Preparation of Hydrogen - Laboratory Method (PDF)

# PREPARATION OF HYDROGEN - Laboratory Method [Contd.] ## By action of dilute acid on zinc ### Laboratory preparation of hydrogen by action of dil. HCl on zinc. - **REACTION**: Zn + 2HCl [dil.] → ZnCl2 + H2 [g] - **REACTANTS**: - Granulated zinc - in flat bottom flask [X]. - Dilute hyd...

# PREPARATION OF HYDROGEN - Laboratory Method [Contd.] ## By action of dilute acid on zinc ### Laboratory preparation of hydrogen by action of dil. HCl on zinc. - **REACTION**: Zn + 2HCl [dil.] → ZnCl2 + H2 [g] - **REACTANTS**: - Granulated zinc - in flat bottom flask [X]. - Dilute hydrochloric acid - added through thistle funnel [Y]. - **PROCEDURE**: - Granulated zinc is placed in the flat bottom flask and hydrochloric acid [or dil. sulphuric acid) is added slowly - from the thistle or dropping funnel. - A brisk effervescence is seen with the evolution of - hydrogen gas. - **PURIFICATION**: - Granulated zinc being impure, on treatment with - dilute HCl or H2SO4 evolves in traces- gaseous impurities. - These impurities may be further removed by - passage of the impure gas through three washer bottles & a U-tube. - **Washer bottle 1**: Contains AgNO3 solution which absorbs Impurity - Arsine [AsH3] and phosphine [PH3] - **Washer bottle 2**: Contains Pb(NO3)2 solution which absorbs Impurity - Hydrogen sulphide [H₂S] - **Washer bottle 3**: Contains KOH solution which absorbs Impurity - NO2, CO2, SO2 - **U-Tube 4**: Contains anhydrous CaCl₂ which absorbs Impurity - Moisture - **PRECAUTIONS**: - No leakage of gas should take place & no flame must be near the apparatus. - Hydrogen is collected after all the air in the apparatus is allowed to escape [pure hydrogen burns quietly in air, hence its purity can be tested]. - The end of the thistle funnel should dip below the level of the dil. acid in the flask 'X' or the hydrogen gas may escape out through the thistle funnel 'Y'. - **COLLECTION**: Hydrogen gas is collected by - the downward displacement of water. ## PREPARATION OF HYDROGEN - Laboratory Method [Contd.] ### APPARATUS - In the above laboratory preparation of hydrogen an airtight apparatus is used. - Hydrogen forms an explosive mixture with air - hence the complete apparatus is airtight preventing any leakage of the gas. - A naked flame should not be brought near the apparatus since - it may be the cause of an explosion which may take place if the gas leaks. - The lower end of the thistle funnel should dip below the level of the dilute acid- in the flask thereby minimizing the chance of any leakage of the hydrogen gas through the thistle funnel. ### REACTANTS - Granulated zinc on reaction with dilute acid evolves hydrogen. - Granulated zinc is commercial zinc obtained from molten zinc. - It may contain traces of impurities which has a slight catalyzing effect on the reaction. - Addition of traces of copper [II] sulphate to the reaction medium also enhances the speed of the reaction. - The preferred acid is dilute hydrochloric or sulphuric acid. - Nitric acid being a strong oxidizing agent - oxidizes the hydrogen formed to water & is not used as the acid in the reaction with zinc. ### COLLECTION OF HYDROGEN - Hydrogen is collected by the downward displacement of water. - Hydrogen is almost insoluble in water [100 vols. of water dissolve about 2 vols. of hydrogen at s.t.p.]