Name: reaction order and rate constants
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 4 min 37 s
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

3.2.1 9C.3.1.1.2

Concentration Dependence

Das Maß dafür, wie schnell eine Reaktion abläuft, wird als Reaktionsgeschwindigkeit bezeichnet. Bei einer einstufigen Reaktion ist die Geschwindigkeit gleich der Konzentrationsänderung jedes Reaktanten oder Produkts im Laufe der Zeit, multipliziert mit dem Kehrwert des entsprechenden stöchiometrischen Koeffizienten. Sie können sich die Konzentrationsänderung im Laufe der Zeit wie folgt vorstellen: die Konzentration zum Zeitpunkt t abzüglich der Ausgangskonzentration dividiert durch t. Die Reaktionsgeschwindigkeiten sind immer positiv, so dass die Reaktantenausdrücke negative Vorzeichen haben. Diese Raten sind bei mehrstufigen Reaktionen möglicherweise nicht gleich, aber wir können diese Beziehung dennoch als Schätzung für die Gesamtreaktion verwenden.
Das Geschwindigkeitsgesetz oder die Geschwindigkeitsgleichung beschreibt die Beziehung zwischen der Geschwindigkeit der Reaktion und den Reaktantenkonzentrationen. In dieser Gleichung ist k die Geschwindigkeitskonstante, A und B sind die beiden Reaktanten und m und n sind ihre jeweiligen Reaktionsordnungen. Die Reaktionsordnung beschreibt die Beziehung zwischen der Konzentration eines Reaktanten und der Geschwindigkeit und steht in keinem Zusammenhang mit der Stöchiometrie. Es ist wichtig, sich daran zu erinnern, dass die Reaktionsordnung nicht mit dem Koeffizienten des Reaktanten in der ausgeglichenen Gleichung identisch ist.
Wenn an einer Reaktion zwei oder mehr Reaktanten beteiligt sind, müssen wir die Reaktionsreihenfolge für jeden Reaktanten berücksichtigen. Die Gesamtreaktionsordnung ist gleich der Summe der Reaktionsordnungen der Reaktanten. Wenn beispielsweise Reaktant A erster Ordnung und Reaktant B nullter Ordnung ist, ist die Gesamtreaktionsordnung eins. Die gebräuchlichsten Reaktionsordnungen in einfachen Reaktionen sind null Ordnung, erste Ordnung und zweite Ordnung. Gehen wir sie am Beispiel einer unimolekularen Reaktion durch.
Wenn die Reaktion nullter Ordnung ist, hat die Reaktantenkonzentration keinen Einfluss auf die Reaktionsgeschwindigkeit. Die Reaktionsgeschwindigkeit ist also gleich k, der Geschwindigkeitskonstante. Ein Diagramm der Reaktionsgeschwindigkeit in Bezug auf die Konzentration ist eine horizontale Linie.
In Beziehungen erster Ordnung ist die Reaktantenkonzentration linear mit der Reaktionsgeschwindigkeit verbunden. Somit ist die Rate gleich der Geschwindigkeitskonstante multipliziert mit der Reaktantenkonzentration. Ein Diagramm der Reaktionsgeschwindigkeit in Bezug auf die Konzentration ist linear, mit k als Steigung.
Wenn die Reaktion zweiter Ordnung ist, besteht eine quadratische Beziehung zwischen der Konzentration und der Geschwindigkeit. Somit ist die Rate gleich der Geschwindigkeitskonstante multipliziert mit der Konzentration zum Quadrat. Ein Diagramm der Rate in Bezug auf die Konzentration ist parabolisch, mit k als Steigung.
Die Geschwindigkeitskonstante k ist ein temperaturabhängiger Wert, der die Aktivierungsenergie der Reaktion mit der Reaktionsgeschwindigkeit in Beziehung setzt. Dies werden Sie im nächsten Laborexperiment untersuchen.
Die Reaktionsgeschwindigkeiten werden typischerweise in Mol pro Liter und Sekunde angegeben. Die Rate hat die gleichen Einheiten, unabhängig von der Reaktionsreihenfolge. Die Geschwindigkeitskonstante hat also je nach Reaktionsreihenfolge unterschiedliche Einheiten. Wenn zum Beispiel A die erste Ordnung und B die Nullordnung ist, dann haben wir eine Instanz von Mol pro Liter in der Geschwindigkeitsgleichung. Daher muss die Geschwindigkeitskonstante in inversen Sekunden angegeben werden.
Wie bestimmen wir also die Reaktionsreihenfolge? Die Reaktionsreihenfolge muss experimentell mit einer Reihe von Versuchen bestimmt werden. Wenn Sie zwei Reaktanten haben, besteht eine Methode darin, die Konzentration eines Reaktanten konstant zu halten, den anderen zu variieren und zu bestimmen, wie lange es dauert, eine bestimmte Menge des Produkts herzustellen. Der gleiche Vorgang wird für den zweiten Reaktanten wiederholt. Sie können dann die Reihenfolge für jeden Reaktanten schätzen, indem Sie die Rate im Vergleich zu den unterschiedlichen Konzentrationen der Reaktanten darstellen und prüfen, ob sie wie ein Diagramm null, erster oder zweiter Ordnung aussieht. Wenn Sie Ihre Daten mit der entsprechenden Tarifgleichung abgleichen, können Sie auch k berechnen.
In diesem Labor bestimmen Sie die Reaktionsordnungen von zwei Reaktanten, indem Sie ihre Konzentrationen variieren und festlegen, wie lange es dauert, bis die Reaktion fortschreitet, um die Lösung undurchsichtig zu machen.

