What is chemical reaction? Smart Definition

Photo of a Spontaneous ignitions reaction of sodium in water.



Definition
Chemical reaction is a process in which atoms of elements exchanges electrons (valence electron), with each other to form a new substance.


It is a process involved in the combination of one or more substances and transformed into one or more new substances. 

Energy is released (exothermic reaction or negative heat change) or is absorbed (endothermic reaction or positive heat change), but there will be no loss in total molecular weight. 

When, for example, water is decomposed, its molecules, [2H2O(g) --> 2H2(g) + O2(g) ]each of which consists of one atom of oxygen and two of hydrogen, are broken down; the hydrogen atoms then combine in pairs to form hydrogen molecules and the oxygen atoms to form oxygen molecules as you can see from the above balanced equation. In a chemical reaction, substances lose their characteristic properties. Water, for example, a liquid which neither burns nor supports combustion, is decomposed to yield flammable hydrogen 2H2(g) and combustion-supporting oxygen O2(g).

 In some reactions heat is given off (exothermic reactions as stated above), and in others heat is absorbed (endothermic reactions as stated above). 

Furthermore, the new substances formed differ from the original substances in the energy they contain. Chemical reactions are classified according to the kind of change that takes place.

 When a compound, which consists of two or more elements or groups of elements, is broken down into its constituents, the reaction is called simple decomposition. As you can see in water 2H2O(g) --> 2H2(g) + O2(g). 

When two compounds react with one another to form two new compounds, the reaction is called double decomposition. Like the case of Silver Nitrate and Sodium Chloride AgNO 3 + NaCl → AgCl + NaNO 3. 
In so-called replacement reactions the place of one of the elements in a compound is taken by another element reacting with the compound.

 When elements combine to form a compound, the reaction is termed chemical combination. 

Oxidation (gain of oxygen). And reduction (loss of oxygen) reactions are extremely important. 




Reversible reactions are those in which the chemical change taking place may be paralleled by another change back to the original substances. This is common among weak acids and weak bases for Example  carbonic acid will react with water as follow: H2CO3 (l) + H2O(l)  HCO3 (aq) + H3O+(aq).

Temperature, pressure, and the concentration of the substances involved and, sometimes, upon the use of a chemical called a catalyst. In some chemical reactions.



  

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