Chemical reactions transform matter from reactants into products. All transformations involve a change in energy because chemical bonds in the reactants must be broken and new bonds must be formed to create the products. This process requires or releases energy, most often as heat. Understanding this energy exchange determines the fundamental type of reaction taking place.
The Core Difference: Direction of Energy Flow
The distinction between the two main reaction types depends entirely on the direction of heat movement between the reacting chemicals and their environment. Chemists define the “system” as the specific chemical reaction being studied, and the “surroundings” as everything else, such as the container, the air, and a person’s hand.
Exothermic reactions are defined by the release of energy from the system into the surroundings. Since energy is leaving the reaction mixture, the surroundings gain this released energy, which typically causes the temperature of the container and its environment to increase. The energy released when new bonds are formed in the products is greater than the energy required to break the bonds in the reactants.
Conversely, endothermic reactions absorb energy from the surroundings into the system. This means the surroundings lose heat to the reacting chemicals. This energy is necessary because more energy is required to break the reactant bonds than is released by forming the product bonds. The net result is a cooling effect in the immediate environment.
Observable Physical Signs of Transfer
The most direct way to identify the type of energy transfer is by observing the temperature change of the surroundings. In an exothermic process, the release of thermal energy causes the reaction vessel or its immediate environment to feel warm or hot to the touch. A thermometer placed in the surroundings would show a noticeable temperature increase as the reaction proceeds.
For an endothermic reaction, the absorption of heat from the environment makes the container feel cold. The reaction actively draws thermal energy away from the surroundings, including a person’s hand. This cooling effect can be pronounced enough in some reactions to cause phase changes, such as the freezing of water vapor on the outside of the vessel.
Other rapid signs of energy release are common with exothermic processes, such as the emission of light or sound. Combustion reactions, for instance, are highly exothermic and release energy as both intense heat and visible light, sometimes accompanied by audible cues like a pop or hiss. Endothermic reactions rarely show such dramatic visual releases, but they may cause the surrounding air to cool and condense, leading to visible fog or condensation.
Quantifying Energy Change With Enthalpy
Beyond sensory observation, chemists use a quantitative measure called enthalpy, represented by the symbol H, to describe the heat content of a system at constant pressure. The comparison between the heat content of the products and the reactants is called the change in enthalpy, or Delta H. The sign of this value precisely classifies the reaction type.
When a reaction is exothermic, the system loses energy, meaning the products have less stored chemical energy than the reactants. This energy loss results in a negative Delta H value. Conversely, an endothermic reaction involves the system gaining energy, so the products possess a higher energy content than the starting reactants.
The energy absorbed by an endothermic reaction is represented by a positive Delta H value, reflecting the gain in the system’s heat content. These energy differences can be visualized on a reaction coordinate diagram. Measuring this change is typically done using a device called a calorimeter, which precisely tracks the heat exchange with the surroundings.
Everyday Examples of Reactions
Many familiar processes illustrate these energy dynamics, offering practical context for the concepts. Burning any fuel, like the wood in a fireplace or the gas in a stove, is a combustion reaction that is highly exothermic, as it releases significant heat and light into the environment. The setting of concrete, a process called cement hydration, is also an exothermic reaction, which is why large concrete structures can feel warm as they cure.
An easily observed endothermic process is the action of an instant cold pack used for sports injuries. These packs typically contain ammonium nitrate that, when dissolved in water, absorbs heat from the surrounding water and the area of injury to create a cooling effect. Photosynthesis in plants is another large-scale endothermic reaction, as plants must absorb energy from sunlight to convert carbon dioxide and water into glucose and oxygen.