Why Are Decomposition Reaction Called the Opposite of Combination Reaction?

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In decomposition reaction, a compound breaks down into two or more simple items, whereas in incorporation reaction, two or more items combine to form an end product. Consequently, both the reactions are diametrically opposed.

For example, in a combination reaction, simple molecules of hydrogen and oxygen combine to form a water molecule.

2H2 (g) + O2 (g) → 2H2O (l)

While in decomposition reaction, reverse process takes place—​i.e., water decomposes to hydrogen and oxygen.

2H2O (l) → 2H2 (g) + O2 (g)

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Decomposition Reactions

Decomposition reactions are chemical reactions in which a molecular compound is broken down into simpler compounds by converting all of the bonds within the molecule to single bonds.

For example, when sugar breaks down it becomes carbon dioxide and water. The decomposition reaction can be represented as follows:

C6H12O6 → 6CO2 + 12H2O

In this case there would be six double bonds on each glucose unit, but only one bond between two units. If we were to break up these molecules further then we could get more simple products such as CO2 or H2O.

Even when one or more of the products are still compounds, a reaction is still considered to be a decomposition process. A metal carbonate decomposes into a metal oxide and carbon dioxide gas during the process of oxidation. Calcium carbonate, for example, decomposes into calcium oxide and carbon dioxide, as follows:

CaCO3 → CaO+CO2

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Combination Reactions

Combination reactions are reactions where bonds are broken and new bonds are formed between two or more reactants. In a combination reaction, atoms and/or molecules do what the name implies: they combine to form the product. Combination reactions are always exothermic. For example, barium metal and fluorine gas will form the salt barium fluoride in an exergonic reaction.

Ba + F2 → BaF2

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