An endothermic change is a process, either chemical or physical, that actively draws energy from its surroundings, typically in the form of heat. This absorption of thermal energy means energy must be continuously supplied for the change to proceed. Understanding endothermic changes is foundational to the study of thermodynamics.
The Mechanism of Energy Absorption
To understand energy flow, scientists define the system as the material undergoing the change and the surroundings as everything outside of it. In an endothermic change, the system acts as an energy sink, absorbing thermal energy directly from the surroundings. This required energy is used to break existing bonds, which can be chemical bonds or weaker intermolecular forces holding molecules together.
Because the system is constantly pulling heat from its environment, the surroundings experience a noticeable drop in temperature. For instance, a reaction taking place in a beaker will feel cold to the touch as the system absorbs thermal energy. This net flow of energy into the system means the overall energy change of the process is positive, requiring more energy to break initial bonds than is released when new ones are formed.
Everyday Physical and Chemical Examples
Endothermic changes are responsible for the cooling sensation experienced during many common events, particularly those involving a change of state. When liquid water evaporates from the skin, it requires a significant energy input to overcome intermolecular attractions. This necessary energy is drawn directly from the skin, which causes the cooling sensation when we sweat.
The melting of ice is another common physical change, where solid water absorbs heat from the air to transition into liquid water. Instant cold packs rely on an endothermic chemical change, often involving the dissolution of a salt like ammonium nitrate in water. This process rapidly pulls heat from the surrounding water, causing the pack’s temperature to drop. On a planetary scale, photosynthesis is a complex endothermic process where plants absorb light energy to convert carbon dioxide and water into glucose and oxygen.
Distinguishing Endothermic from Exothermic Change
Endothermic change represents one half of the fundamental energy equation in nature; the opposing process is known as an exothermic change. While an endothermic process absorbs heat from the surroundings, an exothermic process releases heat, often in the form of heat or light, into the surroundings. This release of energy causes the temperature of the surroundings to increase.
Exothermic reactions occur when the energy released by forming new bonds is greater than the energy required to break the initial bonds. Common examples include the combustion of fuel, such as burning wood or natural gas, which releases significant heat and light. The chemical reaction inside a hand warmer, typically the oxidation of iron, is another everyday exothermic example that intentionally warms the surroundings.