konzentrationsabhängigkeit

Chemische Kinetik und das Gesetz der Reaktionsgeschwindigkeit
Chemische Kinetik bezieht sich auf die Geschwindigkeit oder Geschwindigkeit einer chemischen Reaktion. Die Geschwindigkeit hängt vom Mechanismus, der Komplexität und der Anzahl der Reaktanten in der Reaktion ab. Auch die Reaktantenkonzentration spielt eine wichtige Rolle für die Geschwindigkeit einer Reaktion.
Das Tarifgesetz quantifiziert diesen Zusammenhang durch Experimente. Jeder Reaktant trägt durch einen Faktor, der als Reaktionsordnung bekannt ist, zur Geschwindigkeit der Reaktion bei. Dieser Faktor kann von null bis zwei reichen und hängt von der Beziehung zwischen diesem Reaktanten und der Geschwindigkeit der Reaktion ab.
r = k[A]m[B]n für aA + bB + ... → cC
In dieser Gleichung ist r die Reaktionsgeschwindigkeit, k die Reaktionsgeschwindigkeitskonstante, [A] und [B] sind die Konzentrationen der Reaktanten A und B, und m und n sind die Reaktionsordnungen der Reaktanten A bzw. B. Die Reaktionsgeschwindigkeit r ist definiert als die Änderung der Konzentration des Produkts mit der Zeit und hat Einheiten von Mol pro Liter pro Sekunde.
<h4>Reihenfolge einer Reaktion</h4>
Die Ordnung der Reaktion beschreibt die Leistungsabhängigkeit, die die Konzentration der Reaktanten von der Reaktionsgeschwindigkeit hat. Die Gesamtreaktionsordnung ist die Summe der Reaktionsordnungen für jeden der Reaktanten. Es ist wichtig, sich daran zu erinnern, dass die Reaktionsordnung NICHT mit den stöchiometrischen Faktoren der ausgeglichenen chemischen Gleichung zusammenhängt. Mit anderen Worten, die Stöchiometrie kann nicht verwendet werden, um die Reaktionsordnungen für eine chemische Reaktion zu bestimmen. Reaktionsordnungen müssen experimentell bestimmt werden
Um die Reaktionsordnung zu bestimmen, wird die Konzentration des zweiten Reaktanten konstant gehalten, während die Konzentration des ersten Reaktanten variiert wird. Die Zeit, die es dauert, bis die Reaktion eintritt, wird in Sekunden gemessen; Diese Zeit entspricht jedoch nicht der Zeit, die die Reaktion braucht, um zur Vollendung zu gelangen. Stattdessen ist dies die Zeit, die es dauert, bis die Reaktion beginnt. Anschließend werden die Reaktionszeiten für die verschiedenen Reaktantenkonzentrationen verglichen, um die Reihenfolge zu bestimmen. Die gleiche Reihe von Reaktionen wird durchgeführt, um die Ordnung des zweiten Reaktanten zu bestimmen, wobei die Konzentration des ersten Reaktanten konstant gehalten und die Konzentration des zweiten Reaktanten variiert wird.
Durch den Vergleich der Reaktionszeiten kann die Reaktionsreihenfolge bestimmt werden. Wenn zum Beispiel die Reaktionszeit trotz Änderungen der Reaktantenkonzentration konstant bleibt, dann ist der Reaktant nullter Ordnung. Dies bedeutet, dass die Reaktionsgeschwindigkeit gleich dem Reaktionsgeschwindigkeitskoeffizienten k ist, der Einheiten von M/s haben muss. Wenn sich die Reaktionszeit linear mit Konzentrationsänderungen ändert (d. h. eine Verdopplung der Konzentration beeinflusst die Reaktionszeit um den Faktor 2), dann ist der Reaktant erster Ordnung. Das bedeutet, dass die Reaktionsgeschwindigkeit gleich dem Produkt aus dem Reaktionsgeschwindigkeitskoeffizienten und der Konzentration des Reaktanten ist. In diesem Fall hat k Einheiten von 1/s. Wenn schließlich die Reaktionszeit um den Faktor 4 beeinflusst wird, wenn die Konzentration eines Reaktanten verdoppelt wird, ist der Reaktant zweiter Ordnung. Das bedeutet, dass die Geschwindigkeit der Reaktion gleich der Geschwindigkeitskonstante mal dem Quadrat der Reaktantenkonzentration ist, wodurch eine quadratische Beziehung gebildet wird. In diesem Fall müssen die Einheiten von k 1/M⋅s sein.
Die Gesamtordnung der Reaktion ist die Summe der einzelnen Reaktionsbefehle. Wenn die Reaktion z.B. in Bezug auf A erster Ordnung ist, dann ist m = 1. Und wenn die Reaktion in Bezug auf B nullter Ordnung ist, dann ist n = 0. Die Gesamtordnung der Reaktion ist erste Ordnung, weil m + n = 1 ist.
<h4>Konstante der Reaktionsgeschwindigkeit</h4>
Die Geschwindigkeitskonstante k ist spezifisch für die Reaktion und die Temperatur, bei der die Reaktion durchgeführt wird. Die Geschwindigkeitskonstante wird aus der Versuchsreihe bestimmt. Unter Verwendung der gemessenen Rate und der anfänglichen Reaktantenkonzentrationen wird die Geschwindigkeitsgleichung verwendet, um die Geschwindigkeitskonstante zu lösen. Die Gangkonstante ist temperaturabhängig und wird durch die Arrhenius-Gleichung definiert. Diese Gleichung beschreibt die Beziehung zwischen k und der Aktivierungsenergie Ea, der Temperatur T und der idealen Gaskonstante R. Die Konstante A ist eine Konstante der Proportionalität und nicht zu verwechseln mit dem Reaktanten A.
<img src="/CDNSource/lm/labs/50/50_Concepts_2.jpg">
<h4>Referenzen</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr., P.M., Townsend, J.R. (2012). Chemie und chemische Reaktivität. Belmont, Kalifornien: Brooks/Cole, Cengage Lernen.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Chemie: Die molekulare Natur von Materie und Veränderung. Boston, MA: McGraw Hill.</li>
</ol>

