Name: elementary steps in reaction mechanisms
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 6 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.5.1

Reaction Mechanisms

Een chemische reactie wordt vaak weergegeven door een algeheel uitgebalanceerde chemische vergelijking die de reactanten en producten aangeeft. De eigenlijke reactie is echter vaak complexer en verloopt in meerdere stappen. Deze reactie van stikstofmonoxide met waterstof onder vorming van stikstofgas en water vindt bijvoorbeeld plaats in drie verschillende opeenvolgende stappen.Deze stappen worden het reactiemechanisme genoemd. Elke stap in het reactiemechanisme wordt een elementaire reactie genoemd en vertegenwoordigt de interactie, zoals het verbreken of vormen van een binding, tussen de reagerende soorten. Specifieke moleculen, zoals distikstofdioxide en lachgas, worden gevormd tijdens de ene elementaire stap en verbruikt tijdens een andere.Dergelijke soorten worden reactietussenproducten genoemd. Reactie-tussenproducten zijn producten met lage energiewaardes van een elementaire reactie. Ze zijn vaak van korte duur, wat hun afwezigheid in het productmengsel verklaart.Reactie-tussenproducten zijn niet hetzelfde als geactiveerde complexen. Geactiveerde complexen zijn energetische overgangstoestanden die alleen bestaan tijdens de omzetting van reactanten in producten. Door de elementaire stappen te combineren, ontstaat de vergelijking voor de algehele chemische reactie.Hier worden de reactietussenproducten geëlimineerd en verschijnen ze daarom niet in de algemene chemische vergelijking. De verschillende elementaire reacties kunnen met verschillende snelheden verlopen. De langzaamste elementaire stap bepaalt de algehele reactiesnelheid.Hier is de reactie van distikstofdioxide met waterstofgas de snelheidsbeperkende stap. Elementaire reacties kunnen gewoonlijk worden gekarakteriseerd als drie typen, afhankelijk van het aantal reagerende moleculen of de moleculariteit. In een unimoleculaire reactie verandert een enkel reactantmolecuul in een of meerdere producten.Bij een bimoleculaire reactie reageren twee verschillende moleculen. Een termoleculaire reactie, hoewel zeer zeldzaam, omvat drie individuele moleculen die reageren om tussenproducten of producten op te leveren. In tegenstelling tot de snelheidsvergelijking voor een algehele chemische reactie, die experimenteel wordt bepaald, kunnen snelheidsvergelijkingen voor elementaire reacties worden voorspeld uit de stoichiometrische coëfficiënten van hun reactanten.Kortom, de moleculairiteit van een elementaire reactie komt overeen met de algehele volgorde van de reactie van de elementaire stap. Daarom zijn unimoleculaire reacties vaak reacties van de eerste orde, zijn bimoleculaire reacties van de tweede orde en zijn termoleculaire reacties van de derde orde. Een goed begrip van reactiemechanismen en kinetiek helpt chemici bij het identificeren en optimaliseren van chemische reacties.

