Difference between revisions of "Chemical reaction" - New World Encyclopedia

Latest revision as of 14:43, 5 December 2023

Vapors of hydrogen chloride in a beaker and ammonia in a test tube meet to form a cloud of a new substance, ammonium chloride.

A chemical reaction is a process that leads to the interconversion of chemical substances.^[1] The substances initially involved in a chemical reaction are called the reactants, and the substances produced by the reaction are called the products. Given that chemical reactions are usually characterized by a chemical change, they yield one or more products that are, in general, different from the reactants.

Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds. However, the general concept of a chemical reaction, in particular the notion of a chemical equation, is applicable to transformations of elementary particles and nuclear reactions.

A series of different chemical reactions may be performed to synthesize a desired product. In biochemistry, sets of chemical reactions catalyzed by enzymes make up metabolic pathways, in which syntheses and decompositions ordinarily impossible under conditions within a cell are performed.

Chemical reactions take place within each living organism, allowing the organism to survive, grow, and reproduce. In addition, researchers and chemical engineers utilize chemical reactions to produce a vast array of materials, including petrochemicals, agrochemicals, ceramics, polymers and rubber (elastomers), oleochemicals (oils, fats, and waxes), explosives, fragrances, and flavors. Modern society is highly dependent on these products.

Chemical equations

A chemical reaction is symbolically represented by a chemical equation, wherein one set of substances, called the reactants, is converted into another set of substances, called the products. The reactants and products are shown using their chemical formulas, and an arrow is used to indicate the direction of the reaction. The reactants are usually placed to the left of the arrow, and the products are placed to the right. If the reaction is irreversible, a single arrow is used; if the reaction is reversible, a double arrow (pointing in opposite directions) is used.

For example, the combustion of methane in oxygen may be represented by the following equation:

CH₄ + 2 O₂ → CO₂ + 2 H₂O

This equation represents an irreversible reaction in which one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.

Reaction types

The large diversity of chemical reactions and approaches to their study results in the existence of several concurring, often overlapping, ways of classifying them. Below are examples of widely used terms for describing common kinds of reactions.

Isomerisation, in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition; see stereoisomerism
Direct combination or synthesis, in which two or more chemical elements or compounds unite to form a more complex product:

N₂ + 3 H₂ → 2 NH₃

Chemical decomposition or analysis, in which a compound is decomposed into smaller compounds or elements:

2 H₂O → 2 H₂ + O₂

Single displacement or substitution, characterized by an element being displaced out of a compound by a more reactive element:

2 Na(s) + 2 HCl(aq) → 2 NaCl(aq) + H₂(g)

Metathesis or Double displacement reaction, in which two compounds exchange ions or bonds to form different compounds:

NaCl(aq) + AgNO₃(aq) → NaNO₃(aq) + AgCl(s)

Acid-base reactions, broadly characterized as reactions between an acid and a base, can have different definitions depending on the acid-base concept employed. Some of the most common are:

Arrhenius definition: Acids dissociate in water releasing H₃O⁺ ions; bases dissociate in water releasing OH^- ions.
Brønsted-Lowry definition: Acids are proton (H⁺) donors; bases are proton acceptors. Includes the Arrhenius definition.
Lewis definition: Acids are electron-pair acceptors; bases are electron-pair donors. Includes the Brønsted-Lowry definition.

Redox reactions, in which changes in oxidation numbers of atoms in involved species occur. Those reactions can often be interpreted as transferences of electrons between different molecular sites or species. In the following example of a redox rection, I₂ (iodine) is reduced to I^- (iodide anion), and S₂O₃^2- (thiosulfate anion) is oxidized to S₄O₆^2-:

2 S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2 I⁻(aq)

Combustion, a kind of redox reaction in which any combustible substance combines with an oxidizing element, usually oxygen, to generate heat and form oxidized products. The term combustion is usually used for only large-scale oxidation of whole molecules, i.e. a controlled oxidation of a single functional group is not combustion.

