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Examples of Combination Reaction

A chemical reaction involves the conversion of some starting materials which are called as reactants to different chemical compounds which are called as products. Chemical reactions lead to the formation and breaking of chemical bonds between atoms in the molecules. Let’s discuss a particular type of chemical reaction which are named as combination or synthesis reaction. Direct combination or synthesis reaction involves the combination of two or more chemical species and form a more complex product. The general combination reaction can be written as; 

A + B → AB

Such reactions involve the combination of two or more substances but form only one substance as the final product. For example; the combination of iron and sulphur results the formation iron (II) sulphide as given below;

8 Fe + S8 → 8 FeS

Since such reactions form a new chemical substance therefore they are also called as synthesis reactions. Combination reactions can be classified in three types;
  • Reaction between elements.
  • Reaction between elements and compounds.
  • Reaction between compounds.
The reaction of sodium with chlorine form sodium chloride. It shows the combination of two elements; sodium and chlorine element. One is metal here and another is non-metal whereas product is an ionic compound.

2Na(s) + Cl2(g) → 2NaCl (g) 

The combination of two non-metals forms covalent compounds such as combination of solid sulphur and oxygen gas forms sulphur dioxide gas.
S(s)  + O2(g) → SO2(g)

The combination reaction of oxygen molecule with carbon monoxide gas forms carbon dioxide. It is an example of combination of elements and compounds.
O2(g) + 2CO(g) → 2CO2(g)

Similarly the combination of two compounds can form a new compound such as combination of calcium oxide with carbon dioxide gas forms calcium carbonate.
CaO(s) + CO2(g) → CaCO3(s)

Combination or synthesis reactions are exothermic in nature because they involve the formation of new bonds and release a large amount of energy in the form of heat.Another example of combination reaction is the formation of calcium hydroxide by the reaction of quick lime with water. Calcium hydroxide is also called as slaked lime. It is highly vigorous and exothermic reaction.

CaO(s) + H2O(l)  Ca(OH)2(s)

Some of the combustion reactions are also example of combination reaction such as combustion of coal or hydrogen in the presence of oxygen result the formation of carbon dioxide and water respectively. Both are example of combination reactions. 

C(s)  + O2(g) → CO2(g)
2H2(g) + O2(g) → 2H2O (l)

When ammonia gas reacts with hydrogen chloride gas, it forms ammonium chloride which is a white colour solid.

NH3(g) + HCl (g) NH4Cl(s)

Formation of sulphuric acid form the combination of sulphur trioxide and water is also a combination reaction which is again highly exothermic reaction. 
SO3(g) + H2O(l) H2SO4(l)

Formation of black ferrous sulphide form solid iron and sulphur is also a combination reaction.

Fe(s) + S(S) FeS(s)

Another example of combination reaction is Heber’s process which involves the combination of nitrogen and hydrogen gas to form ammonia.
N2(g) + H2(g) NH3(g)

Similarly combination of aluminium with oxygen forms aluminium oxide and combination of potassium with chlorine gas forms potassium chloride.
4 Al(s) + 3 O2(g) → 2 Al2O3(s)
2 K(s) + Cl2(g) → 2 KCl(s)