WordNet

make a request for something; "Order me some flowers"; "order a work stoppage"
bring order to or into; "Order these files"
established customary state (especially of society); "order ruled in the streets"; "law and order"
place in a certain order; "order the photos chronologically"
a degree in a continuum of size or quantity; "it was on the order of a mile"; "an explosion of a low order of magnitude" (同)order_of_magnitude
the act of putting things in a sequential arrangement; "there were mistakes in the ordering of items on the list" (同)ordering
(architecture) one of original three styles of Greek architecture distinguished by the type of column and entablature used or a style developed from the original three by the Romans
a group of person living under a religious rule; "the order of Saint Benedict" (同)monastic order
a commercial document used to request someone to supply something in return for payment and providing specifications and quantities; "IBM received an order for a hundred computers" (同)purchase order
a body of rules followed by an assembly (同)rules of order, parliamentary law, parliamentary procedure
(biology) taxonomic group containing one or more families
(often plural) a command given by a superior (e.g., a military or law enforcement officer) that must be obeyed; "the British ships dropped anchor and waited for orders from London"
a request for something to be made, supplied, or served; "I gave the waiter my order"; "the companys products were in such demand that they got more orders than their call center could handle"
issue commands or orders for (同)prescribe, dictate
give instructions to or direct somebody to do something with authority; "I said to him to go home"; "She ordered him to do the shopping"; "The mother told the child to get dressed" (同)tell, enjoin, say
(mechanics) the equal and opposite force that is produced when any force is applied to a body; "every action has an equal and opposite reaction"
a response that reveals a persons feelings or attitude; "he was pleased by the audiences reaction to his performance"; "John feared his mothers reaction when she saw the broken lamp"
a bodily process occurring due to the effect of some antecedent stimulus or agent; "a bad reaction to the medicine"; "his responses have slowed with age" (同)response
doing something in opposition to another way of doing it that you dont like; "his style of painting was a reaction against cubism"
an idea evoked by some experience; "his reaction to the news was to start planning what to do"
extreme conservatism in political or social matters; "the forces of reaction carried the election"
having a systematic arrangement; especially having elements succeeding in order according to rule; "an ordered sequence"
logical or comprehensible arrangement of separate elements; "we shall consider these questions in the inverse order of their presentation" (同)order, ordination

PrepTutorEJDIC

〈C〉《しばしば複数形で》『命令』,指図,指令 / 〈U〉(物事の)『順序』,順番 / 〈U〉(自然界の)『秣序』 / 〈U〉(社会の)治安,秣序 / 〈U〉整頓(せいとん)された状態,きちんとしていること《good,badなどを伴って》(一般に物事の)調子, 状態 / 〈U〉(商品などの)『注文』《+『for』+『名』》 / 〈C〉調文品 / 〈C〉(レストランなどでの料理の)一盛り / 〈C〉種類,等級,品資(kind, sort) / 〈C〉(動植物分類の)目(もく) / 〈C〉聖職者の階級;《複数形で》聖職 / 〈C〉《しばしばO-》教団,教派;(ある特殊な)社会 / 〈C〉《しばしばO-》勲位,勲章 / 〈C〉(古代ギリシャの)建築様式,柱式 / …‘を'『命令する』,指図する;〈人〉‘に'命令する / 《方向を表す副詞[句]を伴って》〈人〉‘に'行くように命じる / 〈商品など〉‘を'『注文する』;…‘を'あつらえる / …‘を'整頓(せいとん)する,きちんとする / 命令する;注文する
{名}(…に対する)『反応』《+『to』+『名』》 / 〈U〉(…に対する)(政治的・社会的な)『反動』,逆コース《+『against』+『名』》 / 〈U〉〈C〉化学反応,化学変化 / 〈U〉〈C〉(物理学で)反作用
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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2012/12/12 22:05:41」(JST)

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In chemical kinetics, the order of reaction with respect to a given substance (such as reactant, catalyst or product) is defined as the index, or exponent, to which its concentration term in the rate equation is raised.^[1] For the typical rate equation of form , where [A], [B], ... are concentrations, the reaction orders (or partial reaction orders) are x for substance A, y for substance B, etc. The overall reaction order is the sum x + y + ....

For example, the chemical reaction between mercury (II) chloride and oxalate ion

2 HgCl₂(aq) + C₂O₄^2-(aq) → 2 Cl^-(aq) + 2 CO₂(g) + Hg₂Cl₂(s)

has the observed rate equation^[2]

r = k[HgCl₂]¹[C₂O₄^2-]²

In this case, the reaction order with respect to the reactant HgCl₂ is 1 and with respect to oxalate ion is 2; the overall reaction order is 1 + 2 = 3. As is true for many reactions, the reaction orders (here 1 and 2 respectively) differ from the stoichiometric coefficients (2 and 1). Reaction orders can be determined only by experiment. Their knowledge allows conclusions to be drawn about the reaction mechanism, and may help to identify the rate-determining step.

Elementary (single-step) reactions do have reaction orders equal to the stoichiometric coefficients for each reactant, but complex (multi-step) reactions may or may not have reaction orders equal to their stoichiometric coefficients.

Orders of reaction for each reactant are often positive integers, but they may also be zero, fractional, or negative.