. - Even though hydrogen is lighter than air – [1 litre of H₂ weighs 0.09 g. at s.t.p.] it is not collected by downward displacement of air since it forms an explosive mixture with air. - Pure dry hydrogen is collected over mercury or in a flask previously evacuated thereby having absence of air. ### PURIFICATION OF HYDROGEN - Hydrogen is purified by passage through different solutions. - Granulated zinc on reaction with dilute acids imparts traces of - gaseous impurities which are removed by passage through different solutions. - Arsine [AsH3] & phosphine [PH3] – through silver nitrate solution, - Hydrogen sulphide [H₂S] - through lead nitrate solution, - Nitrogen dioxide, carbon dioxide & sulphur dioxide – through KOH solution Moisture using a - drying agent i.e. fused calcium chloride. ## PREPARATION OF HYDROGEN - Industrial Methods [Contd.] ### INDUSTRIAL METHOD - Bosch Process - **STEP I Reaction**: Production of - water gas - C + H2O → CO + H2 - Δ - [coke] [steam] [water gas] - Reactants: White hot coke & steam Endothermic - Temperature: Around 1000°C - Process: Passage of steam over white hot coke [carbon] - Chamber: Specially designed convertor - **STEP II Reaction**: Reduction of steam to hydrogen - by carbon monoxide. - CO + H2 + H2O → CO2 + 2H2 + Δ - [water gas] [excess steam] - Reactants: Water gas & excess steam exothermic - Temperature: Around 450°C - Catalysts: Iron [III] oxide [Fe2O3], promoter chromic oxide [Cr2O3] - Process: Excess steam is mixed with water gas & - passed over a catalyst at elevated temperatures. [CO is converted to CO2 with a further yield of hydrogen]. - **STEP III Reaction**: Separation of - Carbon dioxide [CO2] & unreacted Carbon monoxide [CO] from the above mixture. - **Process**: Removal of - **CO2**: By dissolving mixture in water under pressure [30 atmospheres], or caustic potash solution [2KOH + CO2 → K2CO3 + H2O] - **CO**: By dissolving mixture in – ammoniacal.cuprous chloride solution. [CuCl + CO + 2H2O → CuCl.CO.2H2O] ### INDUSTRIAL METHODS - By electrolysis of water or brine & from natural gas or CaH2 - **By electrolysis of - Water**: - Acidified water on electrolysis- liberates hydrogen at the cathode. - 2H2O → 2H2 + O2 - [acidified] [cathode] [anode] - Electrolysis of brine [NaCl soln.] - Very pure hydrogen is obtained as a by-product during electrolysis of brine. - **From natural gas & from calcium hydride – CaH2**: - Methane present in natural gas [obtained from petroleum ] - reacts with steam to give hydrogen - CH4 + H2O → CO + 3H2 - [methane] [steam] - UnnnctedCO-removed similarly as in-Boschprocess. - CaH2 + 2H2O → Ca(OH)2 + 2H2 - [calcium hydride] ## USES OF HYDROGEN ### IN HYDROGENATION REACTIONS - **Hydrogenation of oil**: - **Addition of hydrogen to organic compounds in presence of catalyst e.g. Pt or Ni under high pressure at about 200°C is called - hydrogenation.** - **Vegetable oils [palm oil] turn to semi solid fats by hydrogenation.** - **Hydrogenation of coal**: - **Passage of hydrogen under high pressure over powdered coal in presence of catalyst at a suitable temperature.** ### Reason/Conditions for use - **Certain metals like – platinum, nickel, gold & palladium readily adsorb - large volumes of hydrogen on their surface.** - **The phenomenon is called - occlusion a property useful for hydrogenation.** - **Hydrogenation of coal leads to - conversion of coal to a product similar to petroleum containing - a higher percentage of hydrogen.** ### IN EXTRACTION OF METALS - **Hydrogen when passed over heated metal oxides of less active metals e.g. zinc, iron, lead & copper, reduces the oxides of the metals to free metals, a process useful in – metallurgy.** ### Reducing agent - ZnO + H2 → Zn + H2O - Fe2O3 + 3H2 → 2Fe + 3H2O - CuO + H2 → Cu + H2O ### IN WELDING AND CUTTING METALS - **Oxygen burns in an atmosphere of hydrogen to produce an oxy-hydrogen flame.** - **The flame is used for welding & cutting.** ### Hydrogen-oxygen mixture on burning - **produces an exothermic reaction and the temperature of the flame is around 2800°C, which makes it useful for welding or cutting.** ### OXIDATION - REDUCTION REACTIONS. - **Oxidation involves- removal of hydrogen from a substance.** - **Reduction involves- addition of hydrogen to a compound.** ### Oxidation reaction - H2S + Cl2 → S [oxidised product] +2HCl ### Reduction reaction - Br2 +H2S → 2HBr [reduced product] + S