Study of the First Element — Hydrogen — Question 17
Back to all questionsWhen hydrogen is passed over a black solid compound A, the products are 'a colourless liquid' and 'a reddish-brown metal B.'
Substance B is divided into two parts, each placed in separate test tubes.
Dilute HCl is added to one part of substance B and dilute HNO3 to the other.
(a) Name the substances A and B.
(b) Give two tests for the colourless liquid formed in the experiment.
(c) What happens to substance A when it reacts with hydrogen ? Give reasons for your answer.
(d) Write an equation for the reaction between hydrogen and substance A.
(e) Is there any reaction between substance B and dilute hydrochloric acid ? Give reasons for your answer.
(a) A = copper oxide [CuO],
B = copper [Cu]
(b) The liquid does not change the colour of blue or red litmus paper, showing neutral nature.
It changes the colour of cobalt chloride paper from blue to pink colour. Hence, the liquid produced is water.
(c) Black copper oxide (A) is reduced to copper, by losing oxygen. This happens because, hydrogen, the reducing agent reduces copper oxide to copper.
(d) CuO + H2 Cu + H2O
(e) As copper is less reactive than hydrogen and lies below it in the metal reactivity series, hence it cannot displace it from HCl. Hence, there is no reaction between them.
Cu + HCl ⟶ No reaction
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.