Final answer:
A spontaneous reaction is one that is thermodynamically favorable, but may not be fast, while a fast or instantaneous reaction occurs quickly and might not be spontaneous. The difference lies in thermodynamics, favoring product formation without continuous energy input, versus kinetics, determining reaction rate.
Step-by-step explanation:
It's important not to equate the spontaneity of a reaction with its speed. While a spontaneous reaction is one where product formation is favored under given conditions, it does not necessarily mean that the reaction will occur quickly. Spontaneity is a thermodynamic quantity, indicating whether a process will occur without needing an ongoing input of energy. This is separate from the kinetics of a reaction, which deals with the rate or speed at which a reaction proceeds.
For example, the combustion of paper is spontaneous because it moves to a more stable energy state by releasing energy, but it will not occur without sufficient activation energy, such as heat from a flame. Therefore, even though the reaction is favorable, it will not proceed rapidly without this energy. In essence, a reaction can be spontaneous but may still require a condition, like heat, to overcome the activation energy barrier and occur.
In contrast, fast reactions or instantaneous reactions are those that proceed rapidly and might or might not be spontaneous. These reactions are typically characterized by a sudden release of energy and can quickly reach completion once started but saying a reaction is fast gives no direct indication of its spontaneity.