The P-Block Elements (Compounds Of Group 17 Elements)

Chlorine

Chlorine ($Cl_2$) is a highly reactive greenish-yellow gas belonging to the halogen group.

Preparation

1. Laboratory Preparation:

• Action of $HCl$ on Oxidizing Agents: Reacting concentrated $HCl$ with oxidizing agents like manganese dioxide ($MnO_2$), potassium permanganate ($KMnO_4$), or potassium dichromate ($K_2Cr_2O_7$).

$MnO_2(s) + 4HCl(conc.) \xrightarrow{heat} MnCl_2(aq) + Cl_2(g) + 2H_2O(l)$

$2KMnO_4(s) + 16HCl(conc.) \rightarrow 2KCl(aq) + 2MnCl_2(aq) + 5Cl_2(g) + 8H_2O(l)$

• Action of $HCl$ on Bleaching Powder:

$CaOCl_2(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + Cl_2(g)$

2. Commercial Production (Electrolysis of Brine): This is the most important industrial method.

Diaphragm Cell Process / Chlor-alkali Process: Electrolysis of brine (concentrated aqueous solution of $NaCl$) produces chlorine gas at the anode, hydrogen gas at the cathode, and sodium hydroxide in the solution.

Anode (Oxidation): $2Cl^-(aq) \rightarrow Cl_2(g) + 2e^-$

Cathode (Reduction): $2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq)$

Overall Reaction: $2NaCl(aq) + 2H_2O(l) \xrightarrow{electrolysis} 2NaOH(aq) + Cl_2(g) + H_2(g)$

Other Methods:

• Electrolysis of molten $NaCl$ (produces $Na$ metal at cathode and $Cl_2$ at anode).
• Deacon's process (oxidation of $HCl$ using air with $CuCl_2$ catalyst), though less common now.

Properties

Physical Properties:

• Greenish-yellow gas.
• Pungent, suffocating smell.
• Denser than air.
• Can be liquefied at room temperature under pressure (boiling point 239 K / -34°C).
• Moderately soluble in water, forming chlorine water. It disproportionates slightly in water.

$Cl_2(g) + H_2O(l) \rightleftharpoons HCl(aq) + HOCl(aq)$

• Reacts with $SO_2$ in presence of water to form $HCl$ and $H_2SO_4$.

$Cl_2(g) + SO_2(g) + 2H_2O(l) \rightarrow 2HCl(aq) + H_2SO_4(aq)$

Chemical Properties:

1. Oxidizing Agent: Chlorine is a strong oxidizing agent due to its high electronegativity and tendency to gain an electron.

• With Metals: Combines vigorously with most metals to form metal chlorides.

$2Fe(s) + 3Cl_2(g) \rightarrow 2FeCl_3(s)$

• With Non-metals:

$P_4(s) + 6Cl_2(g) \rightarrow 4PCl_3(l)$

$PCl_3(l) + Cl_2(g) \rightarrow PCl_5(s)$

$2H_2(g) + Cl_2(g) \xrightarrow{UV \ light \ or \ heat} 2HCl(g)$

• With Hydrogen Sulfide:

$Cl_2(g) + H_2S(g) \rightarrow 2HCl(g) + S(s)$

• With Ammonia: Reacts to form nitrogen and hydrogen chloride. With excess ammonia, it can form nitrogen trichloride ($NCl_3$), which is explosive.

$2NH_3(g) + 3Cl_2(g) \rightarrow N_2(g) + 6HCl(g)$

$NH_3(g) + 3Cl_2(g) \rightarrow NCl_3(l) + 3HCl(g)$

• With Organic Compounds: Undergoes substitution and addition reactions with hydrocarbons.

2. Bleaching Agent: Chlorine acts as a bleaching agent in the presence of moisture. The bleaching action is due to the liberation of nascent oxygen.

$Cl_2 + H_2O \rightarrow HCl + HOCl$ (Hypochlorous acid)

$[O]$ (nascent oxygen) + Coloured matter $\rightarrow$ Colourless matter

3. Formation of Acids:

• Forms hydrogen chloride ($HCl$) with hydrogen.
• Forms hypochlorous acid ($HOCl$) and hydrochloric acid ($HCl$) in water.
• Forms oxoacids like $HClO_3$ (chloric acid) and $HClO_4$ (perchloric acid) when reacted with water under specific conditions or by reacting with bases.

