Name: stoichiometry, rates, and equilibrium
Uploaded: 2023-04-30T18:28:32.000Z
Duration: 2 min 26 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.A

Chemical Reactions

Betrachten Sie eine ausgewogene chemische Reaktion, wie z. B. die Verbrennung von Wasserstoffgas. Hier besteht die quantitative Beziehung zwischen den Reaktanten und Produkten – H2, O2 und H2O – darin, dass zwei Moleküle von H2 mit einem Molekül von O2 reagieren, um zwei Moleküle H2O zu erzeugen. Diese quantitative Beziehung wird als Stöchiometrie bezeichnet.
In einer chemischen Reaktion interagieren Reaktanten miteinander, um Produkte zu bilden. Der Reaktant, der vollständig verbraucht wird, ist der begrenzende Reaktant, und der Reaktant, der in einer Menge vorhanden ist, die größer ist als notwendig, um vollständig mit dem begrenzenden Reaktanten zu reagieren, ist der überschüssige Reaktant.
Eine chemische Reaktion beinhaltet die chemische Umwandlung von Reaktanten in Produkte. Mit fortschreitender Reaktion nimmt die Konzentration der Reaktanten ab, während die Konzentration der Produkte zunimmt. Diese Varianz in den Konzentrationen von Reaktanten und Produkten kann in einem Diagramm als Funktion der Zeit dargestellt werden.
Die Geschwindigkeit, mit der eine Reaktion abläuft, wird als Reaktionsgeschwindigkeit bezeichnet. Er misst die Geschwindigkeit des Verschwindens von Reaktanten oder die Geschwindigkeit des Auftretens von Produkten und wird in Einheiten der Molarität pro Sekunde ausgedrückt.
Eine reversible chemische Reaktion ist eine Reaktion, bei der die Umwandlung von Reaktanten in Produkte und die Umwandlung von Produkten zurück in Reaktanten gleichzeitig erfolgt. Ein Doppelpfeil zwischen den Reaktanten und Produkten zeigt seine reversible Natur an.
Wenn die Geschwindigkeit der Bildung von Produkten oder die Geschwindigkeit der Vorwärtsreaktion gleich der Geschwindigkeit der Bildung von Reaktanten oder der Geschwindigkeit der Umkehrreaktion ist, wird das chemische Gleichgewicht erreicht.
Der Ausdruck der Gleichgewichtskonstante wird als die molaren Konzentrationen der Produkte C und D über den Reaktanten A und B im Gleichgewicht geschrieben, die jeweils auf ihre jeweiligen stöchiometrischen Koeffizienten angehoben werden. Wenn er gelöst ist, ist der Ausdruck gleich der Gleichgewichtskonstante Kc.
Ein Ausdruck in der gleichen Form kann auch für die Reaktanten und Produkte bei beliebiger Konzentration geschrieben werden, und die berechnete Größe wird als Reaktionsquotient Qc bezeichnet.

chemische reaktionen

Eine ausgeglichene chemische Gleichung liefert Informationen über die chemischen Formeln der an der chemischen Veränderung beteiligten Reaktanten und Produkte. Die Stöchiometrie einer Reaktion hilft dabei, vorherzusagen, wie viel des Reaktanten benötigt wird, um die gewünschte Produktmenge zu produzieren, oder in manchen Fällen, wie viel Produkt aus einer bestimmten Menge des Reaktanten gebildet wird.

Die relativen Mengen an Reaktanten und Produkten, die in einer ausgewogenen chemischen Gleichung dargestellt werden, werden oft als stöchiometrische Mengen bezeichnet. In der Realität liegen die Reaktanten jedoch nicht immer in den stöchiometrischen Mengen vor, die in der ausgeglichenen Gleichung angegeben sind.

Bei einer chemischen Reaktion ist der Reaktant, der zuerst verbraucht wird und die Menge des gebildeten Produkts begrenzt, der limitierende Reaktant, während die andere Substanz zum überschüssigen Reaktanten wird. Um die vollständige Umwandlung des anderen Reaktanten in das Produkt sicherzustellen, wird häufig ein Überschuss eines oder mehrerer Reaktanten verwendet.

Betrachten Sie die Reaktion zur Bildung von Wasser, dargestellt durch die Gleichung:

<img alt="Eq1" src="/files/ftp_upload/11691/11691_Equation1.svg" style="text-align: center; height: 20px;">

Die Stöchiometrie zeigt an, dass zwei Mol Wasserstoff und ein Mol Sauersubstanz unter Bildung von zwei Mol Wasser reagieren; das heißt, Wassersubstanz und Sauersubstanz verbinden sich im Verhältnis 2:1.

Stellen Sie sich vor, es wären 5 Mol Wassersubstanz und 2 Mol Sauersubstanz vorhanden. Das Verhältnis der Reaktanten beträgt nun 5:2 (oder 2,5:1), was größer ist als das stöchiometrische Verhältnis von 2:1. Wassersubstanz ist daher im Überschuss vorhanden und Sauersubstanz ist der limitierende Reaktant. Durch die Reaktion des gesamten bereitgestellten Sauersubstanzen (2 Mol) werden 4 Mol der bereitgestellten 5 Mol Wassersubstanz verbraucht, sodass 1 Mol Wassersubstanz nicht umgesetzt wird. Die Berechnung der molaren Mengen jedes bereitgestellten Reaktanten und deren Vergleich mit den in der ausgeglichenen chemischen Gleichung dargestellten stöchiometrischen Mengen ist eine Möglichkeit, die limitierenden und überschüssigen Reaktanten zu identifizieren.

