Spontaneous Reaction

Many people believe that the term spontaneous in 'spontaneous reaction' is associated with the speed of the reaction in question, which is absolutely wrong. A look at the list of spontaneous reactions, and you will realize that some of these reactions are so slow that they span over a period of millions of years. The decaying of a diamond to graphite is an apt example of the same. Basically, the rate of reaction is not dependent on its spontaneity, but is dependent on its chemical kinetics. As you go ahead with this write-up, you will get a better understanding of the entire concept.

What is a Spontaneous Reaction?

Spontaneous reaction is defined as a reaction which takes place on its own, without the involvement of any external factor to facilitate it. It is also defined as a time-evolution of a system in which it releases free energy and moves to a lower, but thermodynamically more stable, state of energy. Though in rare cases, an external trigger may be required at the beginning of the process to start it. But once it starts, it continues on its own. This is in stark contrast to non-spontaneous reaction - wherein some external factor has to continuously come into play to make sure that the process continues.

The direction of a spontaneous process is determined by the laws of thermodynamics - which govern the relations between states of energy in a closed system. Even though spontaneous and non-spontaneous - both reactions are possible, only the former tends to occur naturally while the latter has to be triggered and supported. More importantly, the reaction of a spontaneous reaction is always of a non-spontaneous nature.

In chemistry, spontaneous reaction is quite popular in context of endothermic and exothermic reactions. Most of the reactions that we observe are exothermic reactions, which are chemical reactions accompanied by the evolution of heat. That being said, there do exist some endothermic reactions, i.e. chemical reactions accompanied by the absorption of heat, which qualify to be spontaneous reactions by nature.

Spontaneous Reaction Examples

There exists a lengthy list of chemical reactions in everyday life which are obvious spontaneous reactions. One of the best examples of spontaneous reaction is rusting of an iron nail. If you observe closely, you will realize that although the iron nail rusts slowly, it rusts continuously without any external factor coming into play. Similarly, an example of reaction which requires a trigger is burning of wood. Once the wood is set on fire (trigger) it continues to burn on its own without any other factor coming into play so as to facilitate the burning process.

An example of quick spontaneous reaction is combustion of hydrogen (as well as burning of wood,) while an example of slow reaction is graphite turning into a diamond (as well as rusting of nail.) While endothermic reactions absorb heat from the surroundings, exothermic reactions release heat into the surroundings. One of the best examples of endothermic spontaneous reaction is ammonium nitrate dissolving in water - wherein it absorbs heat and makes water/solution cold. On the other hand, one of the best example of exothermic reaction is the chemical reaction between sodium and chlorine which emits heat in form of flames.

To sum it up, spontaneous reaction or spontaneous process is a process which can take place on its own under the given set of conditions. That being said, you need to understand that the spontaneity in case of spontaneous reaction refers to the feasibility of a process, and not its speed.