Question

Is paper burning with a bright flame endothermic or exothermic? Plastics becoming brittle after being left in the sun? A firecracker exploding?

Answer 1

The burning of paper is certainly an exothermic chemical reaction.

Explanation:

A piece of plastic that becomes brittle after exposure to sunlight has undergone an endothermic reaction. Why? Because energy in the form of sunlight is required to be put into the system to weaken the strong `C-C` bonds of a plastic. On the other hand, an exploding firecracker (and you don't see many of them around these days, alas my lost youth!) is an exothermic reaction.

Another way to consider reaction energetics is to consider energy transfer. Energy is released when bonds are FORMED; energy is required, i.e. absorbed, when bonds are broken. When we respire, we inhale dioxygen gas, and combust this with sugar to form STRONG `C=O` bonds in carbon dioxide, and strong `O-H` bonds in `OH_2`. These bonds are stronger than the bonds you break in the sugar so the net process is EXOTHERMIC.

And to cite another common process. Green plants utilize solar energy to form simple sugars using carbon dioxide and water. Energy is required to break these strong bonds, and the sun provides it.