Die Reaktionsgeschwindigkeit ist die Änderung der Menge eines Reaktanten oder Produkts pro Zeiteinheit. Die Reaktionsgeschwindigkeiten werden daher durch Messung der Zeitabhängigkeit einer Eigenschaft bestimmt, die mit den Mengen an Reaktanten oder Produkten in Verbindung gebracht werden kann.

<img alt="Figure1" src="/files/ftp_upload/11691/11691_Figure1.png" style="text-align: center; height: 300px;">

Die Geschwindigkeit einer chemischen Reaktion kann in einem Diagramm als Variation der Konzentrationen von Reaktanten und Produkten als Funktion der Zeit dargestellt werden.

Eine reversible chemische Reaktion stellt einen chemischen Prozess dar, der sowohl in Vorwärtsrichtung (von links nach rechts) als auch in Rückwärtsrichtung (von rechts nach links) abläuft. Der Status einer reversiblen Reaktion lässt sich bequem durch Auswertung ihres Reaktionsquotienten (Q) beurteilen. Für eine reversible Reaktion beschrieben durch

<img alt="Eq2" src="/files/ftp_upload/11691/11691_Equation2.svg" style="text-align: center; height: 20px;">

Der Reaktionsquotient wird direkt aus der Stöchiometrie der ausgeglichenen Gleichung abgeleitet

<img alt="Eq3" src="/files/ftp_upload/11691/11691_Equation3.svg" style="text-align: center; height: 44px;">

wobei der Index c die Verwendung molarer Konzentrationen im Ausdruck bezeichnet.

<img alt="Figure2" src="/files/ftp_upload/11691/11691_Figure2.png" style="text-align: center; height: 300px;">

Wenn die Geschwindigkeiten der Hin- und Rückreaktion gleich sind, bleiben die Konzentrationen der Reaktanten- und Produktspezies über die Zeit konstant und das System befindet sich im Gleichgewicht. Ein spezieller Doppelpfeil wird verwendet, um die Reversibilität der Reaktion hervorzuheben.

Dieser Text ist adaptiert aus <a href="https://openstax.org/books/chemistry-2e/pages/4-3-reaction-stoichiometry">OpenStax Chemistry 2e, Abschnitt 4.3: Reaction Stoichiometry</a>; <a href="https://openstax.org/books/chemistry-2e/pages/4-4-reaction-yields">Abschnitt 4.4: Reaction Yield</a>; <a href="https://openstax.org/books/chemistry-2e/pages/12-1-chemical-reaction-rates#12163">Abschnitt 12.1: Chemical Reaction Rates</a>; <a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">Abschnitt 13.1 Chemical Equilibria</a>, <a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Abschnitt 13.2 Equilibrium Constants</a><a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">.</a>

Chemische Reaktionen

Eine ausgeglichene chemische Gleichung liefert die Informationen über die chemischen Formeln der Reaktanten und Produkte, die an der chemischen Veränderung beteiligt sind. Ein...

Education

Lehre

JoVE Core

Organic Chemistry

Organische Chemie

Unit 1

Einheit 1

Thermodynamics and Chemical Kinetics

Thermodynamik und chemische Kinetik

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

stoichiometry, rates, and equilibrium

Stoichiometry, reaction rates, and equilibrium explain how chemical reactions are measured and predicted. A balanced chemical equation shows the chemical formulas of the reactants and products in a chemical change. It also gives the relative amounts of each substance involved.

Reaction stoichiometry helps predict how much reactant is needed to make a desired amount of product. It can also show how much product forms from a given amount of reactant. The amounts shown by a balanced equation are called stoichiometric amounts.

In real reactions, reactants are not always present in the exact amounts shown by the equation. The limiting reactant is the substance that gets used up first. It limits how much product can form. The other substance is the excess reactant. Extra reactant is often added so the other reactant can be fully converted into product.

Water formation is a clear example. The balanced equation shows that 2 moles of hydrogen react with 1 mole of oxygen to make 2 moles of water. This is a 2:1 ratio. If 5 moles of hydrogen and 2 moles of oxygen are available, the ratio is 5:2, or 2.5:1. Hydrogen is then in excess, and oxygen is the limiting reactant. All 2 moles of oxygen use 4 moles of hydrogen, leaving 1 mole of hydrogen unreacted.

Reaction rate is the change in the amount of a reactant or product per unit time. Scientists find reaction rates by measuring how a property changes over time if that property can be linked to reactant or product amounts. A rate graph can show how reactant and product concentrations vary with time.

Some reactions are reversible, which means they can proceed in both the forward and reverse directions. The reaction quotient, Q, helps assess the status of a reversible reaction. It is based on the stoichiometry of the balanced equation, and the subscript c shows that molar concentrations are used in the expression. When the forward and reverse rates are equal, concentrations stay constant and the system is at equilibrium. A double arrow is used to show that the reaction is reversible.

Stoichiometry, Rates, and Equilibrium

chemical reactions

Stoichiometry, reaction rates, and equilibrium explain how chemical reactions are measured and predicted. A balanced chemical equation shows the chemical formulas of the reactants and products in a chemical change. It also gives the relative amounts of each substance involved. Reaction stoichiometry helps predict how much reactant is needed to make a desired amount of product. It can also show how much product forms from a given amount of reactant. The amounts shown by a balanced equation are...

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Matter And Its Interactions

HS-PS1-6

Tennessee Academic Standards

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IPC.8.D

Science concepts. The student knows that changes in matter affect everyday life. The student is expected to:

Stoichiometry, Rates, and Equilibrium

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