reactiemechanismen

Chemische reacties vinden vaak stapsgewijs plaats, waarbij twee of meer afzonderlijke reacties achter elkaar plaatsvinden. Een uitgebalanceerde vergelijking geeft de reagerende soort en de productsoort aan, maar onthult geen details over hoe de reactie op moleculair niveau plaatsvindt. Het reactiemechanisme (of reactiepad) geeft details over het precieze, stapsgewijze proces waarmee een reactie plaatsvindt. De afbraak van ozon lijkt bijvoorbeeld een mechanisme te volgen dat uit twee stappen bestaat: <img alt="Eq1" src="/files/ftp_upload/11381/11381_Equation_1.png" style="text-align: center; height: 70px;"> Elk van de stappen in een reactiemechanisme wordt een elementaire reactie genoemd. Deze elementaire reacties vinden in volgorde plaats, zoals weergegeven in de stapvergelijkingen, en ze vormen samen de uitgebalanceerde chemische vergelijking die de algehele reactie beschrijft: <img alt="Eq2" src="/files/ftp_upload/11381/11381_Equation_2.png" style="text-align: center; height: 50px;"> Merk op dat het zuurstofatoom dat in de eerste stap wordt geproduceerd, tijdens de tweede wordt verbruikt en niet als product in de totale reactie verschijnt. Dergelijke soorten die in één stap worden geproduceerd en in een volgende stap worden geconsumeerd, worden reactietussenproducten genoemd. Hoewel de algemene reactievergelijking aangeeft dat twee ozonmoleculen reageren en drie zuurstofmoleculen opleveren, omvat het feitelijke reactiemechanisme niet de directe botsing en reactie van twee ozonmoleculen. In plaats daarvan valt één O3 uiteen en levert O2 en een zuurstofatoom op, en een tweede O3-molecuul reageert vervolgens met het zuurstofatoom en levert twee extra O2-moleculen op. In tegenstelling tot gebalanceerde vergelijkingen die een algehele reactie vertegenwoordigen, zijn de vergelijkingen voor elementaire reacties expliciete representaties van de chemische verandering. Een elementaire reactievergelijking geeft de werkelijke reactant(en) weer die het verbreken/maken van bindingen ondergaat, en het/de gevormde product(en). De snelheidswet voor een elementaire reactie kan dus rechtstreeks worden afgeleid uit de uitgebalanceerde chemische vergelijking. Dit is echter niet het geval voor typische chemische reacties, waarvoor de snelheidswetten alleen betrouwbaar kunnen worden bepaald door middel van experimenten. Unimoleculaire elementaire reacties De moleculaire aard van een elementaire reactie is het aantal soorten reactanten (atomen, moleculen of ionen). Een unimoleculaire reactie omvat bijvoorbeeld de reactie van een enkele reactant om een of meer productmoleculen te produceren: <img alt="Eq3" src="/files/ftp_upload/11381/11381_Equation_3.png" style="text-align: center; height: 50px;"> De snelheidswet voor een unimoleculaire reactie is van de eerste orde; snelheid = k [A]. Een unimoleculaire reactie kan een van de vele elementaire reacties in een complex reactiemechanisme zijn. De reactie (O3 (g) → O2 (g) + O) illustreert bijvoorbeeld een unimoleculaire elementaire reactie die plaatsvindt als onderdeel van een tweestapsreactiemechanisme. Sommige unimoleculaire reacties kunnen echter de enige stap zijn van een eenstapsreactiemechanisme. (Met andere woorden: een ‘totale’ reactie kan in sommige gevallen ook een elementaire reactie zijn.) De ontleding in de gasfase van cyclobutaan, C4H8, tot ethyleen, C2H4, wordt bijvoorbeeld weergegeven door de chemische vergelijking: <img alt="Eq3A" src="/files/ftp_upload/11381/11381_Equation_3A.png" style="text-align: center; height: 50px;"> Deze vergelijking vertegenwoordigt de algehele reactie en beschrijft een unimoleculair elementair proces. De snelheidswet die uit deze vergelijking wordt voorspeld, blijkt, ervan uitgaande dat het een elementaire reactie is, dezelfde te zijn als de snelheidswet die experimenteel is afgeleid voor de algehele reactie, en toont eerste-orde gedrag: <img alt="Eq4" src="/files/ftp_upload/11381/11381_Equation_4.png" style="text-align: center; height: 70px;"> Deze overeenkomst tussen waargenomen en voorspelde snelheidswetten geeft aan dat het voorgestelde unimoleculaire, uit één stap bestaande proces een redelijk mechanisme is voor de butadieenreactie. Bimoleculaire elementaire reacties Bij een bimoleculaire reactie zijn twee soorten reactanten betrokken. Bijvoorbeeld: <img alt="Eq5" src="/files/ftp_upload/11381/11381_Equation_5.png" style="text-align: center; height: 70px;"> In het eerste type, waarbij de twee reactantmoleculen verschillend zijn, is de snelheidswet van de eerste orde in A en van de eerste orde in B (over het geheel genomen van de tweede orde) <img alt="Eq6" src="/files/ftp_upload/11381/11381_Equation_6.png" style="text-align: center; height: 50px;"> In het tweede type, waarin twee identieke moleculen botsen en reageren, is de snelheidswet van de tweede orde in A: <img alt="Eq7" src="/files/ftp_upload/11381/11381_Equation_7.png" style="text-align: center; height: 50px;"> Sommige chemische reacties vinden plaats via mechanismen die bestaan uit een enkele bimoleculaire elementaire reactie. Een voorbeeld is de reactie van stikstofdioxide met koolmonoxide: <img alt="Eq8" src="/files/ftp_upload/11381/11381_Equation_8.png" style="text-align: center; height: 50px;"> Bimoleculaire elementaire reacties kunnen ook betrokken zijn als stappen in een meerstapsreactiemechanisme. De reactie van atomaire zuurstof met ozon is de tweede stap van een tweestaps ozonafbraakmechanisme: <img alt="Eq9" src="/files/ftp_upload/11381/11381_Equation_9.png" style="text-align: center; height: 50px;"> Termoleculaire elementaire reacties Een elementaire termoleculaire reactie omvat de gelijktijdige botsing van drie atomen, moleculen of ionen. Termoleculaire elementaire reacties zijn ongebruikelijk omdat de kans dat drie deeltjes tegelijkertijd botsen zeer zeldzaam is. Er zijn echter enkele gevestigde termoleculaire elementaire reacties. De reactie van stikstofoxide met zuurstof lijkt termoleculaire stappen te omvatten: <img alt="Eq10" src="/files/ftp_upload/11381/11381_Equation_10.png" style="text-align: center; height: 70px;"> Op dezelfde manier lijkt de reactie van stikstofoxide met chloor termoleculaire stappen te omvatten: <img alt="Eq11" src="/files/ftp_upload/11381/11381_Equation_11.png" style="text-align: center; height: 70px;"> Vaak is een van de elementaire stappen in een meerstapsreactiemechanisme aanzienlijk langzamer dan de andere. Omdat een reactie niet sneller kan verlopen dan de langzaamste stap, beperkt deze stap de snelheid waarmee de totale reactie plaatsvindt. De langzaamste elementaire stap wordt daarom de snelheidsbepalende stap (of snelheidsbepalende stap) van de reactie genoemd.

