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

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For other uses, see Chemical reaction (disambiguation).

Vapours 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 results in the interconversion of chemical substances.^[1] The substance or substances initially involved in a chemical reaction are called reactants. Chemical reactions are usually characterized by a chemical change, and they yield one or more products which 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, although the general concept of a chemical reaction, in particular the notion of a chemical equation, is applicable to transformations of elementary particles, as well as nuclear reactions.

Different chemical reactions are used in combinations in chemical synthesis in order to get a desired product. In biochemistry, series of chemical reactions catalyzed by enzymes form metabolic pathways, by which syntheses and decompositions ordinarily impossible in conditions witphin a cell are performed.

Reaction types

Template:Cleanup-section

The large diversity of chemical reactions and of 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)

Precipitation Reactions where species in solution combine to form a solid product (precipitate). A typical example would be the reaction of methatesis described above.
Acid-base reactions, broadly characterized as reactions betwwen 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 definiton: 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. A typical example of redox rection is:

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

In which I₂ is reduced to I^- and S₂O₃^2- (thiosulfate anion) is oxidized to S₄O₆^2-.

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 used usually 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

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

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

Some branches of chemistry include any detectable changes in chemical conformation in the reaction types, while others consider these changes merely as physical properties of a compound.

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 best described as a stepwise series of elementary reactions.

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, the amount of the substance being used,
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.

References
ISBN links support NWE through referral fees

↑ International Union of Pure and Applied Chemistry. "chemical reaction". Compendium of Chemical Terminology Internet edition.

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[1] International Union of Pure and Applied Chemistry. "chemical reaction". Compendium of Chemical Terminology Internet edition.

[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.
+{{otheruses}}
-==Types==
+[[Image:Hydrochloric acid ammonia.jpg|right|180px|thumb|Vapours 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 results in the interconversion of [[chemical substance]]s.<ref>{{GoldBookRef|title=chemical reaction|file=C01033}}</ref> The substance or substances initially involved in a chemical reaction are called [[reactants]]. Chemical reactions are usually characterized by a [[chemical change]], and they yield one or more [[Product (chemistry)|products]] which 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 bond]]s, although the general concept of a chemical reaction, in particular the notion of a [[chemical equation]], is applicable to [[Feynman diagram|transformations of elementary particles]], as well as [[nuclear reaction]]s.
-There are five major classifications of chemical reactions. Some common and widely known are:
+Different chemical reactions are used in combinations in [[chemical synthesis]] in order to get a desired product. In [[biochemistry]], series of chemical reactions [[catalysis|catalyzed]] by [[enzymes]] form [[metabolic pathways]], by which syntheses and decompositions ordinarily impossible  in conditions witphin a cell are performed.
+== Reaction types ==
+{{Cleanup-section|date=October 2007}}
-*[[Isomerization]] ('''A &rarr; B''') in which a [[Chemical compound|compound]] undergoes a structural rearrangement without any change in its net atomic composition;
+[[list of reactions|The large diversity of chemical reactions]] and of 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:
-*[[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.
+*[[Isomerisation]], in which a chemical compound undergoes a structural rearrangement without any change in its net atomic composition; see [[stereoisomerism]]
+*[[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)
+*[[Precipitation]] Reactions where species in solution combine to form a solid product (precipitate). A typical example would be the reaction of methatesis described above.
+*[[Acid-base]] reactions, broadly characterized as reactions betwwen an [[acid]] and a [[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]] definiton: 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. A typical example of redox rection is:
+: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)
+In which I<sub>2</sub> is reduced to I<sup>-</sup> 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>.
+:*[[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 used usually only large-scale oxidation of whole molecules, 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>]]
+* [[Organic reaction]]s encompass a wide assortment of reactions involving [[Organic compound|compounds]] which have [[carbon]] as the main element in their molecular structure. The reactions an organic compound may take part are largely defined by its [[functional group]]s. Defined in opposition to [[Inorganic chemical reaction|inorganic reactions]].
-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:
+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 [[enolate]]s
+*[[Radical (chemistry)|Radical]] reactions, e.g. combustion at high temperature
+*Reactions of [[carbene]]s
-*[[organic reaction]]s, which encompass several different kinds of reactions involving compounds which have [[carbon]] as the main element in their molecular structure.
+Some branches of chemistry include any detectable changes in [[chemical conformation]] in the reaction types, while others consider these changes merely as physical properties of a compound.
-*[[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.
+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 best described as a [[stepwise reaction|stepwise]] series of [[elementary reaction]]s.
-==Energy and reactions==
+== Chemical kinetics ==
-===Net change in energy===
+{{main|Chemical kinetics}}
-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'''
+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]], the amount of the substance being used,
+*[[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]].
-(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 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]].
-====Exothermic reactions====
+==See also==
-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.
+*[[List of reactions]]
+*[[Organic reaction]]
-====Endothermic reactions====
+*[[Inorganic chemical reaction]]
-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).
+*[[List of important publications in chemistry]]
+*[[Stoichiometry]]
+*[[Stoichiometric coefficient]]
+*[[Transition state theory]]
+*[[Gas stoichiometry]]
+*[[Thermodynamic databases for pure substances]]
-====Exceptions to the rule====
+== References ==
-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).
+<references/>
-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
-===Reactive intermediates===
+[[Category:Physical sciences]]
-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]]
-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==
-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,
-*[[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]].
-==Reversibility and spontaneity==
-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.
-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.
-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.
-==See also==
-*[[Quantum electrochemistry]]
-*[[Chemical synthesis]]
-*[[Chemical equation]]
-*[[List of reactions]]
-*[[Thermodynamics]]
-*[[Thermochemistry]]
-*[[Chemical kinetics]]
 [[Category:Chemistry]]
-[[category:Physical sciences]]
-{{credit|23071591}}
+{{credit|165126019}}