What differentiates an endothermic reaction from an exothermic reaction?

The distinction between endothermic and exothermic reactions is fundamentally about the flow of energy, specifically heat, during the reactions. An endothermic reaction is characterized by the absorption of heat energy from its surroundings, which results in a decrease in the temperature of the environment where the reaction occurs. This type of reaction requires energy input to proceed, which is often seen in processes such as photosynthesis or the melting of ice.

Conversely, an exothermic reaction results in the release of heat energy back into the surroundings, causing an increase in temperature. This occurs in reactions such as combustion, where products formed are at a lower energy state than the reactants, leading to the liberation of heat and often light.

Understanding the flow of heat in these reactions helps categorize them: endothermic reactions are driven by the need for energy (absorbing heat), whereas exothermic reactions are often spontaneous due to the release of energy (releasing heat).