Methane, physical and chemical properties of alkanes
Point 8 of 9 — Introduction to Organic Chemistry, Wöhler’s Theory and AlkanesQuestion bank: 0 questions (0 with pictures)
08 Methane, physical and chemical properties of alkanes
Preparation of methane in the laboratory
Heating sodium acetate withsoda lime(NaOH + CaO): CH₃COONa + NaOH —CaO/heat→ CH₄ ↑ + Na₂CO₃. CaO makes the mixture porous and prevents the tube from fusing.
It is collected over water because it isinsoluble in waterand lighter than air.
Physical properties
The first four members (C₁–C₄) are gases, C₅–C₁₇ areliquids and the higher ones are solids; boiling point rises with molecular mass.
Non-polar, so they are insoluble in water and soluble in organic solvents; they are lighter than water and burn with a luminous flame.
Chemical properties
Combustion: CH₄ + 2O₂ → CO₂ + H₂O + heat. Complete combustion gives CO₂ and H₂O; incomplete combustion gives CO (poisonous) or free carbon (soot).
Reaction with halogens — substitution: indiffused sunlight hydrogen atoms are replaced one after another: CH₄ + Cl₂ → CH₃Cl + HCl → CH₂Cl₂ → CHCl₃ → CCl₄. Methane is unreactive in the dark, hence the old name paraffin "little affinity".
Uses of halogeno-derivatives: CHCl₃ (chloroform) as an anaesthetic and solvent, CCl₄ as a solvent and in fire extinguishers, Freon in refrigerators, and iodoform as an antiseptic.
Catalytic cracking: long-chain alkanes are broken at about 500 °C with a catalyst into shorter alkanes and alkenes — the industrial route to petrol and to unsaturated hydrocarbons: C₈H₁₈ → C₄H₁₀ + C₄H₈.
Obtaining fragmented carbon (carbon black): incomplete combustion of methane gives carbon black, used in rubber tyres, inks and paints: CH₄ —(limited air)→ C + 2H₂.
Obtaining water gas: passing steam over red-hot coke gives a mixture of CO and H₂ (water gas), used as a fuel and as a source of hydrogen: C + H₂O → CO + H₂.
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