Are Glow Sticks Endothermic or Exothermic?

Glow sticks give off light but not heat. Because energy is released, the glow stick reaction is an example of an exergonic (energy-releasing) reaction. However, it is not an exothermic (heat-releasing) reaction because heat isn't released. You can think of exothermic reactions as a type of exergonic reaction. All exothermic reactions are exergonic, but not all exergonic reactions are exothermic. 

Endothermic reactions absorb heat. While glow sticks don't absorb heat and aren't endothermic, they are affected by temperature. The rate at which the chemical reaction proceeds slows as the temperature is reduced and speeds as the temperature is increased. This is why glow sticks last longer if you refrigerate them. If you place a glow stick in a bowl of hot water, the rate of the chemical reaction will be increased. The glow stick will glow more brightly, but it will stop working more quickly.

If you truly want to classify the glow stick reaction, it's an example of chemiluminescence. Chemiluminescence is light produced from a chemical reaction. It is sometimes called cool light because heat does not need to be produced.

How a Glow Stick Works

A typical glow stick or light stick contains two separate liquids. There is a hydrogen peroxide solution in one compartment and a phenyl oxalate ester with a fluorescent dye in another compartment. When you snap the glow stick, the two solutions mix and undergo a chemical reaction. This reaction does not emit light, but it produces enough energy to excite the electrons in the fluorescent dye. When the excited electrons fall from a higher energy state to a lower energy state, they emit photons (light). The color of the glow stick is determined by the dye that is used.