Endothermic and exothermic is a common phenomenon in chemical reactions, and there are many ways to judge endothermic and exothermic, and several commonly used methods are introduced below.
1.Judging by the essence of the chemical reaction.
The essence of a chemical reaction is the breaking of old bonds and the formation of new ones. If the old bond is broken to absorb the energy and the new bond is formed to release the energy, then the reaction is endothermic;If the old bond breaks to absorb the energy and the new bond is formed to release the energy, then the reaction is exothermic.
2.Judging by the chemical equation.
Depending on whether or not endothermic or exothermic substances appear in the chemical equation, it can be judged whether the reaction is endothermic or exothermic. For example, some common endothermic reactions include the reaction of C with water vapor, the reaction of carbon with carbon dioxide, the decomposition of metal oxides, etc.;Some common exothermic reactions include the reaction of metals with acids, bases, or salts, acid-base neutralization reactions, etc.
3.It is judged by the energy difference between the reactant and the product.
If the energy of the reactant is higher than the energy of the product, then the reaction is endothermic;If the energy of the reactant is lower than the energy of the product, then the reaction is exothermic.
4.Judging by experimental phenomena.
In chemical experiments, it is possible to judge whether a reaction is endothermic or exothermic by observing the experimental phenomena. For example, when heated, if the color of the substance changes, bubbles are produced, or precipitation is produced, etc., the reaction may be endothermic;If the substance turns black or red when heated, etc., then the reaction may be an exothermic reaction.
5.Judging by the energy state of matter.
In a chemical reaction, the state of the substance also affects the endothermic or exothermic of the reaction. For example, reactions between solid substances are generally more endothermic than those between liquid or gaseous substances;Whereas, reactions between liquid substances are generally more exothermic than reactions between gaseous substances.
In short, there are many ways to judge endothermic and exothermic, and it is necessary to choose the appropriate method according to the specific situation.