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Lesson 4: Iron Chemical Properties, Reactions and Oxides

Chemistry

Lesson 4: Iron Chemical Properties, Reactions and Oxides

Overview

Overview

🌳 Concept tree

  1. Iron Chemical Properties, Reactions and Oxides
  2. ├── Physical properties
  3. ├── Chemical properties
  4. ├── Iron oxides
  5. ├── Summary of the oxides
  6. └── Typical "give reasons" for this chapter
Lesson 4: Iron Chemical Properties, Reactions and Oxides · 1 / 7
Physical properties

Physical properties

  • Pure iron has no industrial importance because it is rather soft, malleable, ductile and magnetic; m.p. 1538 °C; density 7.87 g/cm3.
  • Its properties depend on its purity and on the nature of the impurities.
  • Many steels and iron alloys are made with specific properties.
Lesson 4: Iron Chemical Properties, Reactions and Oxides · 2 / 7
Chemical properties

Chemical properties

  • Iron does not reach the maximum state (+8) that would correspond to removing all 8 electrons of 4s and 3d.
  • States above +3 are unimportant; +2 comes from the 4s electrons and +3 gives the stable half-filled 3d5.
Comparison aspectMain ideaKeywords
Airred-hot iron + dry air/oxygen → magnetic oxide: 3Fe + 2O₂ → Fe₃O₄. (In damp air at ordinary temperature iron rusts to hydrated iron(III) oxide.)ironoxygenmagneticoxide
Water vapourat red heat (about 500 °C): 3Fe + 4H₂O(steam) ⇌ Fe₃O₄ + 4H₂heat4H₂OsteamFe₃O₄
Non-metals
  • 2Fe + 3Cl₂ → 2FeCl₃ (heat)
  • Fe + S → FeS (heat)
3Cl₂2FeCl₃heat
Acids
  • Dilute HCl or H₂SO₄ give iron(II) salts and hydrogen
  • not iron(III)
  • because the hydrogen released reduces any Fe3+ formed: Fe + H₂SO₄ → FeSO₄ + H₂
  • Fe + 2HCl → FeCl₂ + H₂. Hot concentrated H₂SO₄ gives a mixture of Fe(II) and Fe(III) sulphates with SO₂: 3Fe + 8H₂SO₄ → FeSO₄ + Fe₂(SO₄)₃ + 4SO₂ + 8H₂O. Concentrated nitric acid makes iron passive: a thin
  • protective oxide layer forms and stops further reaction. The layer can be removed by abrasion or by dilute hydrochloric acid
DiluteH₂SO₄giveiron
Comparison aspectMain ideaKeywords
Worked example 7.
  • 5.6 g of iron are heated with 3.2 g of sulphur. Moles: Fe = 5.6/56 = 0.1 mol
  • S = 3.2/32 = 0.1 mol. Fe + S → FeS
  • 1:1
  • so 0.1 mol FeS = 0.1 × 88 = 8.8 g is formed with nothing left over. The product is not attracted by a magnet
  • unlike the original iron
ironheatedsulphurMoles
Worked example 8.
  • Volume of hydrogen at STP obtained when 11.2 g of iron dissolve in excess dilute sulphuric acid: moles Fe = 11.2/56 = 0.2 mol
  • Fe + H₂SO₄ → FeSO₄ + H₂ gives 0.2 mol H₂ = 0.2 × 22.4 = 4.48 L
Volumehydrogenobtainediron
Lesson 4: Iron Chemical Properties, Reactions and Oxides · 3 / 7
Iron oxides

