User:Milton Beychok/Sandbox: Difference between revisions

From Citizendium
Jump to navigation Jump to search
imported>Milton Beychok
imported>Milton Beychok
No edit summary
Line 1: Line 1:
{{TOC|right}}
A '''chemical reaction''' is a process that leads to the transformation of one set of [[Chemistry|chemical]] substances to another. The one or more substances present at the start of a reaction are called '''''reactants''''' and the one or more substances present at the end of the reaction are called '''''products'''''. The study of chemical reactions is part of the field of [[science]] called [[chemistry]].


Chemical reactions can result in [[molecule]]s attaching to each other to form larger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangement of [[atom]]s within or across molecules. Chemical reactions usually involve the making or breaking of [[Bond (chemical)|chemical bonds]].
Chemical reactions can be either spontaneous<ref name=Rosengarten>[http://www.youtube.com/watch?v=m1nKEz2DPC0 Chemistry Tutorials: Entropy, Enthalpy and Spontaneous Reactions]</ref> and require no input of [[energy]], or non-spontaneous<ref name=Rosengarten/> which often require the input of some type of energy such as [[heat]], [[light]] or [[electricity]]. Classically, chemical reactions are strictly transformations that involve the movement of [[electron]] in the forming and breaking of [[chemical bond]]s. A more general concept of a chemical reaction would include [[nuclear reactions]] and [[Elementary particle|elementary particle reactions]].
==Energy changes in reactions==
{{Image|ExoEndo Reax.png|right|400px|Energy level diagrams of exothermic and endothermic reactions<ref>{{cite book|author=Paul Collison, David Kirby and Averil Macdonald|title=Nelson Modular Science, Volume 2|edition=|publisher=Nelson Thorne Ltd.|year=2002|id=ISBN 0-7487-6247-7}}</ref>}}
In terms of the [[energy]] changes that take place during chemical reactions, a reaction may be either '''''exothermic''''' or '''''endothermic''''' ... terms which were first coined by the [[France|French]] chemist [[Marcellin Berthelot]] (1827 − 1907). The meaning of those terms and the difference between them are discussed below and illustrated in the adjacent diagram of the energy profiles for exothermic and endothermic reactions.
===Exothermic reactions===
Exothermic chemical reactions release energy. The released energy may be in the form of heat, light (e.g., flame), electricity (e.g., [[battery]] discharge),  [[sound]] and [[shock wave]]s (e.g., [[explosion]]) .... either singly or in combinations.
A few examples of exothermic reactions are:
*[[Mixing]] of [[acid]]s and [[alkali]]s (releases heat)
*[[Combustion]] of [[fuel]]s (releases heat and light)
===Endothermic reactions===
Endothermic chemical reactions absorb energy. The energy absorbed may be in various forms just as is the case with exothermic reactions:
A few examples of endothermic reactions are:
*Dissolving [[ammonium nitrate]] (NH<sub>4</sub>NO<sub>3</sub>) in [[water]] (H<sub>2</sub>O) (absorbs heat and cools the surroundings)
*[[Electrolysis]] of water to form [[hydrogen]] (H<sub>2</sub>) and [[oxygen]] (O<sub>2</sub>) [[gas]]es (absorbs electricity)
*[[Photosynthesis]] of [[chlorophyll]] plus water plus sunlight to form [[carbohydrates]] and oxygen (absorbs light)
== Reaction types ==
The common kinds of classical chemical reactions include:<ref>In the following [[chemical equation]]s, (aq) indicates an aqueous solution, (g) indicates a gas and (s) indicates a solid. Superscripts with a positive sign (+)  indicate an [[Ion|cation]] and superscripts with a negative sign (−) indicate an [[Ion|anion]].</ref>
*[[Isomerization]], 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 2 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]] in which a compound is decomposed into elements or smaller compounds:
:2 [[Water|H<sub>2</sub>O]] → 2 [[Hydrogen|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|double displacement]], in which two compounds exchange [[ion]]s or [[Chemical bond|bonds]] to form different compounds:
:[[Sodium chloride|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|Arrhenius]] definition: Acids dissociate in water releasing H<sub>3</sub>O<sup>+</sup> ions; bases dissociate in water releasing OH<sup>−</sup> ions.
**[[Acid-base|Brønsted-Lowry]] definition: Acids are proton (H<sup>+</sup>) donors; bases are proton acceptors. Includes the Arrhenius definition.
**[[Acid-base|Lewis]] definition: Acids are electron-pair acceptors; bases are electron-pair donors. Includes the Brønsted-Lowry definition.
*[[Redox reaction]]s, in which changes in the [[oxidation number]]s of atoms in the involved species occur. Those reactions can often be interpreted as transferences of electrons between different molecular sites or species. An example of a redox reaction is:
:2 S<sub>2</sub>O<sub>3</sub><sup>2−</sup>(aq) + [[Iodine|I]]<sub>2</sub>(aq) → S<sub>4</sub>O<sub>6</sub><sup>2−</sup>(aq) + 2 I<sup>−</sup>(aq)
:In which [[iodine]] (I<sub>2</sub>) is [[Redox|reduced]] to the iodine [[anion]] (I<sup>−</sup>) and the [[thiosulfate]] anion (S<sub>2</sub>O<sub>3</sub><sup>2−</sup>) is [[Redox|oxidized]] to the [[tetrathionate]] anion (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 [[Oxidation|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).
:[[Methane|CH<sub>4</sub>]] + 2 [[Oxygen|O]]<sub>2</sub> → [[Carbon dioxide|CO<sub>2</sub>]] + 2 [[Hydrogen|H]]<sub>2</sub>O
*[[Disproportionation]] with one reactant forming two distinct products varying in oxidation state.
: 2 Sn<sup>2+</sup> → [[Tin|Sn]] + Sn<sup>4+</sup>
*[[Organic reaction]]s encompass a wide assortment of reactions involving [[organic compound]]s which are chemical compounds having [[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.
==References==
<references/>

Revision as of 20:53, 11 April 2010

Retrieved from "https://citizendium.org/wiki/index.php?title=User:Milton_Beychok/Sandbox&oldid=389113"