ICSE Chemistry Ch 06 - Hydrogen PDF

## **ICSE Chemistry-9** ### **Preparation of Hydrogen** #### **From Water** - When metal reacts with water whether cold, hot or vapour, hydrogen gas and oxides of those metals are produced. - Certain reactive metals like potassium, sodium, calcium etc. react directly with cold water present in...

## **ICSE Chemistry-9** ### **Preparation of Hydrogen** #### **From Water** - When metal reacts with water whether cold, hot or vapour, hydrogen gas and oxides of those metals are produced. - Certain reactive metals like potassium, sodium, calcium etc. react directly with cold water present in the surroundings to form their hydroxides. 2K + 2H₂O → 2KOH + H₂ 2Na + 2H₂O → 2NaOH + H₂ Ca + 2H₂O → Ca(OH)₂ + H₂ **Experiment 1: Reaction of sodium with water** * **Aim:** To show that sodium metal reacts with cold water and liberates hydrogen gas. * **Materials Required:** A beaker half filled with water at room temperature, a piece of sodium metal, phenolphthalein solution. * **Reaction Involved:** 2Na + 2H₂O → 2NaOH + H₂ * **Procedure:** Take a beaker half filled with water at room temperature and add a few drops of phenolphthalein solution. Then place a piece of sodium metal in the water. * **Observations:** 1. Sodium metal immediately starts to melt and form a red coloured solution. 2. A highly inflammable gas evolves which self ignites after some time. 3. A bright yellow flame is seen. #### **From Dilute Acids** - Reactive metals easily react with dilute acids and liberate hydrogen gas, but less reactive metals like copper, gold, etc. cannot react with dilute acids. - Hydrochloric acid and sulphuric acid themselves get reduced into nitrogen monoxide or nitrogen dioxide and do not liberate hydrogen. * **Action of dilute sulfuric acid:** Mg + H₂SO₄ → MgSO₄ + H₂ Zn + H₂SO₄ → ZnSO₄ + H₂ Fe + H₂SO₄ → FeSO₄ + H₂ 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂ * **Action of dilute hydrochloric acid:** Mg +2HCl → MgCl₂ + H₂ Zn + 2HCl → ZnCl₂ + H₂ Fe + 2HCl → FeCl₂ + H₂ 2Al + 6HCl → 2AlCl₃ + 3H₂ * **Action of nitric acid:** Very dilute nitric acid (1% acid) can liberate hydrogen on reaction with moderate reactive metals like magnesium and manganese. Mn + 2HNO₃ → Mn(NO₃)₂ + H₂ Mg + 2HNO₃ → Mg(NO₃)₂ + H₂ - Other metals when react with nitric acid, they normally liberate nitrogen monoxide or nitrogen dioxide (as per concentration of acids) due to oxidizing action of acid. 3Zn + 8HNO₃ → 3Zn(NO₃)₂ + 4H₂O + 2NO 3Fe + 8HNO₃ →3Fe(NO₃)₂ + 4H₂O + 2NO **Table 6.1: Reaction of metals with acids** | Metal | Reaction | Nature of reaction | |---|---|---| | K | 2K + 2HCl → 2KCl + H₂ | Explosive reaction | | Na | 2Na +2HCl → 2NaCl + H₂ | Explosive reaction | | Ca | Ca + 2HCl → CaCl₂ + H₂ | Vigorous reaction | | Mg | Mg + 2HCl → MgCl₂ + H₂ | Vigorous reaction | | Al | 2Al + 3HCl → 2AlCl₃ + H₂ | Less vigorous | | Zn | Zn + 2HCl → ZnCl₂ + H₂ | Less vigorous | | Fe | Fe + 2HCl → FeCl₂ + H₂ | Normal reaction | | Pb | Pb + 2HCl → PbCl₂ + H₂ | Normal reaction | | H | Reacts with concentrated acids | -| | Cu | No reaction due to low reactivity | - | | Hg | - | - | | Ag | - | - | | Au | - | - | - Zn is the most preferred metal as Na/K reacts violently, Ca and Mg are expensive, Al forms a protective coating, iron has to be heated. H₂ cannot be prepared from Cu, Hg, Ag, and Au due to low reactivity. #### **From Alkalis** - When metals like aluminum, zinc, and lead react with alkalis then it forms the respective soluble salts of metals and liberates hydrogen gas. * **Action of Sodium Hydroxide:** Zn + 2NaOH → Na₂ZnO₂ + H₂ 2Al + 6NaOH → 2NaAlO₃ + 3H₂ Pb + 2NaOH → Na₂PbO₂ + H₂ * **Action of Potassium Hydroxide:** 2Al + 6KOH → 2K₃AlO₃ + 3H₂ Zn + 2KOH → K₂ZnO₂ + H₂ Pb + 2KOH → K₂PbO₂ + H₂ - As you can see that some metals like aluminum, zinc, and lead can react with acids as well as bases to give their respective salts and liberate hydrogen. This is so because these metals are amphoteric in nature. Amphoteric metals are those metals which can react with both acids and bases. ### **Laboratory Preparation Of Hydrogen** * **Principle:** Zinc displaces hydrogen from sulphuric acid. * **Reactants:** Zinc powder/Granulated zinc, dilute sulphuric acid. * **Reaction:** Zn(s) + H₂SO₄ → ZnSO₄(aq) + H₂ Zn(s) + 2HCl(aq) → ZnCl₂(aq.) + H₂ * **Steps:** 1. Set up the apparatus as shown in the figure. 2. Add the sulphuric acid through the thistle funnel to the zinc metal. * **Observation:** Effervescence is seen due to the release of hydrogen gas till the reaction is completed. The gas thus released passes through the delivery tube and reaches the gas jar kept in the water trough. * **Collection of H₂ gas:** Hydrogen gas is collected in the upper regions of the gas jar by downward displacement of water as it is slightly soluble in water. * **Precautions:** 1. Concentrated H₂SO₄ is not used because it acts as an acid as well as an oxidizing agent and consequently its reduction product, SO₂ gas is formed, and H₂ undergoes oxidation to form water, which is not a desired product. Zn + 2H₂SO₄(conc.) → ZnSO₄ + 2H₂O + SO₂ 2. Pure zinc is not used in the preparation of dihydrogen because being non-porous in nature, the reaction between Zn and H₂SO₄ is slow. The presence of impurities however, makes zinc porous and speed up the reaction. 3. The apparatus used for the reaction and collection of hydrogen gas should be airtight because hydrogen gas forms an explosive mixture with air. 4. The gas jar used for the collection of hydrogen should be completely filled so that the last traces of any other gas can be removed. * **Removal of Impurities:** The hydrogen gas made by this method contains some impurities like water vapour, carbon dioxide, sulphur dioxide, etc. These impurities have to be removed from the collected mixture to get pure hydrogen. This can be done by the following ways: 1. The impurity of water vapour can be removed by passing the collected mixture through drying agents like phosphorus pentoxide or anhydrous calcium chloride. Anhydrous CaCl₂ + H₂O → Hydrated CaCl₂ P₂O₅ + 3H₂O → 2H₃PO₄ (Phosphoric acid) 2. The impurities of carbon dioxide, sulphur dioxide, and oxides of nitrogen come from sulfuric acid. These impurities can be removed in the following ways: CO₂ + 2KOH → K₂CO₃ +H₂O SO₂ + 2KOH → K₂SO₃ + H₂O 2NO₂ + 2KOH → 2KNO₂ + H₂O 3. The impurity of hydrogen sulphide comes from dilute sulfuric acid. This can be removed by passing the gas over lead acetate solution. (CH₃COO)₂Pb + H₂S → PbS + 2CH₃COOH 4. The impurities of phosphine (PH₃) and Arsine (AsH₃) enter the gaseous mixture through zinc. These impurities can be removed by passing the gas through silver nitrate solution. PH₃ +3AgNO₃ → Ag₃P + 3HNO₃ AsH₃ +3AgNO₃ → Ag₃As + 3HNO₃ - Zinc is the most suitable metal for the preparation of hydrogen gas in the laboratory. This is because its reaction with acids is neither too fast nor too slow. The other metals are not suitable due to their nature of the reaction with the acids. This can be seen by the following observations: 1. Sodium and potassium are very expensive and have violent and explosive reactions with the acids. 2. Calcium and magnesium are also expensive and the liberation of hydrogen gas on their reaction with acids is very fast, which makes its collection difficult. 3. Aluminium metal’s surface generally forms a protective covering of aluminium oxide which lowers its reactivity. 