Iron oxides

1. Iron(II) oxide, FeO→(black powder, insoluble in water)
Comparison aspectMain ideaKeywords
Preparation
  • heating iron(II) oxalate without air: FeC₂O₄ → FeO + CO + CO₂
  • or reducing higher oxides with H₂ or CO under controlled conditions: Fe₂O₃ + H₂ → 2FeO + H₂O
  • Fe₂O₃ + CO → 2FeO + CO₂
heatingironoxalatewithout
Propertieseasily oxidized in hot air: 4FeO + O₂ → 2Fe₂O₃. Reacts with acids to give iron(II) salt and water: FeO + H₂SO₄ → FeSO₄ + H₂Oeasilyoxidized4FeO2Fe₂O₃
2. Iron(III) oxide, Fe₂O₃
  • (red-brown
  • found in nature as hematite)
brownfoundnaturehematite
Comparison aspectMain ideaKeywords
Preparation
  • (i) adding an alkali to an iron(III) salt gives reddish-brown Fe(OH)₃: FeCl₃ + 3NH₄OH → Fe(OH)₃↓ + 3NH₄Cl
  • heating the hydroxide above 200 °C: 2Fe(OH)₃ → Fe₂O₃ + 3H₂O. (ii) Heating iron(II) sulphate: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
addingalkaliironsalt
Properties
  • insoluble in water
  • used as a red pigment in paints
  • reacts with hot concentrated acids to give iron(III) salts: Fe₂O₃ + 3H₂SO₄ → Fe₂(SO₄)₃ + 3H₂O
insolublewaterusedpigment
3. Black (magnetic) iron oxide, Fe₃O₄(found as magnetite)foundmagnetite
Comparison aspectMain ideaKeywords
•A mixed oxide of iron(II) and iron(III) (FeO·Fe₂O₃)mixedoxideironFe₂O₃
Preparation
  • action of hot air or steam on red-hot iron
  • or partial reduction of iron(III) oxide: 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂
actionsteamironpartial
Properties
  • strongly magnetic
  • with hot concentrated acids it gives both iron(II) and iron(III) salts
  • proving it is a mixed oxide: Fe₃O₄ + 4H₂SO₄ → FeSO₄ + Fe₂(SO₄)₃ + 4H₂O. On heating in air it is oxidized to iron(III) oxide: 4Fe₃O₄ + O₂ → 6Fe₂O₃. Further reduction by H₂ or CO (about 400–700 °C) gives lower oxides and finally iron
stronglymagneticconcentratedacids
Worked example 9 (the mixed oxide).
  • If 46.4 g of Fe₃O₄ (M = 232
  • 0.2 mol) are dissolved in excess hot concentrated sulphuric acid
  • they give 0.2 mol of FeSO₄ (30.4 g) and 0.2 mol of Fe₂(SO₄)₃ (80 g). This ratio 1 : 1 of Fe(II) salt to Fe(III) salt shows that for every three Fe atoms in the oxide one is Fe2+ and two are Fe3+
  • matching FeO·Fe₂O₃
Fe₃O₄dissolvedexcessconcentrated
Lesson 4: Iron Chemical Properties, Reactions and Oxides · 4 / 7
Summary of the oxides

Summary of the oxides

Oxide Colour Occurs as Fe oxidation state With hot conc. acid gives
FeO black — +2 Fe(II) salt + water
Fe₂O₃ red-brown hematite +3 Fe(III) salt + water
Fe₃O₄ black, magnetic magnetite +2 and +3 (mixed) Fe(II) and Fe(III) salts
Lesson 4: Iron Chemical Properties, Reactions and Oxides · 5 / 7
Typical "give reasons" for this chapter

Typical "give reasons" for this chapter

Comparison aspectMain ideaKeywords
Chromium and copper have abnormal electronic configurationsa half-filled (3d5) or full (3d10) sublevel makes the atom more stablehalffilledfull3d10
Chromium is active but resists atmospheric attacka non-porous oxide layer of larger volume than the metal forms on its surfaceporousoxidelayerlarger
Cu, Ag and Au are transition elements
  • their ions (Cu2+
  • Au3+) have incomplete d sublevels
ionsincompletesublevels
•The decrease in atomic radius across the first series is small: increasing nuclear attraction is opposed by increasing repulsion between 3d electronsdecreaseatomicradiusacross
Transition elements have high melting and boiling pointsstrong metallic bonds involving both 4s and 3d electronsstrongmetallicbondsinvolving
Most transition-element compounds are attracted by a magnetthey contain unpaired d electronscontainunpairedelectrons
•Iron reacts with dilute acids to give iron(II) not iron(III) salts: the liberated hydrogen reduces Fe3+ to Fe2+Ironreactsdiluteacids
Scandium does not form Sc2+it loses all three outer electrons to reach the stable argon structurelosesthreeouterelectrons
Iron(III) chloride is paramagneticFe3+ is 3d5 with five unpaired electronsfiveunpairedelectrons
Concentrated nitric acid makes iron passivea thin protective oxide layer forms on the surfacethinprotectiveoxidelayer
Most transition elements are catalysts
  • their 4s and 3d electrons form temporary bonds with reactants
  • lowering the activation energy
electronsformtemporarybonds
Lesson 4: Iron Chemical Properties, Reactions and Oxides · 6 / 7
Part 7