C₁₀H₈+ 12 O₂ → 10 CO₂ + 4 H₂O

CH₂S + 6 F₂ → CF₄ + 2 HF + SF₆

Reactions can also be classified according to their mechanism, some typical examples being:

Reactions of ions, e.g. disproportionation of hypochlorite
Reactions with reactive ionic intermediates, e.g. reactions of enolates
Radical reactions, e.g. combustion at high temperature
Reactions of carbenes

Organic reactions

Organic reactions encompass a wide assortment of reactions involving compounds that have carbon as the main element in their molecular structure. The reactions in which an organic compound may take part are largely defined by its functional groups.

There is no limit to the number of possible organic reactions and mechanisms. However, certain general patterns are observed that can be used to describe many common or useful reactions. Each reaction has a stepwise reaction mechanism that explains how it happens. Organic reactions can be organized into several basic types, with some reactions fitting into more than one category. Some of the basic types of organic chemical reactions are noted below.

Addition reactions, including such reactions as halogenation, hydrohalogenation, and hydration. Based on the mechanism, the main addition reactions are classified as electrophilic, nucleophilic, or radical addition.

Elimination reactions, including processes such as dehydration.

Substitution reactions are divided into several types: nucleophilic aliphatic substitution with SN1, SN2 and SNi reaction mechanisms; nucleophilic aromatic substitution; nucleophilic acyl substitution; electrophilic substitution; electrophilic aromatic substitution; and radical substitution.

Organic redox reactions are redox reactions specific to organic compounds and are very common.

Rearrangement reactions are divided into 1,2-rearrangements, pericyclic reactions, and metathesis reactions.

In condensation reactions, two reactants combine, and a small molecule (usually water) is split off. The opposite reaction, when water is consumed in a reaction, is called hydrolysis. Many polymerization reactions are derived from organic reactions. They are divided into addition polymerizations and step-growth polymerizations.

Chemical kinetics

The rate of a chemical reaction is a measure of how the concentration or pressure of the involved substances changes with time. Analysis of reaction rates is important for several applications, such as in chemical engineering or in chemical equilibrium study. Rates of reaction depends basically on:

Reactant concentrations, which usually make the reaction happen at a faster rate if raised through increased collisions per unit time.
Surface area available for contact between the reactants, in particular solid ones in heterogeneous systems. Larger surface area leads to higher reaction rates.
Pressure, by increasing the pressure, you decrease the volume between molecules. This will increase the frequency of collisions of molecules.
Activation energy, 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 a lower activation energy.
Temperature, which hastens reactions if raised, since higher temperature increases the energy of the molecules, creating more collisions per unit time,
The presence or absence of a catalyst. Catalysts are substances which 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.
For some reactions, the presence of electromagnetic radiation, most notably ultra violet, is needed to promote the breaking of bonds to start the reaction. This is particularly true for reactions involving radicals.

Reaction rates are related to the concentrations of substances involved in reactions, as quantified by the rate law of each reaction. Note that some reactions have rates that are independent of reactant concentrations. These are called zero order reactions.

Notes

↑ Chemical reaction IUPAC Gold Book. Retrieved September 24, 2008.

References
ISBN links support NWE through referral fees

Chang, Raymond. 2006. Chemistry, 9th ed. New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031
Cotton, F. Albert, and Geoffrey Wilkinson. 1980. Advanced Inorganic Chemistry, 4th ed. New York: Wiley. ISBN 0471027758
McMurry, J., and R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0131402080
McMurry, John. 2004. Organic Chemistry, 6th ed. Belmont, CA: Brooks/Cole. ISBN 0534420052
Morrison, Robert T., and Robert N. Boyd. 1992. Organic Chemistry, 6th ed. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-643669-2
Solomons, T.W. Graham, and Craig B. Fryhle. 2004. Organic Chemistry, 8th ed. Hoboken, NJ: John Wiley. ISBN 0471417998

External links

All links retrieved December 5, 2023.

Reactions chemtutor.

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[1] Chemical reaction IUPAC Gold Book. Retrieved September 24, 2008.

