Chemical reaction
Process rearranging atoms with energy change, yielding new substances.
A chemical reaction is the process by which one set of chemical substances is transformed into another. This transformation involves the rearrangement of atoms, and it always comes with an energy change as new products form. Traditionally, these reactions only affect the positions of electrons—breaking and forming chemical bonds between atoms—without altering the atomic nuclei, meaning the elements themselves stay the same. Such reactions are often represented by a chemical equation. The starting substances are called reactants or reagents, and they produce one or more products, which typically have different properties than the reactants did.
Chemical reactions often happen through a series of smaller steps known as elementary reactions; the detailed path of these steps is called the reaction mechanism. They are described by chemical equations that show the starting materials, final products, and sometimes intermediate products and reaction conditions. Each reaction proceeds at a characteristic rate, which depends on temperature and chemical concentration. Some reactions release heat and are called exothermic, while others need heat to occur and are called endothermic. Generally, raising the temperature increases the reaction rate because more thermal energy is available to reach the activation energy needed to break bonds.
Reactions can be classified as redox (involving oxidation and reduction) or non-redox (without oxidation or reduction). Most simple redox reactions fall into categories like combination, decomposition, or single displacement. In chemical synthesis, various reactions are used to obtain a desired product. In biochemistry, a linked series of reactions—where the product of one becomes the reactant for the next—forms metabolic pathways. These are often sped up by protein enzymes, which allow reactions that would be impossible under normal cellular conditions to occur at the temperature and concentrations found inside a cell. The concept of a chemical reaction has also been extended to processes involving entities smaller than atoms, such as nuclear reactions, radioactive decays, and reactions between elementary particles, as described by quantum field theory.
**History**
People have known about chemical reactions since ancient times, including combustion in fire, fermentation, and reducing ores to metals. Early theories of material transformation came from Greek philosophers, like Empedocles’ Four-Element Theory, which held that everything is made of fire, water, air, and earth. During the Middle Ages, alchemists studied chemical transformations, especially trying to turn lead into gold using reactions of lead and lead-copper alloys with sulfur. A major goal for medieval alchemists was the artificial production of chemical substances. For example, works attributed to Jābir ibn Ḥayyān (around 850–950) describe synthesizing ammonium chloride from organic substances, and later alchemists, starting around 1300, produced mineral acids like sulfuric and nitric acids by heating sulfate and nitrate minerals such as copper sulfate, alum, and saltpeter. In the 17th century, Johann Rudolph Glauber made hydrochloric acid and sodium sulfate by reacting sulfuric acid with sodium chloride. The lead chamber process (1746) and the Leblanc process allowed large-scale production of sulfuric acid and sodium carbonate, bringing chemical reactions into industry. Further improvements led to the contact process in the 1880s for sulfuric acid and the Haber process (1909–1910) for ammonia synthesis.
From the 16th century, researchers like Jan Baptist van Helmont, Robert Boyle, and Isaac Newton tried to create theories for observed chemical transformations. In 1667, Johann Joachim Becher proposed the phlogiston theory, which claimed that a fire-like element called “phlogiston” was inside combustible bodies and released during burning. This was proven wrong in 1785 by Antoine Lavoisier, who correctly explained combustion as a reaction with oxygen from the air. In 1808, Joseph Louis Gay-Lussac recognized that gases always react in fixed relationships with each other. Building on this and John Dalton’s atomic theory, Joseph Proust developed the law of definite proportions, which led to the concepts of stoichiometry and chemical equations.
In organic chemistry, it was long believed that compounds from living organisms were too complex to make synthetically. This idea, called vitalism, held that organic matter had a “vital force” that separated it from inorganic materials. That separation ended in 1828 when Friedrich Wöhler synthesized urea from inorganic precursors. Other major contributors to organic chemistry include Alexander William Williamson, who synthesized ethers, and Christopher Kelk Ingold, who, among many discoveries, established the mechanisms of substitution reactions.
**Characteristics**
Some chemical reactions have specific characteristics, such as...
- field
- Chemistry
- known_for
- Transformation of substances via rearrangement of atoms, with energy changes; described by chemical equations; includes exothermic and endothermic reactions; classified as redox or non-redox; fundamen
Lore & Background
Chemical reactions such as combustion, fermentation, and the reduction of ores to metals were known since antiquity. Initial theories of transformation were developed by Greek philosophers, such as the Four-Element Theory of Empedocles. In the Middle Ages, chemical transformations were studied by alchemists, who attempted to convert lead into gold using reactions of lead and lead-copper alloys with sulfur. In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride.
Reader's Guide
Chemical reactions are central to chemistry, enabling the transformation of substances through the rearrangement of atoms and the breaking and forming of chemical bonds. They are characterized by energy changes, such as exothermic reactions that produce heat and endothermic reactions that require heat. Reactions occur at characteristic rates influenced by temperature and concentration, with rates typically increasing with temperature due to greater thermal energy to reach activation energy. Reactions can be classified as redox (involving oxidation and reduction) or non-redox. In biochemistry, consecutive reactions form metabolic pathways, often catalyzed by protein enzymes that increase reaction rates. The concept has been extended to reactions between entities smaller than atoms, including nuclear reactions and radioactive decays.
Did You Know?