Part 7

  • 📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

  • Lesson 4 – Iron Chemical Properties, Reactions and Oxides
  • Detailed Academic Lesson Overview:
  • Iron reacts with non-metals (S, Cl2), dilute acids to produce Iron(II) salts and hydrogen, and concentrated acids.
  • We examine the preparation, oxidation, and acid behaviors of FeO, Fe2O3, and magnetic iron oxide Fe3O4.

🌳 Hierarchy Classification & Conceptual Roadmap

Iron and Iron Oxides Chemistry

├── 1. Pure Iron Reactions

  • │ ├── 2 Fe + 3 Cl2 → 2 FeCl3 (Chlorine is a strong oxidizing agent, producing Fe3+)

│ ├── Fe + S → FeS (Sulfur produces Fe2+)

  • │ ├── Fe + dil. H2SO4 → FeSO4 + H2 (Hydrogen is a reducing agent, preventing Fe3+ formation)
  • │ └── Passivity Reaction with conc. HNO3 forms a microscopically thin, non-porous protective oxide layer

├── 2. Iron(II) Oxide (FeO)

  • │ ├── Prepared by thermal decomposition of iron(II) oxalate in absence of air
  • │ └── Reacts with dilute acids to give Fe(II) salts and water

├── 3. Iron(III) Oxide (Fe2O3 – Hematite)

  • │ ├── Prepared by heating iron(II) sulfate (FeSO4) or iron(III) hydroxide (Fe(OH)3)
  • │ └── Reacts only with hot concentrated acids to give Fe(III) salts

└── 4. Magnetic Iron Oxide (Fe3O4 – Magnetite)

Comparison aspectMain ideaKeywords
├── Mixed oxideFeO · Fe2O3Fe2O3
•└── Reacts with hot conc. H2SO4 to produce a mixture of Fe(II) and Fe(III) sulfatesReactsconcH2SO4produce
•📋 Master Teacher's Blackboard Rules & Exam SecretsMasterTeacherBlackboardRules
RULE 1Iron reacts with dilute acids to give Fe2+ salts because the liberated H2 gas is a reducing agentIronreactsdiluteacids
RULE 2Iron reacts with chlorine gas to give Fe3+ (FeCl3) because chlorine is a strong oxidizing agentIronreactschlorinegive
RULE 3
  • Fe3O4 is a mixed oxide
  • proof: when reacted with hot concentrated acid
  • it yields both Fe2+ and Fe3+ salts (FeSO4 + Fe2(SO4)3 + H2O)
Fe3O4mixedoxideproof
TRAP
  • Heating iron(II) oxalate in AIR yields Fe2O3 (due to rapid atmospheric oxidation of FeO)
  • but heating in the ABSENCE of air yields FeO + CO + CO2
Heatingironoxalateyields
📊 Detailed Comparison MatrixDiagnostic Reaction Matrix of Iron Oxides with AcidsDiagnosticReactionIronOxides
Oxide Reaction with Dilute Acids Reaction with Hot Conc. H2SO4 Behavior on Heating in Air
Iron(II) oxide (FeO) Dissolves giving Fe2+ salt + H2O Reacts giving Fe2+ salt Oxidizes rapidly to Fe2O3 (Hematite)
Iron(III) oxide (Fe2O3) No reaction (Does not dissolve) Dissolves giving Fe2(SO4)3 + H2O Thermodynamically stable (No change)
Magnetite (Fe3O4) No reaction (Does not dissolve) Gives mixed FeSO4 + Fe2(SO4)3 + H2O Oxidizes at high temp to Fe2O3
Lesson 4: Iron Chemical Properties, Reactions and Oxides · 7 / 7