Examples Of Exothermic Reactions

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

Combustion

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

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

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.”

About The Author

Alex Bolano

When Alex isn't nerdily stalking the internet for science news, he enjoys tabletop RPGs and making really obscure TV references. Alex has a Masters's degree from the University of Missouri-St. Louis.

Speak Your Mind!

READ THIS NEXT

Identifying The Impacts Of The El Niño And La Niña Events On Habitat Quality Of Chub Mackerel In The East China Sea

Published by Wei Yu and Xinjun Chen Shanghai Ocean University These findings are described in the article entitled Climate-induced habitat suitability variations of chub mackerel Scomber japonicus in the East China Sea, recently published in the journal Fisheries Research (Fisheries Research 207 (2018) 63-73). This work was conducted by the team of Prof. Xinjun Chen […]

What Is The Rock Cycle: Definition, Diagram, And Examples

The rock cycle is a concept of geology that describes the transition of rocks between the three rock types: igneous, sedimentary, and metamorphic. The cycle outlines how each rock type can be converted to another rock type through geologic processes. Most of us think of rocks as objects which don’t change. We perceive that rocks […]

Where Does Glycolysis Take Place In A Cell?

Glycolysis takes place in the cytoplasm of a cell as the first step in cellular respiration of the Kreb’s cycle. When glycolysis occurs, it breaks down glucose into pyruvic acids in the cytoplasm. There are many different kinds of pathways and processes that occur in our bodies. There are things that are primarily in humans […]

Flexible Substrates For DNA Arrays

Array fabrication technologies date back to the early 1990s when Affymetrix first used ultraviolet light to control the stepwise synthesis of biopolymers from a surface.[1] The most commercially successful result of this technology was high-density arrays of DNA. At the time, DNA arrays were one of the most efficient ways to detect and measure nucleic […]

Bread Mold: How To Identify Types Of Mold

” Bread mold is a type of fungus and is therefore eukaryotic (contains membrane-bound organelles) and multi-cellular. Mold cells are present in a long filamentous structure called a hypha. The mold cells are connected via pores in the septa between cells and are surrounded by a tube-shaped cell wall. A collection of hyphae is called […]

Jatropha curcas: A Renewed Promise To Meet Sustainable Future Energy Needs

Sustainable production of energy and industrial materials from non-food crops can substantially reduce our reliance on non-renewable fossil fuel reserves. Jatropha curcas is a promising non-food crop with oil-rich seeds (30-48%), an easy propagation system, and high adaptability to a wide range of climatic and soil conditions. These attributes have led to the promotion of […]

Cheating In Higher Education

The topic of academic misconduct in higher education has received a lot of attention over the past few decades, and the general findings have been troubling. Although it is still not clear whether academic misconduct has been rising or not, most studies have found that the rate of such behavior is quite high. Researchers have […]