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ICSE Class 8 Chemistry Question 17 of 18

Hydrogen — Question 5

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Question 10

State a reason why, when hydrogen is passed over heated copper oxide, the resultant product formed, differs in colour from the original reactant.

Answer

As hydrogen is a strong reducing agent hence it reduces heated copper (II) oxide to copper. Therefore, the original black copper (II) oxide changes to reddish brown copper.

CuO[black]+H2⟶Cu[reddish brown]+H2O\underset{\text{[black]}}{\text{CuO}} + \text{H}_2 \longrightarrow \underset{\text{[reddish brown]}}{\text{Cu}} + \text{H}_2\text{O}

Hydrogen - Study Guide | Bright Tutorials
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Chemistry | Chapter 8: HydrogenWeb Content / Study Guide

Hydrogen — Interactive Study Guide

Quick Concept Map

HYDROGEN: Lightest element, symbol H, atomic mass 1
PREPARATION: Zn + dil. H2SO4 → ZnSO4 + H2↑
TEST: Pop test (burns with a pop sound)
PROPERTIES: Colourless, odourless, lightest gas, reducing agent
WATER OF CRYSTALLISATION: Fixed water molecules in a crystal (e.g., CuSO4.5H2O)

Lab Preparation Summary

ItemDetail
ReactantsZinc granules + Dilute sulphuric acid
Word equationZinc + Dilute sulphuric acid → Zinc sulphate + Hydrogen
ApparatusFlat-bottomed flask, thistle funnel, delivery tube, gas jar, trough
CollectionDownward displacement of water (H2 is insoluble)
Drying agentConcentrated H2SO4
TestPop test — burns with a “pop” sound

Key Reactions of Hydrogen

ReactionWord EquationType
CombustionHydrogen + Oxygen → WaterCombination
Reducing CuOCopper oxide + Hydrogen → Copper + WaterRedox
With chlorineHydrogen + Chlorine → Hydrogen chlorideCombination
Haber processHydrogen + Nitrogen → AmmoniaCombination

Water of Crystallisation — Key Data

Common NameFormulaColourH2O Molecules
Blue vitriolCuSO4.5H2OBlue → White (on heating)5
Green vitriolFeSO4.7H2OGreen7
Washing sodaNa2CO3.10H2OWhite10
GypsumCaSO4.2H2OWhite2
Epsom saltMgSO4.7H2OWhite7

Hydrogen as Clean Fuel

AdvantagesChallenges
Highest calorific value (150,000 kJ/kg)Highly explosive — difficult to store
Only product is water — zero pollutionExpensive to produce
Can be made from water (renewable)No fuelling station infrastructure
Fuel cells generate electricity directlyNeeds very high pressure or low temperature for storage

Self-Check Questions

  1. Name the acid and metal used in the lab preparation of hydrogen.
  2. How is hydrogen gas tested? What is this test called?
  3. Why is hydrogen called a reducing agent?
  4. What happens when blue copper sulphate crystals are heated? Is this reversible?
  5. State two advantages and two challenges of using hydrogen as a fuel.

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