Decomposition Reactions — Breaking Down Complex Substances
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28件の反応
Decomposition reactions are the reverse of synthesis: a single compound breaks down into two or more simpler substances. The general form is AB -> A + B. These reactions require energy input — heat (thermal decomposition), electricity (electrolysis), or light (photolysis) — to break the chemical bonds holding the compound together. Decomposition is essential in both nature and industry, from digestion to metal extraction.
Reaction Mechanism
Bond breaking requires energy, so decomposition reactions are typically endothermic. The energy input must exceed the bond dissociation energy of the weakest bonds in the compound. Thermal decomposition occurs when heat provides sufficient energy — calcium carbonate decomposes at about 840 degrees C into calcium oxide and carbon dioxide. Electrolytic decomposition uses electrical energy to force non-spontaneous reactions, as in the electrolysis of water (2H2O -> 2H2 + O2). Photolytic decomposition uses light energy, exemplified by silver chloride darkening in sunlight.
Everyday Examples
When you open a bottle of carbonated water, dissolved carbonic acid decomposes into water and carbon dioxide gas — the fizz you see and hear. Hydrogen peroxide in your medicine cabinet slowly decomposes into water and oxygen, which is why it loses effectiveness over time. Food spoilage involves enzymatic decomposition of proteins, fats, and carbohydrates into simpler, often smelly compounds.
産業上の重要性
The thermal decomposition of limestone (CaCO3 -> CaO + CO2) produces quicklime, a key material in steel production, water treatment, and construction. Global production exceeds 350 million tonnes annually. Electrolysis of brine (NaCl solution) produces chlorine, sodium hydroxide, and hydrogen — the chlor-alkali process underpinning the chemical industry. Cracking of hydrocarbons in petroleum refining is thermal decomposition that converts heavy oils into gasoline, diesel, and petrochemical feedstocks.
Safety Note
Some decomposition reactions produce toxic or flammable gases. Heating ammonium nitrate can cause explosive decomposition — the cause of several industrial disasters. Always ensure adequate ventilation and never heat unknown compounds in sealed containers.
Cracking of Octane (Thermal Cracking)
C₈H₁₈ → C₄H₁₀ + C₄H₈
Large hydrocarbon molecules like octane are broken down into smaller, more useful molecules through thermal or catalytic cracking. This process …
Decomposition of Ammonium Nitrate
NH₄NO₃ → N₂O + 2H₂O
Ammonium nitrate decomposes at about 200 C to form nitrous oxide (laughing gas) and water. At higher temperatures or with …
Decomposition of Calcium Hypochlorite
2Ca(OCl)₂ → 2CaCl₂ + O₂
Calcium hypochlorite slowly decomposes to release oxygen, which is why it has strong oxidizing and bleaching properties. The decomposition rate …
Decomposition of Carbonic Acid
H₂CO₃ → H₂O + CO₂
Carbonic acid readily decomposes into water and carbon dioxide gas. This unstable acid exists in equilibrium with dissolved CO2 in …
Decomposition of Iron(II,III) Oxide (Direct Reduction)
Fe₃O₄ + 4CO → 3Fe + 4CO₂
Iron(II,III) oxide (magnetite) is reduced by carbon monoxide to produce metallic iron and carbon dioxide. This is one of the …
Decomposition of Nitrous Oxide (Laughing Gas)
2N₂O → 2N₂ + O₂
Nitrous oxide decomposes at high temperatures (above 600 C) into nitrogen and oxygen. This decomposition releases oxygen, which allows nitrous …
Decomposition of Barium Peroxide
2BaO₂ → 2BaO + O₂
Barium peroxide decomposes when heated above 700 C to form barium oxide and oxygen gas. This reversible reaction was historically …
Decomposition of Sodium Bicarbonate
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Sodium bicarbonate (baking soda) decomposes when heated above 50 C into sodium carbonate, water, and carbon dioxide. This thermal decomposition …
Decomposition of Water (Electrolysis)
2H₂O → 2H₂ + O₂