Konzentrationsabhängigkeit

Chemische Kinetik und das Gesetz der Reaktionsgeschwindigkeit
Chemische Kinetik bezieht sich auf die Geschwindigkeit oder Geschwindigkeit einer chemischen Reaktion. Der Kurs hängt vom Mechanismus ab, ...

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reaction order and rate constants

Chemical kinetics describes the speed of a chemical reaction. The rate depends on the reaction mechanism, the complexity of the reaction, and the number of reactants involved. Reactant concentration also affects how fast a reaction happens. The rate law is the experimental relationship that connects these factors to reaction rate.

In the rate law, each reactant contributes through a reaction order. The general form is r = k[A]^m[B]^n for aA + bB + ... → cC. Here, r is the reaction rate, k is the rate constant, and [A] and [B] are the reactant concentrations. The exponents m and n show the reaction order for each reactant.

The reaction rate is the change in product concentration over time. Its units are moles per liter per second. The overall reaction order is the sum of the individual orders. Reaction order is not the same as the numbers in the balanced chemical equation, so stoichiometric coefficients cannot be used to find it. Reaction orders must be measured experimentally.

To find a reaction order, one reactant concentration is changed while the other is held constant. The reaction time is measured in seconds, but this is the time for the reaction to start, not the time to completion. The times from different concentrations are compared to determine the order. The same process is repeated for the second reactant by switching which concentration is varied.

