Chemical reaction

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another.^[1] When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products are generated. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur.

The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. Reactions often consist of a sequence of individual sub-steps, the so-called elementary reactions, and the information on the precise course of action is part of the reaction mechanism. Chemical reactions are described with chemical equations, which symbolically present the starting materials, end products, and sometimes intermediate products and reaction conditions.

Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Some reactions produce heat and are called exothermic reactions, while others may require heat to enable the reaction to occur, which are called endothermic reactions. Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms.

A reaction may be classified as redox in which oxidation and reduction occur or non-redox in which there is no oxidation and reduction occurring. Most simple redox reactions may be classified as a combination, decomposition, or single displacement reaction.

Different chemical reactions are used during chemical synthesis in order to obtain the desired product. In biochemistry, a consecutive series of chemical reactions (where the product of one reaction is the reactant of the next reaction) form metabolic pathways. These reactions are often catalyzed by protein enzymes. Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperature and concentrations present within a cell.

The general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions, radioactive decays and reactions between elementary particles, as described by quantum field theory.

Evolution of a gas

Formation of a

precipitate

Change in

temperature

Change in

state

The general characteristics of chemical reactions are:

concentrations, which usually make the reaction happen at a faster rate if raised through increased collisions per unit of time. Some reactions, however, have rates that are independent of reactant concentrations, due to a limited number of catalytic sites. These are called zero order reactions.

Reactant

available for contact between the reactants, in particular solid ones in heterogeneous systems. Larger surface areas lead to higher reaction rates.

Surface area

– increasing the pressure decreases the volume between molecules and therefore increases the frequency of collisions between the molecules.

Pressure

which is defined as the amount of energy required to make the reaction start and carry on spontaneously. Higher activation energy implies that the reactants need more energy to start than a reaction with lower activation energy.

Activation energy

which hastens reactions if raised, since higher temperature increases the energy of the molecules, creating more collisions per unit of time,

Temperature

The presence or absence of a . Catalysts are substances that make weak bonds with reactants or intermediates and change the pathway (mechanism) of a reaction which in turn increases the speed of a reaction by lowering the activation energy needed for the reaction to take place. A catalyst is not destroyed or changed during a reaction, so it can be used again.

catalyst

For some reactions, the presence of , most notably ultraviolet light, is needed to promote the breaking of bonds to start the reaction. This is particularly true for reactions involving radicals.

electromagnetic radiation

The speed at which reactions take place is studied by reaction kinetics. The rate depends on various parameters, such as:

Several theories allow calculating the reaction rates at the molecular level. This field is referred to as reaction dynamics. The rate v of a first-order reaction, which could be the disintegration of a substance A, is given by:

Its integration yields:

Here k is the first-order rate constant, having dimension 1/time, [A](t) is the concentration at a time t and [A]₀ is the initial concentration. The rate of a first-order reaction depends only on the concentration and the properties of the involved substance, and the reaction itself can be described with a characteristic half-life. More than one time constant is needed when describing reactions of higher order. The temperature dependence of the rate constant usually follows the Arrhenius equation:

where E_a is the activation energy and k_B is the Boltzmann constant. One of the simplest models of reaction rate is the collision theory. More realistic models are tailored to a specific problem and include the transition state theory, the calculation of the potential energy surface, the Marcus theory and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory.^[20]

Reaction of hydrogen and oxygen to form water.

Monitoring

Mechanisms of monitoring chemical reactions depend strongly on the reaction rate. Relatively slow processes can be analyzed in situ for the concentrations and identities of the individual ingredients. Important tools of real-time analysis are the measurement of pH and analysis of optical absorption (color) and emission spectra. A less accessible but rather efficient method is the introduction of a radioactive isotope into the reaction and monitoring how it changes over time and where it moves to; this method is often used to analyze the redistribution of substances in the human body. Faster reactions are usually studied with ultrafast laser spectroscopy where utilization of femtosecond lasers allows short-lived transition states to be monitored at a time scaled down to a few femtoseconds.^[70]

Chemical equation

Substrate

Chemical reaction model

Chemist

Chemistry

Combustion

Limiting reagent

List of organic reactions

Mass balance

Microscopic reversibility

Organic reaction

Reaction progress kinetic analysis

Reversible reaction

Atkins, Peter W.; Julio de Paula (2006). Physical Chemistry (4th ed.). Weinheim: . ISBN 978-3-527-31546-8.

Wiley-VCH

Brock, William H. (1997). (in German). Braunschweig: Vieweg. ISBN 978-3-540-67033-9.

Viewegs Geschichte der Chemie

Brückner, Reinhard (2004). Reaktionsmechanismen (in German) (3rd ed.). München: Spektrum Akademischer Verlag. 978-3-8274-1579-0.

ISBN

Wiberg, Egon, Wiberg, Nils and Holleman, Arnold Frederick (2001). . Academic Press. ISBN 978-0-12-352651-9.{{cite book}}: CS1 maint: multiple names: authors list (link)

Inorganic chemistry

. Encyclopædia Britannica. Vol. 6 (11th ed.). 1911. pp. 26–33.