A reaction can also have an undefined reaction order with respect to a reactant if the rate is not simply proportional to some power of the concentration of that reactant; for example, one cannot talk about reaction order in the rate equation for a bimolecular reaction between adsorbed molecules:

First order

If a reaction rate depends on a single reactant and the value of the exponent is one, then the reaction is said to be first order. In organic chemistry, the class of S_N1 (nucleophilic substitution unimolecular) reactions consists of first-order reactions. For example,^[3] in the reaction of aryldiazonium ions with nucleophiles in aqueous solution ArN₂⁺ + X⁻ → ArX + N₂, the rate equation is r = k[ArN₂⁺], where Ar indicates an aryl group.

Another class of first-order reactions is radioactive decay processes which are all first order. These are, however, nuclear reactions rather than chemical reactions.

Second order

A reaction is said to be second order when the overall order is two. The rate of a second-order reaction may be proportional to one concentration squared , or (more commonly) to the product of two concentrations . As an example of the first type, the reaction NO₂ + CO → NO + CO₂ is second-order in the reactant NO₂ and zero order in the reactant CO. The observed rate is given by , and is independent of the concentration of CO.^[4]

The second type includes the class of S_N2 (nucleophilic substitution bimolecular) reactions, such as the alkaline hydrolysis of ethyl acetate^[3]:

CH₃COOC₂H₅ + OH⁻ → CH₃COO⁻ + C₂H₅OH.

This reaction is first-order in each reactant and second-order overall: r = k[CH₃COOC₂H₅][OH⁻]

If the same hydrolysis reaction is catalyzed by imidazole, the rate equation becomes^[3] r = k[imidazole][CH₃COOC₂H₅]. The rate is first-order in one reactant (ethyl acetate), and also first-order in imidazole which as a catalyst does not appear in the overall chemical equation.

Pseudo-first order

If the concentration of a reactant remains constant (because it is a catalyst or it is in great excess with respect to the other reactants), its concentration can be included in the rate constant, obtaining a pseudo-first order (or occasionnally pseudo-second order) rate equation. For a typical second-order reaction with rate equation , if the concentration of reactant B is constant then :, where the pseudo-first order rate constant . The second-order rate equation has been reduced to a pseudo-first-order rate equation, which makes the treatment to obtain an integrated rate equation much easier.

For example the hydrolysis of sucrose in acid solution is often cited as a first-order reaction with rate . The true rate equation is third-order, . However the concentrations of both the catalyst H⁺ and the solvent H₂O are normally constant, so that the reaction is pseudo-first order.^[5]

Zero order

For zero-order reactions, the reaction rate is independent of the concentration of a reactant, so that changing its concentration has no effect on the speed of the reaction. This is true for many enzyme-catalyzed reactions, provided that the reactant concentration is much greater than the enzyme concentration which controls the rate. For example, the biological oxidation of ethanol to acetaldehyde by the enzyme liver alcohol dehydrogenase (LADH) is zero order in ethanol.^[6]

Fractional order

In fractional order reactions, the order is a non-integer, which often indicates a complex reaction mechanism. For example, the pyrolysis of ethanal (CH₃-CHO) into methane and carbon monoxide proceeds with an order of 1.5 with respect to ethanal: r = k[CH₃-CHO]^3/2.^[7] The decomposition of phosgene (COCl₂) to carbon monoxide and chlorine has order 1 with respect to phosgene itself and order 0.5 with respect to chlorine: r = k[COCl₂] [Cl₂]^1/2.

Mixed order

In a mixed-order reaction, the order of a reaction changes in the course of a reaction as a result of changing variables such as pH. An example is the oxidation of an alcohol to a ketone by a ruthenate (RuO₄²⁻) and a hexacyanoferrate, the latter serving as the sacrificial catalyst converting Ru(IV) back to Ru(VI):^[8] the disappearing-rate of the ferrate is zero-order with respect to the ferrate at the onset of the reaction (when its concentration is high and the ruthenium catalyst is quickly regenerated) but changes to first-order when its concentration decreases.

Negative order

A reaction rate can have a negative partial order with respect to a substance. For example the conversion of ozone (O₃) to oxygen follows the rate equation , corresponding to second order in ozone and order (-1) with respect to oxygen.

When a partial order is negative, the overall order is usually considered as undefined. In the above example for instance, the reaction is not described as first order even though the sum of the partial orders is 2 + (-1) = 1, because the rate equation is more complex than that of a simple first-order reaction.

References

^ IUPAC's Goldbook definition of order of reaction
^ Petrucci R.H., Harwood W.S. and Herring F.G. General Chemistry (8th ed., Prentice-Hall 2002), p.585-6
^ ^a ^b ^c Kenneth A. Connors Chemical Kinetics, the study of reaction rates in solution, 1990, VCH Publishers
^ Whitten K.W., Galley K.D. and Davis R.E. General Chemistry (4th edition, Saunders 1992), p.638-9
^ I. Tinoco, K. Sauer and J.C. Wang Physical Chemistry. Principles and Applications in Biological Sciences. (3rd ed., Prentice-Hall 1995) p.328-9
^ Tinoco, Sauer and Wang p.331
^ P. Atkins and J. de Paula, Physical Chemistry 8th ed.(W.H. Freeman 2006), p.830
^ Ruthenium(VI)-Catalyzed Oxidation of Alcohols by Hexacyanoferrate(III): An Example of Mixed Order Mucientes, Antonio E,; de la Peña, María A. J. Chem. Educ. 2006 83 1643. Abstract

External links

Chemical kinetics, reaction rate, and order (needs flash player)
Reaction kinetics, examples of important rate laws (lecture with audio).
Rates of Reaction

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