What is the primary difference between endothermic and exothermic reactions?

The primary difference between endothermic and exothermic reactions lies in the direction of heat transfer during the reactions. An endothermic reaction specifically requires heat from its surroundings, which means it absorbs energy in the form of heat. This is why substances involved in endothermic processes can feel cold to the touch, as they take heat away from their environment, leading to a decrease in temperature around them.

Conversely, during an exothermic reaction, energy is released into the surroundings in the form of heat. This release of heat typically raises the temperature of the surrounding environment. An example of an exothermic reaction is combustion, where energy is liberated as heat and light when a substance burns.

The correct understanding of these two types of reactions helps to clarify the behavior of matter and energy transfer in various chemical processes, making it foundational knowledge in thermochemistry.