Study of the First Element — Hydrogen — Question 22
Back to all questionsQuestion 1(1993)
Write balanced equation of the reaction in the preparation of : hydrogen from a solution of potassium hydroxide [other than by electrolysis].
Preparation of hydrogen from potassium hydroxide :
2Al + 2KOH + 2H2O ⟶ 2KAlO2 + 3H2 [g]
Key Concepts Covered
This question tests your understanding of the following concepts from the chapter Study of the First Element — Hydrogen: Question, Balanced, Equation, Reaction, Preparation, Hydrogen. These are fundamental topics in Chemistry that students are expected to master as part of the ICSE Class 9 curriculum.
A thorough understanding of these concepts will help you answer similar questions confidently in your ICSE examinations. These topics are frequently tested in both objective and subjective sections of Chemistry papers. We recommend revising the relevant section of your textbook alongside practising these solved examples to build a strong foundation.
How to Approach This Question
Read the question carefully and identify what is being asked. Break down complex questions into smaller parts. Use the terminology and concepts discussed in this chapter. Structure your answer logically — begin with a definition or key statement, then provide supporting details. Review your answer to ensure it addresses all parts of the question completely.
Key Points to Remember
- Balance chemical equations before solving numerical problems.
- Learn the periodic table trends and exceptions.
- Understand reaction mechanisms, not just outcomes.
- Use correct IUPAC nomenclature in your answers.
Practice more questions from Study of the First Element — Hydrogen — Chemistry, Class 9 ICSE
Chapter Overview: Study of Hydrogen
Hydrogen is the lightest and most abundant element in the universe. This chapter covers its laboratory preparation, collection methods, physical and chemical properties, and uses. In the laboratory, hydrogen is prepared by the action of dilute sulphuric acid on zinc granules using a Woulfe's bottle. Hydrogen is collected by downward displacement of water (as it is insoluble) or upward displacement of air (as it is lighter than air). It is a colourless, odourless, tasteless gas that is highly combustible. Chemically, hydrogen acts as a reducing agent: it reduces metal oxides to metals. It burns in air with a pale blue flame to form water. The pop test (a burning splint produces a "pop" sound) identifies hydrogen. The chapter also discusses the position of hydrogen in the periodic table, isotopes of hydrogen (protium, deuterium, tritium), and industrial applications including the Haber process for ammonia production and hydrogenation of vegetable oils.
Key Reactions
| Reaction | Equation |
|---|---|
| Lab Preparation | Zn + H2SO4(dil.) → ZnSO4 + H2↑ |
| Combustion | 2H2 + O2 → 2H2O (pale blue flame) |
| With CuO | CuO + H2 → Cu + H2O (CuO reduced, black to reddish-brown) |
| With PbO | PbO + H2 → Pb + H2O |
| With Cl2 | H2 + Cl2 → 2HCl (in sunlight, explosive) |
| Haber Process | N2 + 3H2 ⇌ 2NH3 (Fe catalyst, 450°C, 200 atm) |
| With Na | 2Na + H2 → 2NaH (sodium hydride) |
Must-Know Concepts
- H2 is the lightest gas (density 0.09 g/L at STP), colourless, odourless
- Pop test: a burning splint held near H2 gas produces a "pop" sound
- H2 is a reducing agent because it removes oxygen from metal oxides
- Collection: downward displacement of water (insoluble) or upward displacement of air (lightest gas)
- Hydrogenation: addition of H2 to unsaturated oils using Ni catalyst to make saturated fats (vanaspati ghee)
- H2 is NOT collected over conc. H2SO4 as a drying agent (it can be dried by passing through it)
- Nascent hydrogen (newly generated, atomic H) is more reactive than molecular H2
Important Diagrams to Practice
- Laboratory preparation of hydrogen using Woulfe's bottle (Zn + dil. H2SO4)
- Collection of H2 by downward displacement of water
- Reduction of CuO by hydrogen (experimental setup with delivery tube)
Common Mistakes
- Using conc. H2SO4 for lab preparation (only dilute is used; conc. acid oxidises H2 to H2O)
- Saying hydrogen is collected by downward displacement of air (it is upward displacement since H2 is lighter)
- Confusing the pop test result: it is a "pop" sound, not a "squeaky pop" with a glowing splint (that is O2)
- Forgetting to mention that HCl is not used because ZnCl2 forms a layer that stops the reaction
Scoring Tips
- Draw and label the Woulfe's bottle setup clearly with thistle funnel, delivery tube, and trough
- When writing reactions, always mention the physical state and conditions
- For questions on "reducing nature," show the oxide being reduced step by step
- Know uses of hydrogen: rocket fuel, ammonia production, hydrogenation, fuel cells
Frequently Asked Questions
Why is dilute H2SO4 preferred over dilute HCl for preparing hydrogen?
Dilute HCl can also be used, but the ZnCl2 formed is slightly soluble and may coat the zinc surface, slowing the reaction. ZnSO4 formed with H2SO4 is highly soluble and does not interfere with the reaction.
Why is hydrogen considered a future fuel?
Hydrogen produces only water when burned (no CO2 or pollutants), has high energy content per unit mass, and can be produced from water by electrolysis using renewable energy. Fuel cells converting H2 to electricity are already used in vehicles and power plants.
What is nascent hydrogen?
Nascent hydrogen is hydrogen in its atomic form (H), generated at the moment of a reaction (in situ). It is more reactive than molecular hydrogen (H2) because it does not need to break the H-H bond before reacting. It can reduce compounds that molecular hydrogen cannot.