- Chemical reactions are usually characterized by a chemical change, yielding products with properties different from the reactants.
- Some reactions produce heat (exothermic), while others require heat to occur (endothermic).
- The elementary reaction is the smallest division into which a chemical reaction can be decomposed, with no intermediate products.
- Enzymes increase the rates of biochemical reactions, allowing metabolic syntheses and decompositions impossible under ordinary conditions to occur within a cell.
The Atomic Heartbeat
Chemistry, at its core, is the discipline devoted to understanding how matter—built from chemical elements—behaves, changes, and interacts. At the center of this inquiry sit atoms and the bonds they form with one another, and it is precisely in the breaking and reforming of those bonds that chemical reactions unfold. The field is not merely a collection of laboratory observations; it is a recognized academic discipline with its own curricula, degrees, national and international societies, and peer-reviewed journals. It functions simultaneously as a natural science that seeks the rules governing the physical world through empirical method, a physical science focused on non-living systems, and a biological science that traces chemical processes through living organisms. This multi-layered identity means that a single reaction can be examined through the lens of thermodynamics, molecular structure, or cellular function, each perspective revealing a different facet of the same atomic event.
Time, Energy, and the Speed of Change
Not all reactions unfold at the same pace, and the field has developed specialized branches to capture that diversity. Chemical kinetics is dedicated to measuring how fast processes proceed, while thermochemistry and calorimetry focus on the heat absorbed or released during those transformations. At the extreme end of the timescale, femtochemistry investigates reactions that complete in roughly one quadrillionth of a second, pushing the boundaries of what can be observed. Physical chemistry, which draws heavily on infinitesimal calculus and quantum mechanics, provides the mathematical and theoretical scaffolding for understanding the energetics and dynamics behind every reaction. Spectroscopy, meanwhile, studies how matter interacts with radiated energy, offering a window into the electronic and structural changes that accompany a reaction. Together, these sub-disciplines ensure that a chemical reaction is never described by a single number; it is a rich interplay of rate, energy flow, and molecular rearrangement that can be dissected from multiple complementary angles.
Where Worlds Meet
Many of the most fascinating reactions occur not in homogeneous bulk but at boundaries between different phases or media. Electrochemistry examines reactions that take place at the interface of an electron conductor—such as a metal or semiconductor electrode—and an ionic conductor like an electrolyte, where electrons are transferred between the two. Surface science broadens this idea to any interface: solid–liquid, solid–gas, solid–vacuum, or liquid–gas, each presenting unique physical and chemical phenomena. Photochemistry adds another dimension by studying reactions that are triggered by the absorption of light by atoms or molecules, meaning the energy source is electromagnetic rather than thermal. Geochemistry, in turn, applies chemical reasoning to the mechanisms behind major geological systems, where reactions play out over vast spatial and temporal scales. These interface-driven fields remind us that a reaction's environment—what it touches, what energy it receives, what medium it sits in—can be as defining as the atoms involved.
Reactions That Build Life and Medicine
When chemical reactions are directed toward living systems, the stakes and the complexity multiply. Biochemistry maps the chemicals, reactions, and interactions that sustain organisms, linking closely to molecular biology and genetics. Neurochemistry narrows the focus to the transmitters, peptides, proteins, lipids, sugars, and nucleic acids that form, maintain, and modify the nervous system. Medicinal chemistry applies the full toolkit of reaction design to medical and drug-related purposes, while bioorganic chemistry deliberately merges organic and biochemical methods to serve biological questions. Organometallic chemistry, which studies compounds containing at least one bond between a carbon atom and a metal—ranging from alkaline and transition metals to metalloids like boron, silicon, and tin—sits at a fertile intersection where organic and inorganic disciplines overlap. Bioinorganic chemistry further explores the role of metals in biology. Together, these branches show that the same fundamental bond-making and bond-breaking events that define chemistry in a flask are the very processes that keep a cell alive and a drug effective.
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Frequently Asked Questions
What is a chemical reaction?
A chemical reaction is a process in which one or more starting substances are transformed into different products through the rearrangement of atoms. During this rearrangement, chemical bonds are broken and formed, and the process is always accompanied by some release or absorption of energy.
What are the main 'powers' or defining features of a chemical reaction?
Its core abilities include reshuffling atoms into new arrangements, shifting energy (either releasing it as heat or absorbing it from the surroundings), and producing substances with properties distinct from the originals. These transformations are classically limited to electron-level changes in bonding, leaving atomic nuclei untouched.
How is a chemical reaction typically described or 'summarized'?
Chemists capture a reaction with a chemical equation that lists the reactants on one side and the resulting products on the other, balanced so that atom counts match. This shorthand lets you see at a glance what goes in, what comes out, and how much of each is involved.
What categories or 'subtypes' does a chemical reaction fall into?
Reactions can be sorted by energy flow into exothermic (releasing heat) and endothermic (absorbing heat) types, and by electron transfer into redox and non-redox classes. These labels help predict behavior and organize the vast landscape of possible transformations.
Why is the chemical reaction considered a cornerstone of Chemistry Fundamentals?
It is the fundamental mechanism by which matter changes its identity, making it the engine behind everything from metabolism to industrial synthesis. Understanding how atoms rearrange with accompanying energy shifts is essentially the central question the entire field of chemistry revolves around.
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