Question

what is activation energy?

Answer 1

the energy given to start any process is refered to as activation energy

Answer 2

Good answer, very concise. Can I try to add a little on top, with an example?

Let us consider a reversible process, such as opening and closing an umbrella. There are many modern marvels in this field, but I am thinking of the classic umbrella with its curved handle and all that.

In normal conditions, with no wind and such, opening the umbrella requires an effort on our side; it will not happen spontaneously. We will have to actively move, using our muscles, the center piece along the rod, overcoming in the process the tension on the fabric that covers the umbrella, as well as gravity. At some point, we will even have to slide the center piece over a triangular tab attached to a spring, which will also absorb part of the energy our bodies provide to the system, up to a point where the spring will push the triangular piece back out, causing it to lock the umbrella in the open position. The overall balance of the process is endothermic, because it absorbs energy for it to happen.

The opposite process, closing the umbrella, is overall exothermic because the energy required for it to occur is already stored in the system. Once we press the triangular tab that locks the umbrella open, the tension in the fabric as well as gravity will cause the center piece to slide down the rod again, effectively closing the umbrella for us.

The interesting thing is that the last bit of the first process, when the spring pushes the triangular tab out, is exothermic. The energy that was stored in the spring as we slid the center piece up is released once we go past the tab and it comes back out. Even though the whole process is endothermic, its last stage is exothermic.

The opposite must be true for the reverse process. Despite the fact that closing an umbrella is exothermic, the initial stage is endothermic. We first need to provide energy to push the locking tab into the rod, and only then will the tension in the fabric and gravity be able to close the umbrella. That bit of energy required to start an exothermic process is its activation energy.

I hope this helped visualizing the concept. In any case, have fun and enjoy the day.