Uses:

• Disinfectant and bleaching agent.
• Production of $HCl$, $CO$, $CCl_4$, $CHCl_3$, $CFCs$.
• In the manufacture of plastics (PVC), pesticides, refrigerants, and pharmaceuticals.
• Used for producing precious metals from their ores.

Hydrogen Chloride

Hydrogen chloride ($HCl$) is a colorless gas, and its aqueous solution is known as hydrochloric acid, a strong acid.

Preparation

1. Laboratory Preparation:

• Reaction of Sodium Chloride with Concentrated Sulfuric Acid: This is the most common method. Heating sodium chloride ($NaCl$) with concentrated sulfuric acid ($H_2SO_4$) produces hydrogen chloride gas.

$NaCl(s) + H_2SO_4(conc.) \xrightarrow{heat} NaHSO_4(s) + HCl(g)$

• If $NaCl$ is heated with excess concentrated $H_2SO_4$ at higher temperatures, sodium sulfate ($Na_2SO_4$) is formed.

$2NaCl(s) + H_2SO_4(conc.) \xrightarrow{high \ T} Na_2SO_4(s) + 2HCl(g)$

• The gas produced is dried by passing it through concentrated sulfuric acid.

2. Commercial Production:

• Direct combination of hydrogen and chlorine gases.

$H_2(g) + Cl_2(g) \xrightarrow{UV \ light \ or \ heat} 2HCl(g)$

• As a by-product during the chlorination of organic compounds.

Properties

Physical Properties:

• Colorless gas.
• Pungent, suffocating odor.
• Denser than air.
• Highly soluble in water, forming hydrochloric acid.
• Liquefies at 240 K (-33°C) and solidifies at 159 K (-114°C).

Chemical Properties:

1. Acidic Nature: Hydrochloric acid ($HCl$ solution) is a strong acid, completely ionizing in water.

$HCl(g) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)$

• Reacts with bases to form chlorides and water (neutralization).

$NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$

• Reacts with active metals to produce hydrogen gas.

$Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$

• Reacts with metal carbonates and hydrogencarbonates to produce carbon dioxide gas.

$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$

• Reacts with metal sulfides to produce hydrogen sulfide gas.

$FeS(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2S(g)$

• Reacts with metal sulfites to produce sulfur dioxide gas.

$Na_2SO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + SO_2(g)$

2. Oxidizing Action: Concentrated $HCl$ can act as an oxidizing agent, although it is weaker than nitric acid.

• Oxidizes $MnO_2$, $KMnO_4$, $K_2Cr_2O_7$.

$MnO_2(s) + 4HCl(conc.) \rightarrow MnCl_2(aq) + Cl_2(g) + 2H_2O(l)$

3. Aqua Regia: Concentrated $HCl$ mixed with concentrated $HNO_3$ (3:1 ratio) forms aqua regia, which dissolves noble metals like gold and platinum.

Uses:

• Manufacture of chlorides of other metals.
• In steel industry for pickling (removing rust and scale).
• In medicine as a digestive aid.
• In food processing (gelatin production).
• Used in laboratories as a common reagent.

Oxoacids Of Halogens

Halogens, especially chlorine, bromine, and iodine, form a series of oxoacids where the halogen atom is bonded to oxygen atoms.

General Characteristics:

• The central halogen atom exhibits positive oxidation states ($+1, +3, +5, +7$).
• The general formulas are $HXO$, $HXO_2$, $HXO_3$, $HXO_4$.
• The acidity and oxidizing strength generally increase with the oxidation state of the halogen.

Oxoacids of Chlorine:

• Hypochlorous Acid ($HOCl$): Chlorine in $+1$ oxidation state. Weak acid, strong oxidizing and bleaching agent.

$Cl_2 + H_2O \rightleftharpoons HCl + HOCl$

• Chlorous Acid ($HClO_2$): Chlorine in $+3$ oxidation state. Uncommon, unstable acid. Forms chlorites ($ClO_2^-$).
• Chloric Acid ($HClO_3$): Chlorine in $+5$ oxidation state. Strong acid and strong oxidizing agent. Forms chlorates ($ClO_3^-$).

$3Cl_2 + 6NaOH \rightarrow 5NaCl + NaClO_3 + 3H_2O$

• Perchloric Acid ($HClO_4$): Chlorine in $+7$ oxidation state. Strongest oxoacid of chlorine. Very strong oxidizing agent, especially when hot and concentrated. Forms perchlorates ($ClO_4^-$).

