Question

Why are spontaneous processes irreversible?

Answer 1

The second law of thermodynamics expresses that the condition of entropy of the universe will consistently increment after some time. Measure of spontaneous changes occurring in a system is called entropy. Entropy is represented by ‘S’ and it is an extensive property as the value depends on the amount of the substance in the system.

Complete answer:
Chemical reactions which take place without any influence of external factors are spontaneous reactions and the reaction that requires an influence of external factors that cannot take place with the influence of external factors are non-spontaneous reactions.
An irreversible process requires a different path to be taken in order to get back to its original state and a spontaneous process can be reversed only by opting for a different path to get to its original state.
Like for example in a cylinder expansion of a gas into a vacuum is a spontaneous process and if the gas is expanded it will require work from the environment to return back to its original state (the surrounding have to do work on the system $w > 0$. Hence, the path for restoring the system to its original state can be achieved by a different value of w rather than following the path by which the system was changed first.

Note:
Spontaneous changes are natural processes. If the value of $\Delta G$ is positive, the process is a non- spontaneous process and if the value of $\Delta G$ is negative the process will not require input from the surroundings and will be a spontaneous process. A spontaneous process is an irreversible process.