[1]

@@ Line 1: / Line 1: @@
-A '''chemical reaction'''  is a process involving one, two or more [[chemical|substances]] (called [[reactants]]), characterized by a [[chemical change]] and yielding one or more [[Product (chemistry)|product]](s) which are different from the reactants.  Classically, chemical reactions encompass changes that strictly involve the motion of [[electrons]], although the general concept of a chemical reaction (in particular the notion of a [[chemical equation]]) is applicable to [[Feynman diagrams|transformations of elementary particles]], as well as [[nuclear reaction]]s.
+{{Copyedited}}{{Images OK}}{{Approved}}
-==Types==
+[[Image:Hydrochloric acid ammonia.jpg|right|180px|thumb|Vapors of [[hydrogen chloride]] in a beaker and [[ammonia]] in a test tube meet to form a cloud of a new substance, [[ammonium chloride]].]]
-There are five major classifications of chemical reactions. Some common and widely known are:
+A '''chemical reaction''' is a process that leads to the interconversion of [[chemical substance]]s.<ref>[http://goldbook.iupac.org/C01033.html Chemical reaction] IUPAC Gold Book. Retrieved September 24, 2008.</ref> The substances initially involved in a chemical reaction are called the '''[[reactant]]s''', and the substances produced by the reaction are called the '''[[product]]s'''. Given that chemical reactions are usually characterized by a [[chemical change]], they yield one or more [[Product (chemistry)|products]] that are, in general, different from the reactants.
-*[[Isomerization]] ('''A &rarr; B''') in which a [[Chemical compound|compound]] undergoes a structural rearrangement without any change in its net atomic composition;
+Classically, chemical reactions encompass changes that strictly involve the motion of [[electrons]] in the forming and breaking of [[chemical bond]]s. However, the general concept of a chemical reaction, in particular the notion of a [[chemical equation]], is applicable to [[Feynman diagram|transformations of elementary particles]] and [[nuclear reaction]]s.
-*[[Combination reaction|Direct combination]] or [[Chemical synthesis|synthesis]] ('''A + B &rarr; C'''), in which two or more [[Chemical element|elements]] or compounds unite to form a more complex product;
-*[[Chemical decomposition]] or analysis ('''A &rarr; B + C'''), in which a compound is decomposed into smaller compounds;
-*[[Single displacement reaction|Single displacement]] or [[Substitution (chemistry)|substitution]] ('''A + BC &rarr; AC + B'''), characterized by an element being displaced out of a compound by a more [[Reactivity series|reactive]] element;
-*[[Double displacement reaction|Double displacement]] or substitution ('''AC + BD &rarr; AD + BC'''), in which two compounds in [[aqueous]] [[solution]] (usually [[ionic]]) exchange elements or ions to form different compounds.
-The collision of more than two particles into the ordered structure necessary to perform chemical transformations is extremely unlikely; which is why ternary reactions in practice are not observed.  A chemical reaction may require three or more reagents, but the process can generally be decomposed into a stepwise series (or a a set of stepwise series) of the above.
+A series of different chemical reactions may be performed to [[chemical synthesis|synthesize]] a desired product. In [[biochemistry]], sets of chemical reactions [[catalysis|catalyzed]] by [[enzymes]] make up [[metabolic pathways]], in which syntheses and decompositions ordinarily impossible under conditions within a cell are performed.
+{{toc}}
+Chemical reactions take place within each living organism, allowing the organism to survive, grow, and reproduce. In addition, researchers and chemical engineers utilize chemical reactions to produce a vast array of materials, including [[petrochemical]]s, [[agrochemical]]s, [[ceramic]]s, [[polymer]]s and rubber ([[elastomer]]s), [[oleochemical]]s (oils, fats, and waxes), [[explosive]]s, fragrances, and flavors. Modern society is highly dependent on these products.
-The large diversity of chemical reactions makes it difficult to establish simple criteria for functional (as opposed to mechanistic) classification. However, some kinds of reactions have similarities which make it possible to define some larger groups. A few examples are:
+== Chemical equations ==
