Chemical reactions occur around us, from simple processes to complex biological functions. These reactions involve breaking existing chemical bonds within reactants and forming new bonds to create products. This rearrangement is accompanied by energy changes, released or absorbed from the surroundings. Understanding these energy changes is a fundamental concept in chemistry and explains many daily phenomena.
What Endothermic and Exothermic Mean
Chemical reactions are categorized based on how they manage energy with their surroundings. Reactions that release energy, often in the form of heat or light, are known as exothermic reactions. In these reactions, the energy stored in the products is less than the energy contained within the initial reactants, with the excess energy being expelled. A common example is a burning candle, where the chemical bonds in the wax and oxygen break, and new bonds form, releasing noticeable heat and light. Similarly, the process inside a hand warmer, which involves the oxidation of iron, generates warmth as it releases energy.
Conversely, reactions that absorb energy from their surroundings are endothermic reactions. This typically cools the surroundings. For endothermic reactions, products possess a higher energy content than reactants, indicating energy was drawn in. A familiar instance is an instant cold pack, which becomes cold to the touch. Photosynthesis, the process by which plants convert light energy into chemical energy, is another endothermic process requiring solar energy.
How to Determine Energy Flow in a Reaction
Observing temperature changes in the surroundings directly identifies energy flow. If the reaction vessel or its immediate environment becomes warmer, it indicates that energy is being released, making it exothermic. Conversely, if the surroundings feel colder, it signals that the reaction is absorbing energy, making it endothermic. This temperature change happens because heat is either transferred from the system to the surroundings or from the surroundings to the system.
Another way to determine energy flow involves measuring the enthalpy change, symbolized as ΔH. Enthalpy is a thermodynamic property representing the total heat content of a system. For exothermic reactions, ΔH has a negative value, meaning energy has left the system. In contrast, for endothermic reactions, ΔH is positive, indicating the system has absorbed energy. While heat is the most commonly observed form of energy exchange, reactions can also absorb or release energy as light, electricity, or sound.
Common Examples of Energy Changes
Everyday life offers many examples of reactions involving energy changes. Combustion reactions, such as the burning of wood in a fireplace or natural gas in a stove, are classic exothermic examples where significant heat and light are released. The setting of concrete also involves an exothermic reaction, where cement hydration releases heat. Even the rusting of iron, a slower process, is an exothermic reaction releasing energy.
Endothermic processes are equally prevalent. The familiar melting of ice is an endothermic physical change. Instant cold packs, often used for sports injuries, rely on the endothermic dissolution of ammonium nitrate in water for cooling. Photosynthesis, performed by plants to produce glucose, is a biological endothermic process capturing light energy.