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Combination or addition or synthesis is a chemical change in which atoms or molecules of two or more substance combine to form molecules of a single new substance. For e.g.,
- Burning of hydrogen in air or oxygen to produce water.
- Combination of ammonia and hydrogen chloride to produce ammonium chloride.
- Burning of magnesium in air gives magnesium oxide.
Chemical reactions are often accompanied by evolution or absorption of energy.
The chemical reactions, which proceed with evolution of heat energy i.e. when heat is released, are called exothermic reactions.
This is only possible when the energy required to break the bonds of the reactants is less than the energy released in the formation of new bonds of the products.
Examples:
The amount of heat (energy) produced is written along with the products. This indicates that heat is given out.
The chemical reactions, which proceed with the absorption of heat energy, are called endothermic reactions.
Examples:
The amount of heat absorbed is written along with the reactants since it is absorbed.
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