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ICSE Class X Notes 2019 : Chemistry

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Hydrogen chloride Molecular formula: HCl Type of bonding: Covalent Occurrence: Volcanic gases and gastric juices of mammals Methods of preparation: 1. Synthesis: Moist hydrogen gas combines with chlorine in the presence of diffused sunlight. H2 + Cl2 2HCl The reaction is explosive in direct sunlight and negligible in dark and activated Carbon acts as a catalyst in the reaction at dark. 2. By the action of Conc. Sulphuric acid on metallic chlorides: Metallic chloride + H2SO4 {conc.} Salt + HCl (g) Lab Preparation: It is prepared by the action of conc. Sulphuric acid on sodium chloride: C NaHSO HCl NaCl H SO 200 2 4 4 Reactant Used: NaCl and H2SO4 {conc.}: sulphuric acid is used since it is non volatile. Concentrated nitric acid cannot be used since it is highly volatile and may volatize out along with HCl vapours. Temperature of the reaction: Temperature less than or around 2000C is maintained. Temperature above 2000C are not used for the following reasons: 1. Fuel is wasted 2. Sodium sulphate forms a hard crust which sticks to the glass apparatus and is difficult to remove. 200 C 2NaCl H 2 SO 4 Na 2 SO 4 2HCl Drying agent used: Conc. H2SO4 because it does not react with HCl CaO and P2O5 are not used as they react with HCl CaO 2HCl CaCl2 H 2 O ; 2P2 O5 3HCl POCl3 3HPO3 Collection of HCl Gas: Upward displacement of air (since it is heavier than air) It is not collected over water since it is highly soluble in water. Identification: When a rod dipped in ammonium hydroxide is brought near the mouth of jar full of HCl white dense fumes of ammonium chloride is formed. HCl NH 4 OH NH 4 Cl H 2 O Fountain experiment: This experiment demonstrates the high solubility of HCl gas in water. This also demonstrates the acidic nature of HCl gas as the blue litmus forms a red fountain. CHEMICAL PROPERTIES: 1.Thermal dissociation: hydrogen chloride dissociates into hydrogen and chlorine on heating above 5000C. 500 C 2HCl H 2 Cl 2 2.Reaction with Ammonia: It combines with ammonia to form dense white fumes of ammonium chloride. NH 3 HCl NH 4 Cl This is the reaction in which two gases reacts to form solid. HYDROCHLORIC ACID: It is an aqueous solution of HCl Gas. Preparation of hydrochloric acid: Hydrochloric acid in the lab is prepared by a special funnel arrangement: Importance of funnel arrangement: 1. Prevents or minimizes back suction 2. Provides a large surface area for the absorption of HCl gas. AZEOTROPE: A constant boiling mixture is a solution which boils without any change in its composition. HCl attains this when its concentration reaches 22.2% by wt at 1100C. Chemical properties of Hydrochloric acid: 1. Action of metals on acid: metals above hydrogen releases Hydrogen which blows off the burning splinter with a pop sound. This reaction proves that HCl contains hydrogen: Zn 2HCl ZnCl2 H 2 2. Reaction with base: HCl reacts with metallic oxides or hydroxides to form salt and water only. This reaction is called neutralization. CuO 2HCl CuCl2 H 2 O Mg OH 2 2HCl MgCl2 2H 2 O 3. Reaction with carbonates or bicarbonates ( CO3 group): HCl reacts with metallic carbonate and Metallic Bicarbonates to release a colourless gas that is odorless which turns lime water milky but has no effect on potassium dichromate solution that is carbon dioxide ( CO2) along with salt and water. Na 2 CO3 2HCl 2NaCl H 2 O CO2 NaHCO3 HCl NaCl H 2 O CO2 4. Reaction with Sulphites or bisulphites (SO3): HCl reacts with metallic sulphites or bisulphites to release a gas which smells like burning sulphur and turns acidified potassium dichromate solution orange to green and also turns lime water milky, that is sulphur dioxide ( SO2) along with salt and water. K 2 SO3 2HCl 2KCl H 2 O SO2 KHSO3 HCl KCl H 2 O SO2 5. Reaction with metallic sulphide (S-2) : HCl reacts with metallic sulphide to release a colourless gas with rotten egg smell which turns moist blue litmus red and turns moist lead acetate paper silvery black that is hydrogen sulphide ( H2S) along with salt. Na 2 S 2HCl 2NaCl H 2 S ZnS 2HCl ZnCl2 H 2 S 6. Action on thiosulphates S 2 O3 : Dil. HCl reacts with thiosulphate to produce sulphurdioxide and 2 precipitates yellow solid Sulphur. Na 2 S 2 O 3 2HCl 2NaCl SO 2 S H 2 O This reaction distinguishes sulphites from thiosulphate. Sulphites do not produce Sulphur with dil. HCl. 7. Reaction with Nitrates NO3 : Dil. HCl reacts with Silver Nitrate solution to give a thick curdy white ppt. 1 of AgCl which is insoluble in HNO3 but becomes soluble in excess of ammonium hydroxide to form a complex salt diammine silver (I) chloride. AgNO3 HCl AgCl HNO3 AgCl NH 4 OH Ag NH 3 2 Cl H 2 O When AgCl is exposed to sunlight, it blackens since it decomposes into Ag (black powder) and Chlorine. 8. Aqua Regia: It is a mixture having 3 parts of Conc. HCl and 1 part of Conc. HNO3. Aqua regia is a solvent for noble metals like gold (Au) and Platinum (Pt). 3HCl HNO3 NOCl 2H 2 O 2 Cl Nitrosyl chloride nascent chlorine The Nascent chlorine reacts with noble metals to give their respective chlorides. Pt 4 Cl PtCl4 Au 3 Cl AuCl3 Oxidizing agents: A substance that gets reduced either by accepting hydrogen or by removing oxygen is called as oxidizing agents. Some of the oxidizing agents are as follows: Manganese IV oxide (MnO2), lead IV oxide ( PbO2) , Potassium permanganate (KMnO4), Bleaching powder (CaOCl2). 9. Reaction of oxidizing agents with Conc. HCl: When Conc.HCl is treated with oxidizing agents it releases a gas which is greenish yellow in colour having pungent odour that turns moist starch iodide paper blue black that is chlorine ( Cl2) along with salt and water. MnO 2 4HCl MnCl2 2H 2 O Cl 2 PbO 2 4HCl PbCl2 2H 2 O Cl 2 2KMnO4 16HCl 2KCl 2MnCl2 8H 2 O 5Cl 2 Test of HCl: 1. HCl gives thick white dense fumes of NH4Cl when a glass rod dipped in NH4OH solution is held near the vapours of HCl. 2. With AgNO3, it gives white curdy ppt. of AgCl. This reaction differentiates HCl from other acids. 3. Greenish yellow gas chlorine is liberated when Conc. HCl is heated with oxidizing agents. Compiled by: Dinesh Tamang

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