-*[[organic reaction]]s, which encompass several different kinds of reactions involving compounds which have [[carbon]] as the main element in their molecular structure.
+A chemical reaction is symbolically represented by a '''[[chemical equation]]''', wherein one set of substances, called the ''reactants'', is converted into another set of substances, called the ''products''. The reactants and products are shown using their [[chemical formula]]s, and an arrow is used to indicate the direction of the reaction. The reactants are usually placed to the left of the arrow, and the products are placed to the right. If the reaction is irreversible, a single arrow is used; if the reaction is reversible, a double arrow (pointing in opposite directions) is used.
-*[[redox reaction]]s, which involve augmenting or decreasing the electrons associated with a particular atom.
-*[[Combustion]], where a substance reacts with [[oxygen]] gas;
-See [[list of reactions]] for more examples.
+For example, the combustion of [[methane]] in [[oxygen]] may be represented by the following equation:
-==Energy and reactions==
+:CH<sub>4</sub> + 2&nbsp;O<sub>2</sub> &rarr; CO<sub>2</sub> + 2&nbsp;H<sub>2</sub>O
-===Net change in energy===
-According to the [[Third law of thermodynamics]], any [[closed system]] will tend to minimize its [[free energy]]. Without any outside influence, any reaction mixture, too, will try to do the same. For many cases, an analysis of the [[enthalpy]] of the system will give a decent account of the energetics of the reaction mixture.
-The enthalpy of a reaction is calculated using standard [[reaction enthalpy|reaction enthalpies]] and [[Hess' law of constant heat summation]]. Many of these enthalpies may be found in beginners' books on thermodynamics.
-For example, consider the reaction
-'''CH<sub>4</sub> + 2&nbsp;O<sub>2</sub> &rarr; CO<sub>2</sub> + 2&nbsp;H<sub>2</sub>O'''
+This equation represents an irreversible reaction in which one molecule of methane reacts with two molecules of oxygen to produce one molecule of [[carbon dioxide]] and two molecules of [[water]].
-(combustion of [[methane]] in [[oxygen]]). By calculating the amounts of energy required to break all the bonds on the left ("before") and right ("after") sides of the equation using collected data, it is possible to calculate the energy difference between the reactants and the products. This is referred to as &Delta;''H'', where &Delta; (Delta) means difference, and ''H'' stands for [[enthalpy]], a measure of energy which is equal to the heat transferred at constant pressure. &Delta;''H'' is usually given in units of [[Joule|kilojoules]] (kJ) or in [[Calorie|kilocalories]] (kcal).
+== Reaction types ==
-====Exothermic reactions====
+[[list of reactions|The large diversity of chemical reactions]] and approaches to their study results in the existence of several concurring, often overlapping, ways of classifying them. Below are examples of widely used terms for describing common kinds of reactions.
-If &Delta;''H'' is negative for the reaction, then energy has been released often in the form of heat. This type of reaction is referred to as [[exothermic]] (literally, outside heat, or throwing off heat). An exothermic reaction is more favourable and thus more likely to occur. An example reaction is [[combustion]], which we already know from everyday experience, since burning gas in air produces heat.
-====Endothermic reactions====
+*[[Isomerisation]], in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition; see [[stereoisomerism]]
-A reaction may have a positive &Delta;''H''. If a reaction has a positive &Delta;''H'', it consumes energy as the reaction moves towards completion. This type of reaction is called [[endothermic]] (literally, inside heat, or absorbing heat).
+*[[Combination reaction|Direct combination]] or [[Chemical synthesis|synthesis]], in which two or more chemical elements or compounds unite to form a more complex product:
+::[[Nitrogen|N]]<sub>2</sub> + 3 [[Hydrogen|H]]<sub>2</sub> → 2 [[Ammonia|NH<sub>3]]</sub>
+*[[Chemical decomposition]] or '''analysis''', in which a compound is decomposed into smaller compounds or elements:
+::2 [[Water|H<sub>2</sub>O]] → 2 H<sub>2</sub> + [[Oxygen|O]]<sub>2</sub>