Reactiemechanismen

Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates ...

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elementary steps in reaction mechanisms

Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps.

Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced equation for the overall reaction. In the ozone example, an oxygen atom is made in the first step and used up in the second step, so it does not appear in the final equation. A species made in one step and used up in a later step is called a reaction intermediate.

The overall equation may show two ozone molecules forming three oxygen molecules, but the mechanism shows that the two ozone molecules do not react directly with each other. Instead, one O3 molecule breaks down to form O2 and an oxygen atom. Then a second O3 molecule reacts with that oxygen atom to make two more O2 molecules. This is why elementary reaction equations give a more direct picture of bond breaking and bond making than the overall equation does.

For an elementary reaction, the rate law can often be written directly from the balanced equation for that step. For most chemical reactions, the rate law must be found by experiment. That is why elementary reactions are useful when chemists build a mechanism that matches the observed behavior of a reaction.

Elementary reactions are described by molecularity, which is the number of reactant species in the step. A unimolecular reaction has one reactant species and forms one or more products. Its rate law is first order, written as rate = k[A]. A unimolecular step may be part of a larger mechanism, such as O3(g) → O2(g) + O in ozone decomposition. Some unimolecular reactions are also the whole reaction, not just one step. The gas-phase decomposition of cyclobutane, C4H8, to ethylene, C2H4, is one example, and its first-order experimental rate law agrees with the rate law predicted for an elementary single-step process.

A bimolecular reaction involves two reactant species. If the two reactants are different, the rate law is first order in A and first order in B, for a second-order overall rate. If the two reacting molecules are the same, the rate law is second order in A. Some reactions occur by a single bimolecular elementary reaction, such as the reaction of nitrogen dioxide with carbon monoxide. Bimolecular steps can also appear in multistep mechanisms, including the reaction of atomic oxygen with ozone in ozone decomposition.

