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Chemical equilibrium refers to a stable ratio between reactants and products in a reversible chemical reaction. In a reversible reaction, reactants do not completely transform into products; rather, they will slowly stop reacting as chemical equilibrium is reached. The rate of a reaction is affected by many factors, including temperature, phase of matter and presence of a catalyst. Many reactions require an initial input of energy to begin reacting.
More fundamentally, a chemical reaction is the making or breaking of chemical bonds. A chemical bond results when electromagnetic forces between atoms or molecules cause an attraction between them. An ionic bond is when two ions—oppositely-charged atoms—directly attract one another. A covalent bond involves the sharing of electron pairs between atoms. These chemical bonds form new substances with their own chemical properties.
The concept of chemical equilibrium is related to the idea of a reversible reaction. In fact, all chemical reactions are to some degree reversible, so there is no fundamental difference between reactants and products. Some reactions, however, have a non-negligible degree of reversibility—it is in these reactions that chemical equilibrium becomes important. When reversibility is emphasized in scientific notation, the original arrow between reactants and products is replaced by a pair of hooked arrows. These indicate that the reaction occurs in both directions.
The speed at which chemical equilibrium is reached can vary tremendously. Some reactions are complete after less than a second, while others take many years. Although there is no single method of predicting a reaction rate, many factors are known to have important roles.
One such factor is temperature. Higher temperatures allow more energy to enter a system, which typically results in faster reactions. The phase of matter—solid, liquid or gas—can also influence how quickly substances reach chemical equilibrium. Finally, the presence of a catalyst can greatly accelerate a reaction. An enzyme is a type of catalyst important in regulating the metabolism of living things.
Some reactions do not take place even though they are not at chemical equilibrium. This is because many reactions require an activation energy. Liquid hydrogen and oxygen, for example, can remain in physical contact without reacting. A small amount of energy, however, can cause the reactants to essentially explode and release a great deal of energy. The same phenomenon is seen when burning wood—although wood can release much energy when burned, an initial spark or flame is always required to get the process going.