+*[[Single displacement reaction|Single displacement]] or [[substitution (chemistry)|substitution]], characterized by an element being displaced out of a compound by a more [[Reactivity series|reactive]] element:
+::2 [[Sodium|Na]](s) + 2 [[Hydrogen chloride|HCl]][[(aq)]] → 2 [[Sodium chloride|NaCl]](aq) + H<sub>2</sub>(g)
+*[[Metathesis reaction (chemistry)|Metathesis]] or '''Double displacement reaction''', in which two compounds exchange [[ion]]s or bonds to form different compounds:
+::NaCl(aq) + [[Silver nitrate|AgNO<sub>3</sub>]](aq) → [[Sodium nitrate|NaNO<sub>3</sub>]](aq) + [[Silver chloride|AgCl]](s)
+*[[Acid-base]] reactions, broadly characterized as reactions between an [[acid]] and a [[Base (chemistry)|base]], can have different definitions depending on the acid-base concept employed. Some of the most common are:
+:* [[Acid-base#The Arrhenius definition|Arrhenius]] definition: Acids dissociate in water releasing H<sub>3</sub>O<sup>+</sup> ions; bases dissociate in water releasing OH<sup>-</sup> ions.
+:* [[Brønsted-Lowry acid-base theory|Brønsted-Lowry]] definition: Acids are [[proton]] (H<sup>+</sup>) donors; bases are proton acceptors. Includes the Arrhenius definition.
+:* [[Acid-base#The Lewis definition|Lewis]] definition: Acids are electron-pair acceptors; bases are electron-pair donors. Includes the Brønsted-Lowry definition.
+* [[Redox reaction]]s, in which changes in [[oxidation number]]s of atoms in involved species occur. Those reactions can often be interpreted as transferences of electrons between different molecular sites or species. In the following example of a redox rection, I<sub>2</sub> ([[iodine]]) is reduced to I<sup>-</sup> (iodide anion), and S<sub>2</sub>O<sub>3</sub><sup>2-</sup> ([[thiosulfate]] anion) is oxidized to S<sub>4</sub>O<sub>6</sub><sup>2-</sup>:
+::2 S<sub>2</sub>O<sub>3</sub><sup>2−</sup>(aq) + I<sub>2</sub>(aq) → S<sub>4</sub>O<sub>6</sub><sup>2−</sup>(aq) + 2 I<sup>−</sup>(aq)
+*[[Combustion]], a kind of redox reaction in which any combustible substance combines with an oxidizing element, usually oxygen, to generate [[heat]] and form oxidized products. The term combustion is usually used for only large-scale oxidation of whole [[molecule]]s, i.e. a controlled oxidation of a single functional group is not combustion.
+::C<sub>10</sub>H<sub>8</sub>+ 12 O<sub>2</sub> → 10 CO<sub>2</sub> + 4 H<sub>2</sub>O
+::CH<sub>2</sub>S + 6 [[fluorine|F<sub>2</sub>]] → [[carbon tetrafluoride|CF<sub>4</sub>]] + 2 [[hydrogen fluoride|HF]] + [[sulfur hexafluoride|SF<sub>6</sub>]]
-====Exceptions to the rule====
+Reactions can also be classified according to their mechanism, some typical examples being:
-The above rule, "Exothermic reactions are favourable", is usually true. However, there may be situations where exothermic reactions may not be favourable. This happens when the stability obtained due to loss of enthalpy is off set by a corresponding decrease in [[entropy]] (a measure of randomness).
+:*Reactions of ions, e.g. [[disproportionation]] of [[hypochlorite]]
-The exact rule is that a reaction is favourable when the [[Gibbs free energy]] of that reaction is negative where &Delta;''G'' = &Delta;''H'' &minus; ''T''&Delta;''S''; &Delta;''G'' being the change in Gibbs free energy, &Delta;''H'' being the change in enthalpy, and &Delta;''S'' is the change in entropy
+:*Reactions with reactive ionic intermediates, e.g. reactions of [[enolate]]s
+:*[[Radical (chemistry)|Radical]] reactions, e.g. combustion at high temperature
+:*Reactions of [[carbene]]s
-===Reactive intermediates===
+=== Organic reactions ===
-While [[Thermodynamics]] attempts to answer the question: "Will this reaction occur?", another important question "How fast is the reaction?" is left completely unanswered by it. This is because Thermodynamics (or what is now known as [[Equilibrium Thermodynamics]]) tries to understand the reaction in terms of the initial and final states of the reaction mixture. It does not attempt to figure out the process by which a reaction occurs. This field of study is taken up by the field of [[Reaction Kinetics]]
+[[Organic reaction]]s encompass a wide assortment of reactions involving [[Organic compound|compounds]] that have [[carbon]] as the main element in their molecular structure. The reactions in which an organic compound may take part are largely defined by its [[functional group]]s.
