Final answer:
A spontaneous reaction releases free energy and moves to a more stable state, whereas an instantaneous reaction occurs quickly with a sudden release of energy.
Step-by-step explanation:
A spontaneous reaction is one which releases free energy and moves to a more stable state. Instantaneous reactions occur rapidly with sudden release of energy.
For example, the combustion of gasoline is a spontaneous reaction. When ignited, gasoline rapidly reacts with oxygen and releases energy in the form of heat and light.
On the other hand, an instantaneous reaction occurs without any delay. A good example is the explosion of fireworks, where the reaction happens instantaneously, resulting in a sudden release of energy and colorful lights.