Water decomposes into hydrogen and oxygen gases when an electric current is passed through it. This electrolysis reaction is the …
Thermal Decomposition of Copper(II) Carbonate
CuCO₃ → CuO + CO₂
Green copper(II) carbonate decomposes when heated to form black copper(II) oxide and carbon dioxide gas. This is a classic laboratory …
Thermal Decomposition of Mercury(II) Oxide
2HgO → 2Hg + O₂
Mercury(II) oxide decomposes when heated to about 500 C into mercury metal and oxygen gas. This reaction is historically significant …
Thermal Decomposition of Potassium Chlorate
2KClO₃ → 2KCl + 3O₂
Potassium chlorate decomposes when heated (with MnO2 catalyst at ~200 C, or without catalyst at ~400 C) to form potassium …
Decomposition of Ozone
2O₃ → 3O₂
Ozone decomposes into molecular oxygen. In the stratosphere, this reaction is part of the ozone-oxygen cycle that protects Earth from …
Thermal Decomposition of Sodium Carbonate Decahydrate
Na₂CO₃·10H₂O → Na₂CO₃ + 10H₂O
Washing soda (sodium carbonate decahydrate) loses its water of crystallization when heated. This dehydration reaction produces anhydrous sodium carbonate (soda …
Decomposition of Silver Chloride (Photodecomposition)
2AgCl → 2Ag + Cl₂
Silver chloride decomposes when exposed to light, forming metallic silver and chlorine gas. This photosensitivity is the foundation of silver-based …
Thermal Decomposition of Sodium Nitrate
2NaNO₃ → 2NaNO₂ + O₂
Sodium nitrate decomposes when heated above 380 C to form sodium nitrite and oxygen. This reaction illustrates the general behavior …
Decomposition of TNT
2C₇H₅N₃O₆ → 3N₂ + 5H₂O + 7CO + 7C
TNT (trinitrotoluene) decomposes explosively when detonated, producing nitrogen, water, carbon monoxide, and solid carbon (soot). TNT is relatively stable and …
Decomposition of Zinc Carbonate
ZnCO₃ → ZnO + CO₂
Zinc carbonate decomposes when heated to form zinc oxide and carbon dioxide gas. Zinc carbonate (smithsonite) is an important zinc …
Thermal Decomposition of Sulfuric Acid
H₂SO₄ → H₂O + SO₃
Concentrated sulfuric acid decomposes at high temperatures (above 340 C) into water vapor and sulfur trioxide gas. This is the …
Thermal Decomposition of Lead(IV) Oxide
2PbO₂ → 2PbO + O₂
Lead(IV) oxide (lead dioxide) decomposes when heated above 290 C to form lead(II) oxide and oxygen. This decomposition reflects the …
Decomposition of Calcium Carbonate (Calcination)
CaCO₃ → CaO + CO₂
Calcium carbonate (limestone) decomposes when heated above 840 C into calcium oxide (quicklime) and carbon dioxide gas. This is one …
Decomposition of Hydrogen Peroxide
2H₂O₂ → 2H₂O + O₂
Hydrogen peroxide spontaneously decomposes into water and oxygen gas. The reaction is dramatically accelerated by catalysts like manganese dioxide, potassium …
Decomposition of Sodium Azide (Airbag Reaction)
2NaN₃ → 2Na + 3N₂
Sodium azide rapidly decomposes when ignited to produce sodium metal and nitrogen gas. This extremely fast reaction (completing in about …
Decomposition of Nitroglycerin
4C₃H₅N₃O₉ → 12CO₂ + 10H₂O + 6N₂ + O₂
Nitroglycerin decomposes explosively, producing a large volume of hot gases (carbon dioxide, water vapor, nitrogen, and oxygen) almost instantaneously. The …
Thermal Decomposition of Potassium Permanganate
2KMnO₄ → K₂MnO₄ + MnO₂ + O₂
Potassium permanganate, a deep purple crystalline solid, decomposes when heated above 240 C to form potassium manganate, manganese dioxide, and …
Thermal Decomposition of Magnesium Carbonate
MgCO₃ → MgO + CO₂
Magnesium carbonate decomposes when heated above 350 C to form magnesium oxide and carbon dioxide. This reaction occurs at a …
Thermal Decomposition of Ammonium Dichromate (Volcano Reaction)
(NH₄)₂Cr₂O₇ → Cr₂O₃ + N₂ + 4H₂O
Ammonium dichromate decomposes spectacularly when ignited, producing green chromium(III) oxide, nitrogen gas, and water vapor. The green fluffy chromium oxide …
Thermal Decomposition of Sodium Percarbonate
2Na₂CO₃·3H₂O₂ → 2Na₂CO₃ + 3H₂O₂
Sodium percarbonate dissolves in water to release hydrogen peroxide and sodium carbonate. This controlled release of H2O2 makes it an …