-Reactions very seldom occur directly. Usually, reactants must collide to form an [[activated complex]]. This has a higher internal energy than the original reactants combined, having gained some  from the [[kinetic energy]] of the collision. This energy allows for the rearrangement of bonds which constitutes the reaction. In some reactions, the reactants may pass through several [[reactive intermediates]] before becoming products. [[Reaction Kinetics]] attempts to put all these phenomena into perspective.
-==Reaction Rate==
+There is no limit to the number of possible organic reactions and mechanisms. However, certain general patterns are observed that can be used to describe many common or useful reactions. Each reaction has a stepwise [[reaction mechanism]] that explains how it happens. Organic reactions can be organized into several basic types, with some reactions fitting into more than one category. Some of the basic types of organic chemical reactions are noted below.
-The [[reaction rate|rate]] of a chemical reaction is a measure of how [[concentration]] of the involved substances changes with time. Analysis of reaction rates is important for several applications, such as in [[chemical engineering]] or in [[chemical equilibrium]] study. Rates of reaction depends basically on:
-*[[Reactant]] [[concentration]]s, which usually make the reaction happen at a faster rate if raised,
+*[[Addition reaction]]s, including such reactions as [[halogenation]], [[hydrohalogenation]], and [[hydration]]. Based on the mechanism, the main addition reactions are classified as [[electrophilic addition|electrophilic]], [[nucleophilic addition|nucleophilic]], or [[radical addition|radical]] addition.
-*[[Activation energy]], which is defined as the amount of energy required to make the reaction start and carry on spontaneously. Higher activation energy implies that a reaction will be harder to start and, therefore, slower.
-*[[Temperature]], which hastens reactions if raised, because higher temperature means that the involved species will have more energy, thus making the reaction easier to happen,
-*The presence or absence of a [[catalyst]]. Catalysts are substances which 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.
-Reaction rates are related to the [[concentrations]] of substances involved in reactions, as quantified by the [[mass action|law of mass action]].
+*[[Elimination reaction]]s, including processes such as [[dehydration reaction|dehydration]].
-==Reversibility and spontaneity==
+*[[Substitution reaction]]s are divided into several types: [[Nucleophilic substitution|nucleophilic aliphatic substitution]] with [[SN1 reaction|SN1]], [[SN2 reaction|SN2]] and [[SNi]] [[reaction mechanism]]s; [[nucleophilic aromatic substitution]]; [[nucleophilic acyl substitution]]; [[electrophilic substitution]]; [[electrophilic aromatic substitution]]; and [[radical substitution]].
-Every chemical reaction is, in theory, reversible. In a ''forward reaction'' the substances defined as [[reactant]]s are converted to [[product (chemistry)|products]]. In a ''reverse reaction'' products are converted into reactants.
-[[Chemical equilibrium]] is the state in which the forward and reverse reaction rates are equal, thus preserving the amount of reactants and products. However, a reaction in equilibrium can be driven in the forward or reverse direction by changing reaction conditions such as temperature or pressure. [[Le Chatelier's principle]] can be used to predict whether products or reactants will be formed.
+*[[Organic redox reaction]]s are [[redox reaction]]s specific to organic compounds and are very common.
-Although all reactions are reversible to some extent, some reactions can be classified as irreversible. An ''irreversible reaction'' is one that "goes to completion." This phrase means that nearly all of the reactants are used to form products. These reactions are very difficult to reverse even under extreme conditions.
+*[[Rearrangement reaction]]s are divided into [[1,2-rearrangement]]s, [[pericyclic|pericyclic reactions]], and [[olefin metathesis|metathesis]] reactions.
-A reaction is called [[spontaneous process|spontaneous]] if its thermodynamically favoured, by that meaning that it causes a net increase on global [[entropy]]. Spontaneous reactions (in opposition to non-spontaneous reactions) do not need external perturbations (such as energy supplement) to happen. In a system at chemical equilibrium, it is expected to have larger concentrations of the substances formed by the spontaneous direction of the process.
