The latest reviewed version was checked on 18 September 2017. There is 1 pending change awaiting review. The release of energy in chemical reactions occurs predicting products of chemical reactions pdf the reactants have higher chemical energy than the products.
The chemical energy in a substance is a type of potential energy stored within the substance. This stored chemical potential energy is the heat content or enthalpy of the substance.
The collection of substances that is involved in a chemical reaction is referred to as a system and anything else around it is called the surroundings. If the enthalpy decreases during a chemical reaction, a corresponding amount of energy must be released to the surroundings.
Conversely, if the enthalpy increases during a reaction, a corresponding amount of energy must be absorbed from the surroundings. This is simply the Law of Conservation of Energy. Exothermic reactions is when a chemical reaction releases more energy than it absorbs and you can also see this in many way the most are through chemical reactants. Endothermic reactions is when a chemical reaction absorbs more energy than it releases.
You are already familiar with enthalpy: melting ice is endothermic and freezing water is exothermic. When methane burns in air the heat given off equals the decrease in enthalpy that occurs as the reactants are converted to products. The enthalpy difference between the reactants and the products is equal to the amount of energy released to the surroundings. A reaction in which energy is released to the surroundings is called an exothermic reaction.