4. Iron and lead’s reaction with acids is very slow and layers of impurities are generally formed over their surface, which further slows down their reactivity. 5 Cu, Ag, Au will not displace H₂ from acids. ### **Industrial Manufacture of Hydrogen (Bosch Process)** * In this method, steam and CO (water gas) is heated in the presence of iron (III) oxide and chromium (III) oxide (Fe₂O₃/Cr₂O₃) when CO gets oxidized to CO₂. The gaseous mixture produced is compressed to 25 atm. and passed through water when CO₂ dissolves and H₂ escapes out. CO + H₂ + H₂O Fe2O3/Cr2O3. → CO₂ + H₂ Or by steam reforming of hydrocarbons. C(s) + H₂O(g) → CO + H₂ CnH₂n+2+nH₂O(g) → nCO+(2n+1)H₂ * **Removal of Impurities** The hydrogen gas received through the Bosch process contains the impurities of carbon dioxide, carbon monoxide and water vapour. The removal of these impurities is discussed ahead. 1. **Separation of Carbon Dioxide:** The above received gaseous mixture is first compressed to the pressure of 30 atmospheres and then passed through cold water. In the process the gas mixture converts into carbonic acid, and hydrogen gas is separated out. H₂O + CO₂ → H₂CO₃ (Carbonic acid) 2. **Separation of Carbon Monoxide:** The hydrogen gas received after separating carbon dioxide contains carbon monoxide, which can be removed by passing the gaseous mixture over ammoniacal cuprous chloride. CuCl + CO + 2H₂O → CuCl CO₂H₂O + H₂ 3. **Separation of water vapour:** The moisture in the hydrogen gas is removed by cooling the above gaseous mixture to the temperature of -20°C. At this temperature the water vapour freezes to ice, and pure hydrogen is collected. ### **Oxidation and Reduction Reactions (Redox Reaction)** * **Oxidation** is the addition of oxygen or the removal of hydrogen. * **Reduction** is the addition of hydrogen or the removal of oxygen. - **Reducing agents** are those which helps in reduction but itself undergoes oxidation e.g. Na, Mg, Al, H₂O₂, H₂S. - **Oxidising agents** are those which helps in oxidation but itself undergoes reduction, e.g. non-metals like F₂, Cl KNO3, KMnO4, K2Cr2O7, bleaching powder, H2O, nitric acid, ozone. **Experiment 2: Reaction of calcium with water** * **Aim:** To show that calcium metal reacts with cold water. * **Materials Required:** A beaker half filled with water at room temperature, a piece of calcium metal, phenolphthalein solution. * **Reaction Involved:** Ca + 2H₂O → Ca(OH)₂ + H₂ * **Procedure:** The small piece of calcium metal is added to water, it sinks to the bottom of the water. * **Observation:** 1. Colorless gas bubbles evolve at a moderate rate. And a pop sound is heard when a burning Splinter is brought near the gas bubbles. 2. The metal dissolves and a white coloured solution is formed. * **Conclusion/Inference:** 1. The gas evolved is hydrogen as it pops when a glowing splinter is brought near it. 2. The white coloured solution is of calcium hydroxide. 3. Reaction is less violent than sodium. 4. If red litmus paper dips in the beaker, it will turn into blue. #### **Action of Hot Water ** - Some metals are not very reactive, so they do not react with cold water in the surroundings but they react with hot water and form their oxides. * **Reaction with magnesium:** Magnesium reacts with boiling water to give magnesium oxide and liberates hydrogen gas, but this reaction is slow. Mg + H₂O → MgO + H₂ * **Action of steam on iron:** Metals like iron, zinc, aluminum, etc. are less reactive. They react only with steam to form their oxides. Reaction between iron and steam is reversible. 3Fe + 4H₂O Fe₃O₄ + 4H₂ This reaction is reversible in nature. Zn + H₂O → ZnO + H₂ 2Al + 3H₂O → Al₂O₃ + 3H₂