+* In [[condensation reaction]]s, two [[reactant]]s combine, and a small molecule (usually [[water]]) is split off. The opposite reaction, when water is consumed in a reaction, is called [[hydrolysis]]. Many [[polymerization]] reactions are derived from organic reactions. They are divided into [[addition polymerization]]s and [[step-growth polymerization]]s.
+== Chemical kinetics ==
+{{main|Chemical kinetics}}
+The [[reaction rate|rate]] of a chemical reaction is a measure of how the [[concentration]] or [[pressure]] of the involved substances changes with time. Analysis of reaction rates is important for several applications, such as in [[chemical engineering]] or in [[chemical equilibrium]] study. Rates of reaction depends basically on:
+*[[Reactant]] concentrations, which usually make the reaction happen at a faster rate if raised through increased collisions per unit time.
+*[[Surface area]] available for contact between the reactants, in particular solid ones in heterogeneous systems. Larger surface area leads to higher reaction rates.
+*[[Pressure]], by increasing the pressure, you decrease the volume between molecules. This will increase the frequency of collisions of molecules.
+*[[Activation energy]], 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 a lower activation energy.
+*[[Temperature]], which hastens reactions if raised, since higher temperature increases the energy of the molecules, creating more collisions per unit time,
+*The presence or absence of a [[catalyst]]. Catalysts are substances which 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.
+*For some reactions, the presence of [[electromagnetic radiation]], most notably [[ultra violet]], is needed to promote the breaking of bonds to start the reaction. This is particularly true for reactions involving [[radical (chemistry)|radicals]].
+Reaction rates are related to the [[concentrations]] of substances involved in reactions, as quantified by the [[rate equation|rate law]] of each reaction. Note that some reactions have rates that are ''independent'' of reactant concentrations. These are called [[Rate law#Zero-order reactions|zero order reactions]].
 ==See also==
-*[[Quantum electrochemistry]]
-*[[Chemical synthesis]]
-*[[Chemical equation]]
-*[[List of reactions]]
-*[[Thermodynamics]]
-*[[Thermochemistry]]
-*[[Chemical kinetics]]
-[[Category:Chemical reactions|*]]
-[[category:Physical sciences]]
-[[ar:تفاعل كيميائي]]
+* [[Acid]]
-[[ca:Reacció química]]
+* [[Acid-base reaction]]
-[[de:Chemische Reaktion]]
+* [[Base (chemistry)]]
-[[et:Keemiline reaktsioon]]
+* [[Catalyst]]
-[[es:Reacción química]]
+* [[Chemical bond]]
-[[fr:Réaction chimique]]
+* [[Chemical equation]]
-[[ko:화학 반응]]
+* [[Chemical kinetics]]
-[[id:Reaksi kimia]]
+* [[Combustion]]
-[[it:Reazione chimica]]
+* [[Inorganic chemistry]]
-[[nl:Chemische reactie]]
+* [[Organic chemistry]]
-[[nds:Chemische Reaktschoon]]
+* [[Redox]]
-[[ja:化学反応]]
+* [[Stoichiometry]]
-[[nn:Kjemisk reaksjon]]
-[[pl:Reakcja chemiczna]]
+== Notes ==
-[[pt:Reacção química]]
+<references/>
-[[ru:Химическая реакция]]
-[[sr:Хемијска реакција]]
+== References ==
-[[su:Réaksi kimiawi]]
-[[fi:Kemiallinen reaktio]]
+* Chang, Raymond. 2006. ''Chemistry'', 9th ed. New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031
-[[zh:化学反应]]
+* Cotton, F. Albert, and Geoffrey Wilkinson. 1980. ''Advanced Inorganic Chemistry'', 4th ed. New York: Wiley. ISBN 0471027758
+* McMurry, J., and R.C. Fay. 2004. ''Chemistry'', 4th ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0131402080
+* McMurry, John. 2004. ''Organic Chemistry'', 6th ed. Belmont, CA: Brooks/Cole. ISBN 0534420052
+* Morrison, Robert T., and Robert N. Boyd. 1992. ''Organic Chemistry'', 6th ed. Englewood Cliffs, NJ: Prentice Hall. ISBN 0-13-643669-2
+* Solomons, T.W. Graham, and Craig B. Fryhle. 2004. ''Organic Chemistry'', 8th ed. Hoboken, NJ: John Wiley. ISBN 0471417998
+== External links ==
+All links retrieved December 5, 2023.
+* [http://www.chemtutor.com/react.htm Reactions] chemtutor.
+[[Category:Physical sciences]]
+[[Category:Chemistry]]
-{{credit|23071591}}
+{{credits|Chemical_reaction|232294787|Organic_reaction|233791475}}