F (4.0) to 1 (2.5) due to increasing atomic radius and shielding effect.

Fluorine is the smallest halogen with the highest effective nuclear charge and minimal shielding, allowing it to attract bonding electrons strongly

Both increase down the group due to stronger van der Waals. forces from increased molecular size (Fz < Ch < Brz < 12).

Fluorine has weak intermolecular forces due to its small size, while iodine has strong van der Waals forces due to a large number of electrons and larger atomic size.

Fz: pale yellow gas, Clz: greenish- yellow gas, Brz: reddish-brown liquid, l2: violet solid

Halogens absorb visible light and promote electrons to higher energy levels, hence releasing specific wavelengths, showing color

Halogens readily accept electrons to complete their octet. Their oxidizing power decreases down the group: F2 > Cla > Brz > 12.

Fluorine, due to its small size, high electronegativity, and low bond dissociation energy, easily gains electrons.

Clz+H2O=HCl+HOCl It forms hydrochloric acid and hypochlorous acid, showing disproportionation. react

Due to higher bond strength and oxidizing power, fluorine forms HF rapidly, while 12 has a lower affinity and reacts slowly

A reaction in which an element is simultaneously oxidized and reduced. Example Clz+H2O→HCe+HOCl Chlorine is reduced to HCl and oxidized to HOCl. Interhalogen Compounds and Oxides

Many halogen oxides (like ClOz) are used as oxidizing agents and disinfectants. Halogen Acids and Salt

HI is the strongest due to the weakest H-I bond, allowing easier ionization into Ht and I.

The H-F bond is very strong and short, making HF less dissociative in water.

Increases from HF < HCl < HBr < HI due to decreasing bond strength.

Salts formed by neutralizing hydrogen halides with bases, e.g., NaCe from HCl and NaOH.

An explosive reaction occurs forming Hcl: H2 + Cl2 → 2HCl. Uses of Halogens What are common uses of chlorine

Disinfection of water, manufacturing PVC, bleach, and chlorinated solvents. DESCRIPTIVE QUESTIONS (EXERCISE)

Iodine has antimicrobial properties and kills pathogens by disrupting protein and enzyme function. used bromine

retardants, photography, and extraction and water treatment, preparation

Due to fluorine's high reactivity, it cannot be prepared by chemical displacement and requires electrolysis of HF

It acts as an oxidizing agent to liberate Ch from Hcl

Due to their high reactivity, they exist as halide ions (CI, Br, etc.) in combined form with metals or hydrogen. agents. Arrange Fz, Cl, Br2, Iz in increasing order of oxidizing agents. Ans. See Q.3 from theory