Hydrogen Flashcards
Why isn’t the preparation of Hydrogen from the reaction between Cold water and Potassium preferred/
- The reaction is exothermic and extremely violent.
2. The liberated heat ignites Hydrogen.
Why isn’t the preparation of Hydrogen from the reaction between Sodium and Water preferred?
- The reaction is violent and exothermic but not as violent and exothermic as the reaction between Potassium and cold water.
- The sodium melts into a darting globule, making collection of Hydrogen difficult
- Sodium is therefore wrapped in a wire gauze and used in the above preparation.
Why isn’t the preparation of Hydrogen by reaction between Calcium and cold water preferred?
- The reaction is slightly vigorous.
2. However, calcium is expensive.
GR: Potassium, Sodium and Calcium cannot be used to prepare hydrogen from dilute acids
- Potassium, Sodium and Calcium do react with dilute H2SO4 and HCl.
- However, these reactions are extremely explosive so they are not feasible.
Which elements react with dilute acids to produce Hydrogen?
Mg, Al, Zn, Fe
GR: Lead cannot be used for the preparation of Hydrogen from dilute acids
- Lead reacts with dilute Hydrochloric acid and dilute Sulphuric acid to form an insoluble coating of lead chloride and lead sulphate respectively and hence further reaction comes to a stop.
GR: Dilute nitric acid is not used fin the preparation of hydrogen from metals.
- Nitric acid is a powerful oxidising agent and the nascant oxygen formed on its decomposition oxidises the hydrogen to water.
- Mn and Mg can react with extremely dilute Nitric acid at low temperatures to liberate hydrogen because, since the nitric acid is extremely diluted, its oxidising power is extremely reduced.
Chemical reactions for formation of hydrogen by reaction between metals and alkalis.
- Zn + 2NaOH -> Na2ZnO2 + H2
- Al + NaOH + H2O -> NaAlO2 + H2
- Pb + NaOH -> Na2PbO2 + H2
What is the unique nature of Zn, Al and Pb?
The oxides and hydroxides of Zn, Al and Pb are amphoteric in nature(they react with both acids and bases to form salts and water).
Laboratory preparation of Hydrogen
Reactants:
- Granulated Zinc
- Hydrochloric acid
Products:
- Zinc Chloride
- Hydrogen
Procedure:
- Granulated zinc is placed in the flat bottom flask and dilute hydrochloric acid or sulphuric acid is added slowly from the thistle or dropping funnel.
- A brisk effervescence is seen with the evolution of Hydrogen gas.
Precautions:
1. There should be no escape of Hydrogen gas and there should be no flame near the apparatus.
- Hydrogen is collected after all 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 dilute 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.
GR: The apparatus of the laboratory preparation of Hydrogen is airtight.
- Hydrogen is a combustible gas which forms an explosive mixture with air.
- If the apparatus of the laboratory preparation of Hydrogen would not be airtight, there would be a leakage of Hydrogen which would result in an explosion.
GR: Granulated Zinc, not normal Zinc, is used for the laboratory preparation of Hydrogen.
- Granulated zinc is commercial zinc obtained from molten zinc. It contains traces of impurities which have a slight catalysing effect on the reaction.
GR: Hydrogen, despite being lighter than air, is not collected by downwards displacement of air.
Hydrogen forms an explosive mixture with air.
Bosch process
C + H2O ->(1000C) (water gas)[CO + H2] - heat(endothermic)
[CO + H2] + H2O ->(450C, Fe2O3, Cr2O3) CO2 + 2H2 + heat(exothermic)
Electrolysis
2H2O(acidified) ->(electric current) 2H2(cathode) + O2(anode)