If the reaction time stays the same even when concentration changes, the reactant is zero-order. In that case, the rate equals k, and k has units of M/s. If the time changes linearly with concentration, the reactant is first order. Then the rate equals k times the reactant concentration, and k has units of 1/s. If doubling the concentration changes the time by a factor of 4, the reactant is second order. Then the rate equals k times the square of the concentration, and k has units of 1/M·s.

The overall order is the sum of the individual orders. For example, if A is first order and B is zero-order, the total order is first order because m + n = 1. The rate constant, k, is specific to the reaction and the temperature used. It is found from the experimental data by using the measured rate and initial concentrations in the rate equation.

The rate constant also depends on temperature through the Arrhenius equation. This equation relates k to activation energy, Ea, temperature, T, and the ideal gas constant, R. The constant A in the equation is a proportionality constant. It should not be confused with reactant A.

Reaction Order and Rate Constants

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Molecular Orbital (MO) Theory

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
This protocol serves as a guide in the synthesis of two metal complexes ...

Electron Behavior

Electrons are negatively charged subatomic particles that are attracted to an orbit around the positively-charged nucleus of an atom. They reside in ...

effect-of-temperature-change-on-reaction-rate

Effect of Temperature Change on Reaction Rate

The Arrhenius equation,

relates the activation energy and the rate constant, k, for many chemical reactions.
In this equation, R is the ideal gas ...

factors-influencing-the-rate-of-chemical-reactions

Factors Influencing the Rate of Chemical Reactions

A variety of factors influence the rate of chemical reactions. For a chemical reaction to happen, atoms must collide with enough energy to overcome the ...

Concentration and Rate Law

The rate of a reaction is affected by the concentrations of reactants. Rate laws (differential rate laws) or rate equations are mathematical expressions ...

Measuring Reaction Rates

Polarimetry finds application in chemical kinetics to measure the concentration and reaction kinetics of optically active substances during a chemical ...

Temperature Dependence

Chemical Kinetics 
The reaction rate is the speed at which a chemical reaction occurs. The reaction rate is defined as the change in concentration of a ...

Reaction Rate

The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time ...

Chemical Kinetics and the Reaction Rate Law
Chemical kinetics refers to the rate or speed of a chemical reaction. The rate depends on the mechanism, ...

Matter and Its Interactions: Develop a model based on evidence to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy. (P: 2, CC: 5, CI: PS1)

Solution Formation

There is no one solvent that can dissolve every type of solute. Some substances that readily dissolve in a certain solvent might be insoluble in a ...

Energetics of Solution Formation

The formation of a solution is an example of a spontaneous process, which is a process that occurs under specified conditions without energy from some ...

General Properties of Solutions

Many common substances around us exist as a solution, such as ocean water, air, and gasoline. All solutions are mixtures of substances that are composed ...

Solutions and Concentrations

Source: Laboratory of Dr. Michael Evans &#8212; Georgia Institute of Technology
A solution is a homogeneous mixture containing some components in small ...

Solubility

Solubility
Solubility describes how much of a solute can dissolve in a given volume of a specific solvent. Solubility is usually reported in terms of ...

Solution, Solubility, and Solubility Equilibrium
A solution is a homogeneous mixture&#160;composed of a solvent, the major component, and a solute, the ...

Solution Equilibrium and Saturation

Imagine adding a small amount of sugar to a glass of water, stirring until all the sugar has dissolved, and then adding a bit more. You can repeat this ...

Intermolecular Forces in Solutions

The formation of a solution is an example of a spontaneous process, a process that occurs under specified conditions without energy from some external ...

Matter and Its Interactions: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay. (P: 2, CC: 5, CI: PS1)

9C.3.1.1.3

Chemical and Solubility Equilibria

The free energy change associated with dissolving a solute in a liter of solvent is called the free energy of a solution, &#916;Gsolution. The overall ...

Matter and Its Interactions: Apply scientific principles and evidence to provide an explanation about the effects of changing the surface area, agitation, temperature, and concentration of the reacting particles on the rate at which the reaction occurs. (P: 6, CC: 1, CI: PS1)

9C.3.2.1.2

3 Developing possible explanations of phenomena or designing solutions to engineering problems
 3.2 Constructing explanations and designing solutions 
  3.2.1 Students will be able to apply scientific principles and empirical evidence (primary or secondary) to explain the causes of phenomena or identify weaknesses in explanations developed by the students or others.

Reaction Order and Rate Constants

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