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A reaction in which two or more substances combine to produce a single substance is known as combination, or synthesis reaction. They may be represented by X + Y → XY Combination reactions are usually exothermic. For example barium metal and fluorine gas will combine in a highly exothermic reaction to form the salt barium fluoride:
Ba + F↓2→BaF2
Another example is magnesium oxide combining with carbon dioxide to produce magnesium carbonate.
Another example is Iron combining with Sulphur to produce Iron(II) Sulfide.
Often, reaction can fall into more than one category. For example the combustion of magnesium metal is also a combination reaction, it releases magnesium oxide and energy.
2Mg+O2→2MgO + Heat
This reaction is usually exothermic because when the bond forms between the reactants, heat is released.
A combination reaction can be of three types:
|a) Between two elements||C + O2 → CO2||Carbon completely burnt in oxygen yields carbon dioxide|
|b) Between two compounds||2CaO + 2H2O → 2Ca(OH)2||Calcium oxide (lime) combined with water gives calcium hydroxide (slaked lime)|
|c) Between one element and one compound||2CO + O2 → 2CO2||Oxygen combines with Carbon monoxide, Carbon dioxide is formed.|
When a combination reaction occurs between a metal and a nonmetal the product is an ionic solid. An example could be lithium reacting with sulfur to give lithium sulfide. When magnesium burns in air, the atoms of the metal combine with the gas oxygen to produce magnesium oxide. This specific combination reaction produces the bright flame generated by flares.
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