A termolecular reaction involves the simultaneous collision of three atoms, molecules, or ions. These reactions are uncommon because three particles colliding at the same time is very unlikely. Still, a few termolecular elementary reactions are known. The reaction of nitric oxide with oxygen and the reaction of nitric oxide with chlorine both appear to involve termolecular steps.

In many multistep mechanisms, one elementary step is much slower than the others. A reaction cannot proceed faster than its slowest step, so that step controls the overall rate. This slowest step is called the rate-limiting step, or rate-determining step.

Elementary Steps in Reaction Mechanisms

reaction rate

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measuring reaction rates

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Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k. In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n...

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Rate laws connect the speed of a chemical reaction to the concentration of its reactants. The rate constant, k, and the reaction orders are not taken from the chemical equation. They are found experimentally by watching how the rate changes when the reactant concentrations change. One common way to do this is the method of initial rates. In this method, several trials are run with different starting concentrations. The measured rates are then compared to determine the reaction orders and,...

the integrated rate law: the dependence of concentration on time

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Reading Reaction Progress with Integrated Laws

Integrated rate laws connect reaction concentration to time. They help show how much reactant or product is present after a certain period. They can also help estimate how long a reaction must run to reach a given extent. A common use is finding how long radioactive material must be stored before its radioactivity drops to a safe level. These equations come from the differential rate law after it is integrated with respect to time. For a first-order reaction, where rate = k[A], the integrated...

half-life of a reaction

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Reaction half-life is the time needed for half of a reactant to be consumed. After each half-life, half of the remaining reactant is used up. A hydrogen peroxide decomposition example shows this pattern clearly. The concentration drops from 1.000 M to 0.500 M in the first 6.00 hours, then to 0.250 M in the next 6.00 hours, and then to 0.125 M in the third period. Each later 6.00-hour interval removes half of what is left. For first-order reactions, the half-life does not depend on the reactant...

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The Arrhenius equation links activation energy and the rate constant, k, for chemical reactions. It is written as k = Ae−Ea/RT. In this equation, R is the ideal gas constant with a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor depends on collision frequency and the orientation of reacting molecules. Experimental kinetic data can be used to find the activation...

reaction mechanisms

Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced...

rate-determining steps

Reaction Bottlenecks in Chemical Kinetics

Reaction bottlenecks help explain rate-determining steps in multistep reaction mechanisms. In a mechanism with several elementary steps, one step is often much slower than the others. That slowest step is called the rate-limiting step, or rate-determining step. A reaction cannot move faster than its slowest step, so this step controls the overall reaction rate. The idea is like traffic on a four-lane freeway with one lane closed. Cars can only move as fast as the bottleneck allows. In the same...

catalysis

Catalyst Action in Reaction Rates

Catalysts change the rate of a chemical reaction by giving it a faster path. A catalyst increases reaction rate without being consumed during the process. Its effect is often explained with reaction mechanisms and energy diagrams. For an endothermic reaction, the reactants and products start and end at the same energies in both cases. The uncatalyzed path is a one-step mechanism with one transition state. The catalyzed path follows a different mechanism with two transition states and a lower...

enzymes

Enzyme Action and Inhibition

Enzymes are biological catalysts that speed up many reactions in living organisms. They help control cellular metabolism by lowering the activation energy needed for biochemical reactions. When activation energy drops, reaction rates can increase. Enzyme problems can also lead to serious disease. A genetic abnormality that causes G6PD, or glucose-6-phosphate dehydrogenase, deficiency can disrupt the pathway that supplies NADPH to cells. In red blood cells, this can reduce glutathione and...

key terms and concepts

Explore how chemical reactions happen in steps, including elementary reactions, reaction intermediates, molecularity, rate laws, and the rate-limiting step.

Elementary Steps in Reaction Mechanisms | JoVE K12

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Chemical Reactions

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Elementary Steps in Reaction Mechanisms

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