A direct combination reaction (also known as a synthesis reaction) is a reaction where two or more elements or compounds (reactants) combine to form a single compound (product). Such reactions are represented by equations of the following form: X + Y → XY (A+B → AB). The combination of two or more elements to form one compound is called a combination reaction. In other words, when two or more elements or compounds react so as to form one single

compound Compound may refer to: Architecture and built environments * Compound (enclosure), a cluster of buildings having a shared purpose, usually inside a fence or wall ** Compound (fortification), a version of the above fortified with defensive struc ...

, then the chemical reaction that takes place is called a combination reaction. , a)- Between elements , C + O₂ → CO₂ ,

Carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

completely burnt in oxygen yields

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

, - , b) Between compounds , CaO + H₂O → Ca(OH)₂ ,

Calcium oxide Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime''" connotes calcium-containing inorganic ...

(lime) combined with

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

gives calcium hydroxide (slaked lime) , - , c) Between elements and compounds , 2CO + O₂ → 2CO₂ , Oxygen combines with

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

,And carbon dioxide is formed. There is no specific number of reactants in a combination reaction. Combination reactions are usually exothermic because when the bond forms between the reactants, heat is released. For example, ''barium metal'' and fluorine gas will combine in a highly exothermic reaction to form the salt barium fluoride: Ba + F₂ → BaF₂ Another example is magnesium oxide combining with carbon dioxide to produce magnesium carbonate. MgO + CO₂ → MgCO₃ Another example is iron combining with sulfur to produce iron(II) sulfide. Fe + S → FeS When a combination reaction occurs between a metal and a non-metal the product is an ionic solid. An example could be

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...

reacting with sulfur to give

lithium sulfide Lithium sulfide is the inorganic compound with the formula Li2 S. It crystallizes in the antifluorite motif, described as the salt (Li+)2S2−. It forms a solid yellow-white deliquescent powder. In air, it easily hydrolyses to release hydrog ...

. When

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

burns in air, the atoms of the metal combine with the gas

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

to produce

magnesium oxide Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ions ...

. This specific combination reaction produces the bright flame generated by

flare A flare, also sometimes called a fusée, fusee, or bengala in some Latin-speaking countries, is a type of pyrotechnic that produces a bright light or intense heat without an explosion. Flares are used for distress signaling, illumination, ...

s. Combination reactions can also occur in other situations when the two products do not have the same ionic charge. In such a situation, different quantities of each reactant must be used. To denote this in a

chemical equation A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and chemical formulas. The reactant entities are given on the left-hand side and the product entities on the right-hand side with a plus sign between ...

, a coefficient is added to one or more of the reactants so that the total ionic charge of each reactant is the same. For example, Iron (III) Oxide is formed by the following equation: 4Fe + 3 O2 -> 2Fe2O3. This is because in this case, Iron has a charge of 3+ while each Oxygen atom in O2 has a charge of 2-. Oxygen Gas (O2) must be used instead of elemental oxygen (O) because elemental oxygen is a

free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...

that is unstable and combines with other oxygen atoms to form O2. Another example natural conditions. Likewise, the Hydrogen atoms should be in the form H2 because H is an unstable free radical not commonly found in nature; the chemical equation 2H + O2 -> H2O is also generally impossible under natural conditions. There are some types of combination reactions (a) Combination reactions between two elements. (b) Combination reactions between two compounds. (c) Combination reactions between an element and a compound. In all the above reactions, one single compound is formed. For example, (i) Combination Reaction Between two Elements a)

Hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

reacts with

to form water i.e Combustion of Hydrogen forms Water (do note that this reaction is an exothermic one). 2H2(g) + O2 -> 2H2O(l) + heat b)

reacts with chlorine to form Hydrogen chloride(also known as hydrochloric acid) H2(g) + Cl2(g) -> 2HCl(g) (ii) Combination Reaction Between two Compound a)

(quick lime) reacts with

to form calcium hydroxide (stalked lime) CaO(s) + H2O(l) -> Ca(OH)2 Here is another example of synthesis reaction or direct combination reaction , in this there are three reactants and they form a single product . * Nitrogen reacts with water and oxygen to form ammonium nitrate . 2N2(g) + 4H2O(l) + O2(g) -> 2NH4NO3(s)

References

{{Reflist Chemical reactions