$HClO_4$ is the most stable and strongest oxoacid of chlorine.

Oxoacids of Other Halogens:

• Fluorine: Does not form oxoacids because it is the most electronegative element and cannot exhibit positive oxidation states by bonding with oxygen. It forms only one oxide, $OF_2$, where oxygen is $+2$ and fluorine is $-1$.
• Bromine and Iodine: Form oxoacids analogous to chlorine, but they are generally less stable and weaker acids and oxidizing agents. Examples: $HBrO$, $HBrO_3$, $HIO$, $HIO_3$, $HIO_4$.

Acidity Order: For oxoacids with the same halogen, acidity increases with the number of oxygen atoms attached to the halogen ($HOX < HClO_2 < HClO_3 < HClO_4$).

Oxidizing Strength Order: For oxoacids with the same halogen, oxidizing strength generally follows the same trend as acidity.

Interhalogen Compounds

Definition: Interhalogen compounds are molecules that contain bonds between two different halogen atoms. They are formed when halogens react with each other.

Types: Based on the stoichiometry, they can be classified as:

• XY type: Formed between a larger halogen and a smaller, more electronegative halogen (e.g., $ClF$, $BrF$, $ICl$, $IBr$).
• $XY_3$ type: Formed between a larger halogen and a smaller halogen with three atoms (e.g., $ClF_3$, $BrF_3$, $IF_3$).
• $XY_5$ type: Formed between a larger halogen and a smaller halogen with five atoms (e.g., $ClF_5$, $BrF_5$, $IF_5$).
• $XY_7$ type: Formed only by Iodine ($IF_7$).

Formation: Generally formed by the direct combination of halogens, often with one halogen in excess or under specific conditions (temperature, catalyst).

General Trends:

• The more electronegative halogen acts as the central atom in $XY$ and $XY_3$ type compounds (except $BrF_3$).
• Fluorine is the most electronegative halogen, so it appears as the central atom in many interhalogen compounds, or as the surrounding atom in $XY$, $XY_3$, $XY_5$, $XY_7$ types.

Preparation

Direct Combination: Halogens are mixed in specific ratios and sometimes heated or reacted in the presence of a catalyst.

• $Cl_2 + F_2 \rightarrow 2ClF$
• $Br_2 + 3F_2 \rightarrow 2BrF_3$
• $I_2 + 5F_2 \rightarrow IF_5$
• $I_2 + 7F_2 \rightarrow IF_7$

Reaction with Fluorine: Fluorine is the most reactive halogen and readily reacts with other halogens to form interhalogen compounds.

Properties

1. Physical State:

• XY type: Mostly gases or volatile liquids.
• $XY_3$ type: Liquids or solids.
• $XY_5$ type: Solids.
• $XY_7$ type: Gas.

2. Reactivity:

• Interhalogen compounds are generally more reactive than the individual halogens (except fluorine).
• Their reactivity decreases as the electronegativity difference between the constituent halogens decreases.
• Fluorinating agents are stronger than chlorinating, brominating, or iodinating agents.

3. Structure and Geometry: Their structures can be predicted using VSEPR theory.

• XY type: Linear (e.g., $ClF$, $ICl$). $sp^3d$ hybridization of the central atom.
• $XY_3$ type: T-shaped (e.g., $ClF_3$, $BrF_3$). $sp^3d$ hybridization.
• $XY_5$ type: Square pyramidal (e.g., $IF_5$). $sp^3d^2$ hybridization.
• $XY_7$ type: Pentagonal bipyramidal (e.g., $IF_7$). $sp^3d^3$ hybridization.

4. Hydrolysis: Most interhalogen compounds hydrolyze readily in water to form halide ions, hypohalous acids (or oxoacids), and other products.

• $ClF + H_2O \rightarrow HF + HOCl$
• $IF_3 + 3H_2O \rightarrow 3HF + H_3BO_3$ (This is incorrect for $IF_3$, it forms $HIO_3$ and $HF$). Correct: $IF_3 + 3H_2O \rightarrow HIO_3 + 3HF$

Uses:

• Some interhalogen compounds like $ClF_3$ and $BrF_3$ are powerful fluorinating agents.
• Used in organic synthesis.