Chemical Reaction

A chemical reaction is a process that converts chemical species into other chemical species. The starting substances are called reactants, and the resulting substances are products. Central to chemistry, reactions commonly involve rearrangements of atoms, changes in chemical bonds, or transfers of electrons. They may occur in a single elementary event or through a sequence of steps. The broad chemical definition also includes experimentally observable interconversions between different conformations of a molecule. (goldbook.iupac.org)

Chemical and physical change

A chemical change alters the chemical identity of matter, whereas a physical change alters properties such as shape or physical state without necessarily changing its chemical composition. Melting ice, for example, leaves the substance as water; decomposing water into hydrogen and oxygen produces different substances. Physical and chemical changes can accompany one another, as when a reaction produces a gas or a solid precipitate. (openstax.org)

Color changes, gas evolution, and heat release can provide evidence of a reaction, but an observation alone is not a universal test. Boiling also produces bubbles, and physical processes can exchange heat. Identifying a chemical transformation therefore depends on establishing a change in chemical species rather than relying solely on its visible effects. (openstax.org)

Equations and quantitative relationships

A chemical equation represents reactants and products using formulas, with an arrow indicating the direction of transformation. A balanced equation contains the same number of atoms of each element on both sides. Equations involving charged species must also conserve total electric charge. Balancing changes the coefficients preceding formulas, not the subscripts that define the substances themselves. (openstax.org)

For complete methane combustion:

The coefficients indicate that one methane molecule reacts with two oxygen molecules to form one carbon dioxide molecule and two water molecules. They also describe the corresponding ratios of chemical amounts. These quantitative relationships constitute stoichiometry, which connects reactant consumption with product formation. A balanced equation specifies an overall material balance; it does not, by itself, establish the reaction pathway or speed. (openstax.org)

Symbols such as (s), (l), (g), and (aq) indicate solids, liquids, gases, and aqueous solutions. For solution reactions, a net ionic equation omits spectator ions that remain unchanged, displaying only species involved in the net transformation. (openstax.org)

Major reaction classes

Classification groups reactions by shared features rather than imposing one exclusive system. Three widely used categories are precipitation, acid–base, and oxidation–reduction reactions. (openstax.org)

• Precipitation reactions produce a solid from dissolved species. For example, silver ions and chloride ions can combine to form solid silver chloride: $\mathrm{Ag^+(aq)+Cl^-(aq)\rightarrow AgCl(s)}$.
• Acid–base reactions, under the Brønsted–Lowry definition, involve proton transfer. An acid donates a proton, and a base accepts it. Neutralization of a strong acid by a strong base in water can be represented as $\mathrm{H_3O^+ + OH^- \rightarrow 2H_2O}$.
• Oxidation–reduction reactions, or redox reactions, involve changes in oxidation states. Oxidation corresponds to electron loss and reduction to electron gain; these processes occur together. Combustion of methane is a redox reaction. (openstax.org)

These categories describe different aspects of chemical change. Other classifications emphasize the number and arrangement of reactants and products, or specific structural transformations rather than proton or electron transfer. (openstax.org)

Energy and thermodynamic direction

Reactions exchange energy with their surroundings. Under constant pressure with only pressure–volume work, the heat exchanged equals the change in enthalpy, $\Delta H$. An exothermic reaction releases heat and has negative $\Delta H$; an endothermic reaction absorbs heat and has positive $\Delta H$. (openstax.org)

Heat release alone does not determine thermodynamic favorability. At constant temperature and pressure, Gibbs free energy provides the relevant criterion:

where $T$ is absolute temperature and $\Delta S$ is the system’s entropy change. A negative reaction free-energy change favors forward progress under the specified conditions. Thermodynamics determines favorability, not how rapidly that change occurs. (openstax.org)

Reaction rates and mechanisms

Chemical kinetics studies reaction rates and their dependence on conditions. Rates can depend on reactant concentration, temperature, physical state, and catalysts. For reactions between colliding particles, successful encounters require suitable orientation and sufficient energy; merely bringing reactants together does not guarantee conversion. (openstax.org)

The activation energy characterizes the temperature dependence of a rate constant in the Arrhenius description and is commonly associated with the barrier to reaction. A reacting system passes through a transition state, a high-energy configuration along the reaction pathway. Many reactions proceed through multiple elementary steps rather than a single encounter. (openstax.org)

A reaction mechanism describes these steps and any intermediates formed and subsequently consumed. The rate law of an overall reaction generally requires experimental determination: its concentration exponents cannot ordinarily be read directly from the balanced equation. (openstax.org)

Equilibrium and catalysis

Reversible reactions can reach chemical equilibrium, where forward and reverse rates are equal. Reactant and product concentrations then remain constant macroscopically, although molecular transformations continue. Equal rates do not imply equal concentrations. (openstax.org)

Catalysis accelerates a reaction through an alternative mechanism with more favorable kinetic barriers. A catalyst participates in intermediate steps but is regenerated overall. It does not change the reaction’s free-energy difference or equilibrium constant, so it accelerates the approach to equilibrium without altering the equilibrium composition. Catalysts include solid surfaces, dissolved species, and biological enzymes, whose specificity helps control reactions in living systems. (openstax.org)