Why are decomposition reaction called opposite of combination reaction

For example : Combination : 2H₂ + O₂ → 2H₂O

Decomposition: 2H₂O ──Electricity──▶ 2H₂ + O₂

You can clearly see that in first one , hydrogen and oxygen atoms are combine together to form water. But On the second one see that water molecule is decomposed and break down in to hydrogen and oxygen when electricity is passed through water.

Why Are They Opposite Reactions?

Combination Reaction	Decomposition Reaction
Two or more reactants participate	One reactant Breaks
One product formed at reactant.	Usually Two or more products are formed
Substance combine together	Compound breaks apart
Usually releases energy in the form of heat and light	Usually requires energy in the form of heat and electricity
General form: A + B → AB	General form: AB → A + B

Definition of Combination Reaction

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A combination reaction occurs when two or more reactants combine together to form a single product with heat and light. It is released heat and light that are exothermic but some of need electricity, heat and light.

General Equation

$$A + B \rightarrow AB$$

Example

$$2H_2 + O_2 \rightarrow 2H_2O$$

Explanation:

1. 2H₂ + O₂ → 2H₂O
   (Hydrogen + Oxygen → Water)
2. 2Mg + O₂ → 2MgO
   (Magnesium + Oxygen → Magnesium oxide)
3. C + O₂ → CO₂
   (Carbon + Oxygen → Carbon dioxide)
4. 2Ca + O₂ → 2CaO
   (Calcium + Oxygen → Calcium oxide)
5. Fe + S → FeS
   (Iron + Sulphur → Iron sulphide)
6. CaO + H₂O → Ca(OH)₂
   (Quicklime + Water → Slaked lime)
7. N₂ + 3H₂ → 2NH₃
   (Nitrogen + Hydrogen → Ammonia)
8. 2Na + Cl₂ → 2NaCl
   (Sodium + Chlorine → Sodium chloride)
9. SO₃ + H₂O → H₂SO₄
   (Sulphur trioxide + Water → Sulphuric acid)
10. CO₂ + H₂O → H₂CO₃
    (Carbon dioxide + Water → Carbonic acid)

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Real Life Examples of combination Reaction

• when we Burn magnesium ribbon then it forms magnesium oxide.
• Formation of water when hydrogen and oxygen combine.
• Preparation of slaked lime when we add water to quick lime. ( CaO+H2​O→Ca(OH)2​+Heat )
• Formation of rust on iron (It is a slow react with oxygen and moisture with Iron).

Definition of Decomposition Reaction

A decomposition reaction occurs when one compound breaks down into two or more simpler substances.

General Equation

$$AB \rightarrow A + B$$

Example

$$2HgO \rightarrow 2Hg + O_2$$

Explanation:

1. 2H₂O –(Electricity)–> 2H₂ + O₂
   (Water → Hydrogen + Oxygen)
2. CaCO₃ –(Heat)–> CaO + CO₂
   (Calcium carbonate → Calcium oxide + Carbon dioxide)
3. 2HgO –(Heat)–> 2Hg + O₂
   (Mercury(II) oxide → Mercury + Oxygen)
4. 2AgCl –(Sunlight)–> 2Ag + Cl₂
   (Silver chloride → Silver + Chlorine)
5. 2AgBr –(Sunlight)–> 2Ag + Br₂
   (Silver bromide → Silver + Bromine)
6. 2KClO₃ –(Heat, MnO₂)–> 2KCl + 3O₂
   (Potassium chlorate → Potassium chloride + Oxygen)
7. 2Pb(NO₃)₂ –(Heat)–> 2PbO + 4NO₂ + O₂
   (Lead nitrate → Lead oxide + Nitrogen dioxide + Oxygen)
8. 2NaHCO₃ –(Heat)–> Na₂CO₃ + CO₂ + H₂O
   (Sodium bicarbonate → Sodium carbonate + Carbon dioxide + Water)
9. NH₄NO₃ –(Heat)–> N₂O + 2H₂O
   (Ammonium nitrate → Nitrous oxide + Water)
10. 2H₂O₂ –(MnO₂)–> 2H₂O + O₂
    (Hydrogen peroxide → Water + Oxygen)

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