4.13:
Synthesis and Decomposition Reactions
Generally, in a chemical reaction, molecules interact by breaking one set of bonds and forming a new set of bonds.
A redox, or oxidation–reduction, reaction is a type of chemical reaction involving the partial or complete transfer of electrons. In such reactions, one reactant is oxidized and the other is reduced, with an observable change in their oxidation states.
The oxidized element, which has lost electrons, undergoes an increase in oxidation state. The reduced element, which has gained electrons, undergoes a decrease in oxidation state.
Among the most common redox reactions are synthesis and decomposition reactions. The synthesis of proteins from different amino acids and the digestion of proteins into amino acids are important examples.
Synthesis, or combination, reactions involve the formation of bonds between reactants to create a single product. The reactants may include only elements, elements and compounds, or only compounds.
Examples are the combination of elemental hydrogen and oxygen to create water, the addition of carbon monoxide to elemental oxygen to form carbon dioxide, and the combination of calcium oxide and water to form calcium hydroxide.
Notice that in all cases, multiple simpler reactants combined into a single complex product.
A decomposition reaction is the opposite of a synthesis reaction. In decomposition reactions, a single complex reactant breaks down into simpler products like elements, elements and compounds, or just compounds.
Decomposition reactions require an input of some form of energy. For example, under the influence of an electric field, water breaks down to give hydrogen and oxygen.
In the presence of sunlight, hydrogen peroxide decomposes into oxygen and water. Similarly, calcium hydroxide, upon being heated, decomposes into calcium oxide and water.
4.13:
Synthesis and Decomposition Reactions
Synthesis and decomposition are two types of redox reactions. Synthesis means to make something, whereas decomposition means to break something. The reactions are accompanied by chemical and energy changes.
Synthesis Reactions
Synthesis reactions are also called combination reactions. It is a reaction in which two or more substances combine to form a complex substance. Synthesis reactions are generally represented as: A + B → AB or A + B → C. The formation of nitrogen dioxide is a synthesis reaction: 2 NO (g) + O2 (g) → 2 NO2 (g).
In synthesis reactions, the reactants could be all elements (1), or a combination of an element and a compound (2), or all compounds (3).
1) C (s) + O2 (g) → CO2 (g)
2) 2 CO (g) + O2 (g) → 2 CO2 (g)
3) 2 CaO (s) + 2 H2O (l) → 2 Ca(OH)2 (s)
A combination reaction between a metal and a nonmetal always produces an ionic solid. For example, the formation of sodium chloride or table salt from sodium and chlorine is a combination reaction: 2 Na (s) + Cl2 (g) → 2 NaCl (s).
A synthesis reaction is generally accompanied by the release of energy. In the above example of sodium chloride, 787 kJ of heat energy is released.
Decomposition Reactions
Oxygen was first discovered by the scientist Joseph Priestley, in 1774, by heating mercury oxide with a burning glass. The reaction was a result of decomposition. Priestley had broken down mercury(II) oxide with heat into its elements.
The reaction is represented as: 2 HgO (s) → 2 Hg (l) + O2 (g)
Decomposition reactions involve breaking down a more complex substance into two or more smaller substances. This reaction is often represented as: AB → A + B or C → A + B. Decomposition reactions occur everywhere. For instance, the digestion of proteins, fats, and carbohydrates in our food is an important decomposition reaction. Another example is the decomposition of sodium azide into nitrogen gas.
The reaction is represented as: 2 NaN3 (s) → 2 Na (s) + 3 N2 (g)
In the above reaction, although the coefficient 2 indicates two molecules of sodium azide being decomposed, there is only one reactant. It is, therefore, a decomposition reaction. Similar to the synthesis reaction, in a decomposition reaction, the products formed could be all elements (1), or a combination of elements and compounds (2), or all compounds (3).
1) 2 Al2O3 (s) → 4 Al (s) + 3 O2 (g)
2) 2 KClO3 (s) → 2 KCl (s) + 3 O2 (g)
3) NH4Cl (s) → NH3 (g) + HCl (g)