In chemistry, an exothermic reaction refers to a chemical reaction that results in the release of some quantity of energy, normally in the form of light or heat. The opposite of an exothermic reaction is an endothermic reaction, one that takes in heat from the surrounding environment.
The characteristics of an exothermic reaction can be expressed with the general chemical equation: reactants → products + energy; so an exothermic reaction results in the chemical product and a release of energy.
Exothermic reactions are important in technological applications as the released energy can be used to perform physical work on an external system. The most common example of this is in the internal combustion engine of a standard car. Heat released from the combustion reaction of gasoline exerts a physical force on the engine’s pistons causing them to move. The pistons convert that heat energy into mechanical energy, which drives the turning of the car’s wheels. Exothermic reactions are also common in explosive substances.
The release of energy in an exothermic reaction is related to the total quantity of energy contained in a chemical system. It is very difficult to accurately measure the total energy of a chemical system, so instead, scientists measure the change in energy of system over time. This value is known as the enthalpy change and is represented by the variable ΔH. The enthalpy change is equal to the amount of internal energy, plus the energy required to change that system via a chemical reaction. One can think of enthalpy change as: ΔH = energy used in forming product bonds − energy released in breaking reactant bonds.
For all exothermic reactions ΔH<0. The value is negative is because exothermic reactions release energy, so the total energy of the system after an exothermic reaction is less than what it started with. For example, the equation for a burning hydrogen reaction is: 2H2 (g) + O2 (g) → 2H2O (g) and the respective enthalpy change of this reaction is ΔH=-483.6 kJ/mol of O2. The value is negative as the chemical reaction releases heat into the environment.
Examples of Exothermic Reactions
The most obvious and common kind of exothermic reaction encountered in everyday life is combustion. Combustion refers to a high-temperature exothermic reaction that produces oxidized products. Combustion requires the presence of oxygen and heat. For example, the combustion reaction of natural gas (methane) is: CH4[g] + 2 O2[g] -> CO2[g] + 2 H2O[g] + energy. During this reaction, the hydrocarbon and oxygen bonds of the reactants are broken. Since the double bond of the reactant oxygen molecule is much weaker than the single bonds of the carbon dioxide and water products, the reaction release a large amount of heat into the environment. Very often, the heat produced from a combustion reaction is enough to self-catalyze the reaction, so combustion will continue until there are no more reactants left.
Depending on the amount of oxygen available, a combustion reaction can be complete or incomplete. In a complete combustion reaction, there is enough oxygen to produce a molecule of carbon dioxide per unit reaction. In incomplete combustion reactions, a lack of oxygen results in the production of carbon monoxide, a poisonous gas. The products of incomplete combustion can also react with gases in the atmosphere, creating nitric and sulfuric acid rains.
Thermite is a pyrotechnic composition of metal powder and a metal oxide. The most common variety of thermite is made of iron(III) oxide (Fe2O3) and aluminum, but some varieties use boron oxide, copper oxide, or lead oxide. Although the reactants are stable at room temperature, when heated, thermite undergoes an extremely violent exothermic reaction that releases a large amount of heat. Oxygen forms more stable bonds with aluminum than with iron, so when the composition is heated, the aluminum steals oxygen from the iron, releasing the energy stored in its chemical bonds. The general formula for a thermite reaction using iron(III) oxide is: Fe2O3 + 2 Al → 2 Fe + Al2O3.
Thermite reactions burn extremely hot; up to 2500°C for some varieties of thermite. This high heat reaction has a number of industrial applications, most involving working with metals. Thermite reactions are commonly used for welding purposes, as the stable high-temperature reaction generates enough heat to join metals together. Thermite can even be used to weld underwater, as the released heat creates a bubble of gas around the welding torch. Thermite reactions can also be used to purify samples of elements, and a modified thermite reaction was used to produce the Uranium used in the Manhattan project.
Nuclear fission is a special type of exothermic reaction in which the nucleus of a heavy atom splits into pieces, creating lighter elements and releasing energy. During nuclear fission, energy is released in the form of heat, kinetic energy, and gamma photons—a form of high-energy radiation. Nuclear fission reactions are extremely powerful and are the mechanisms underlying nuclear weapons and nuclear reactors. Unlike other kinds of exothermic reactions, which involve the breaking of chemical bonds and the formation of new chemical bonds, fission reactions are the result of breaking nucelar bonds; bonds between the particles in an atomic nucleus. these bonds are extremely energetic, so when they are broken they release a large amount of energy.
Nuclear fission is a form of nuclear transmutation, as the products are different elements than the original atom. Generally, the nuclei produced are of similar atomic size, normally at a ratio of 3:2 atomic masses. Unfortunately, these products are almost always very radioactive, which gives rise to problems storing the waste products of nuclear fission reactions.
Some elements, especially high atomic number elements, will undergo spontaneous fission, but normally fission requires an external input of energy. In nuclear reactors and bombs, heavy atomic nuclei are bombarded with free neutrons, which break the nucleus apart. The resulting fission reaction releases more neutrons. If there is enough of the reactant available and enough neutrons in an enclosed enough space, a self-sustaining a nuclear chain reaction will occur. The required mass of reactant for a self-sustaining nuclear fission reaction is called its “critical mass.”