ICSE Chemistry Class 8 Notes Chapter 7 Hydrogen
→ Main source of light energy in the sun is fusion of hydrogen and sun is mainly made up of hydrogen.
→ The lightest element and most abundant in universe is H2.
→ Hydrogen is present in water [H2O], acids HCl, H2SO4, HNO3 organic compounds, plants and animals.
→ Metals above [H] in activity series of metals can displace [H] from acids and produce H2 gas.
→ Metals [K, Na, Ca] react with cold water and reaction is vigorous in case of Na, K. Hence the method is not preferred for prepration of H2.
2Na + 2H2O→ 2NaOH + H2.
Ca is costly metal.
→ Metals Mg, Al, Zn, Fe, Pb which are above [H] react with hot water or steam to form respective oxides and H2.
Mg + H2O → MgO + H2
→ Metal below [H] like Cu, Hg, Ag, Pt, Au do not react with water even when hot.
→ Metals above [H] react with dil. acids to displace H2 gas. Mg + 2HCl→ MgCl2 + H2
Zn + 2HCl -> ZnCl2 + H2
→ Dil. H2SO4 or dil. HNO3 is not used to prepare H2 from zinc as H2 produced oxidised to water since H2SO4, HNO3 acids are oxidising agents.
→ Zn granules are preferred as it contains impurities and acts as catalyst.
→ Zn, Pb, Al react with alkalis (on boiling) to produce H2.
→ H2 can also be obtained commercialy and electricity is passed through acidulated water. H2 is collected at cathode as H+ ions are +vely charged ions.
→ Water gas [CO + H2] is obtained by passing steam at 1000°C on coke. Water gas reduces steam in presence of Fe2O3 at 450°C to H2.
→ CO2 is soluble in H2O or KOH.
→ H2 is lightest gas, neutral, has no smell and is colourless, is combustible gas.
→ H2 forms explosive mixture with air. .
Think And Answer (Page 88)
In the preparation of hydrogen in the laboratory, the end of the thistle funnel in the flask should be dipped in the acid. Why?
Answer:
In the preparation of hydrogen in the laboratory, the end of the thistle funnel in the flask should be dipped in the acid so that the gas may not escape from the thistle funnel.
Check Your Progress (Page 89)
Answer these questions.
Question 1.
Which is the most abundant element in the universe?
Answer:
Hydrogen.
Question 2.
Who named the gas hydrogen?
Answer:
Antoine Lavoisier, a French chemist, named the gas ‘hydrogen’.
Question 3.
What happens when steam is passed over magnesium?
Answer:
When steam is passed over heated magnesium, hydrogen gas is produced along with the corresponding magnesium oxide.
Question 4.
What is the electrode connected to the negative terminal called?
Answer:
Cathode.
Question 5.
Which metal is used to prepare hydrogen in the laboratory?
Answer:
Zinc.
Question 6.
What happens when a burning splinter is brought near the mouth of a gas jar containing hydrogen gas?
Answer:
Bring a burning splinter near the mouth of the gas jar. Being highly combustible, the gas bums with a pop sound and the splinter gets extinguished.
Question 7.
Which raw materials are used to produce hydrogen by Bosch’s process?
Answer:
Coke and steam.