### Theory:

So far, we have studied chemical reactions that taking place every day. Each reaction involves different types of atoms, and hence, the way they react also differs. Thus, based on how the atoms of the reactants are rearranged, chemical reactions are classified as follows:
1. Combination reaction
2. Decomposition reaction
3. Single displacement reaction
4. Double displacement reaction and
5. Combustion reaction
Combination reaction
A combination reaction occurs when two or more reactants combine to form one product. It is also referred to as a synthesis reaction or composition reaction. For example, when a reactant 'A' combines with 'B', it forms 'AB', as shown below.

Combination reaction

Combination reaction is further classified into three classes depending on the chemical nature of the reactants.

(i). $$Element+Element → Compound$$
(ii). $$Compound + Element → Compound$$
(iii). $$Compound + Compound → Compound$$

 Reaction type Reaction Examples Element + Element → Compound Two elements combine with one another to form a compound in this type of combination reaction. A metal and a non-metal, or two non-metals, may be involved in the reaction. (i). Both the reactants are non-metals $$S_{(g)}+O_{2(g)}→SO_{2(g)}$$ $$H_{2(g)}+Cl_{2(g)}→2HCl_{(g)}$$ (ii). One of the reactants is metal, and another one is a non-metal $$2Na_{(s)}+Cl_{2(g)}→2NaCl_{(s)}$$ Compound + Element → Compound In this case, a compound reacts with an element to produce a new compound. Phosphorus trichloride (compound) reacts with chlorine gas (element) to form phosphorous pentachloride. $$PCl_{3(l)}+Cl_{2(g)}→PCl_{5(s)}$$ Compound + Compound → Compound This is a reaction between two compounds to form a new compound. Reaction of silicon dioxide with calcium oxide to form calcium silicate. $$SiO_{2(s)}+CaO_{(s)}→CaSiO_{3(s)}$$

Most of the combination reaction releases a large amount of energy due to the formation of new bonds. Hence, they are exothermic in nature.