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

Si consideri una reazione chimica bilanciata, come la combustione di idrogeno gassoso. Qui, la relazione quantitativa tra i reagenti e i prodotti - H2, O2 e H2O - è che due molecole di H2 reagiscono con una molecola di O2 per produrre due molecole di H2O. Questa relazione quantitativa è nota come stechiometria.
In una reazione chimica, i reagenti interagiscono tra loro per formare prodotti. Il reagente che viene completamente consumato è il reagente limitante, e il reagente presente in quantità maggiore del necessario per reagire completamente con il reagente limitante è il reagente in eccesso.
Una reazione chimica comporta la trasformazione chimica dei reagenti in prodotti. Man mano che la reazione procede, la concentrazione dei reagenti diminuisce mentre la concentrazione dei prodotti aumenta. Questa varianza nelle concentrazioni di reagenti e prodotti può essere tracciata in un grafico in funzione del tempo.
La velocità con cui una reazione progredisce è chiamata velocità di reazione. Misura il tasso di scomparsa dei reagenti o il tasso di comparsa dei prodotti ed è espresso in unità di molarità al secondo.
Una reazione chimica reversibile è quella in cui la conversione dei reagenti in prodotti e la conversione dei prodotti in reagenti avvengono simultaneamente. Una doppia freccia tra i reagenti e i prodotti indica la sua natura reversibile.
Quando la velocità di formazione dei prodotti, o la velocità della reazione diretta, è uguale alla velocità di formazione dei reagenti, o la velocità della reazione inversa, si raggiunge l'equilibrio chimico.
L'espressione della costante di equilibrio è scritta come le concentrazioni molari dei prodotti, C e D, sui reagenti, A e B, all'equilibrio, ciascuna elevata ai rispettivi coefficienti stechiometrici. Quando risolta, l'espressione è uguale alla costante di equilibrio, Kc.
Un'espressione nella stessa forma può anche essere scritta per i reagenti e i prodotti a qualsiasi concentrazione, e la quantità calcolata è nota come quoziente di reazione, Qc.

reazioni chimiche

Un'equazione chimica bilanciata fornisce le informazioni sulle formule chimiche dei reagenti e dei prodotti coinvolti nella reazione chimica. La stechiometria di una reazione aiuta a prevedere la quantità di reagente necessaria per produrre la quantità desiderata di prodotto o, in alcuni casi, la quantità di prodotto che si formerà da una quantità specifica di reagente.

Le quantità relative di reagenti e prodotti rappresentati in un'equazione chimica bilanciata sono spesso chiamate quantità stechiometriche. Tuttavia, in realtà, non sempre i reagenti sono presenti nelle quantità stechiometriche indicate dall'equazione bilanciata.

In una reazione chimica, il reagente che viene consumato per primo e che limita la quantità di prodotto formato, è il reagente limitante, mentre l'altra sostanza diventa il reagente in eccesso. Spesso viene utilizzato un eccesso di uno o più reagenti per garantire la completa conversione dell'altro reagente nel prodotto.

Consideriamo la reazione per la formazione dell'acqua rappresentata dall'equazione:

<img alt="Eq1" src="/files/ftp_upload/11691/11691_Equation1.svg" style="text-align: center; height: 20px;">

La stechiometria indica che due moli di idrogeno e una mole di ossigeno reagiscono per produrre due moli di acqua; cioè, idrogeno e ossigeno si combinano in un rapporto 2:1.

Immagina se fossero presenti 5 moli di idrogeno e 2 moli di ossigeno. Il rapporto dei reagenti è ora 5:2 (o 2,5:1), che è maggiore del rapporto stechiometrico di 2:1. L'idrogeno, quindi, è presente in eccesso e l'ossigeno è il reagente limitante. La reazione di tutto l'ossigeno fornito (2 mol) consumerà 4 mol delle 5 mol di idrogeno fornite, lasciando 1 mol di idrogeno non reagito. Calcolare le quantità molari di ciascun reagente fornito e confrontarle con le quantità stechiometriche rappresentate nell'equazione chimica bilanciata è un modo per identificare i reagenti limitanti ed in eccesso.

La velocità di reazione è la variazione della quantità di un reagente o di un prodotto nell'unità di tempo. Le velocità di reazione vengono quindi determinate misurando la dipendenza dal tempo di alcune proprietà che possono essere correlate alle quantità di reagenti o prodotti.

<img alt="Figure1" src="/files/ftp_upload/11691/11691_Figure1.png" style="text-align: center; height: 300px;">

La velocità di una reazione chimica può essere rappresentata in un grafico come la variazione delle concentrazioni di reagenti e prodotti in funzione del tempo.

Una reazione chimica reversibile rappresenta un processo chimico che procede sia in avanti (da sinistra a destra) che indietro (da destra a sinistra). Lo stato di una reazione reversibile viene convenientemente stimato valutando il suo quoziente di reazione (Q). Per una reazione reversibile descritta da

<img alt="Eq2" src="/files/ftp_upload/11691/11691_Equation2.svg" style="text-align: center; height: 20px;">

il quoziente di reazione si deriva direttamente dalla stechiometria dell'equazione bilanciata as

<img alt="Eq3" src="/files/ftp_upload/11691/11691_Equation3.svg" style="text-align: center; height: 44px;">

dove il pedice c denota l'uso delle concentrazioni molari nell'espressione.

<img alt="Figure2" src="/files/ftp_upload/11691/11691_Figure2.png" style="text-align: center; height: 300px;">

Quando le velocità delle reazioni dirette e inverse sono uguali, le concentrazioni delle specie reagente e prodotto rimangono costanti nel tempo e il sistema è all'equilibrio. Una speciale doppia freccia viene utilizzata per enfatizzare la natura reversibile della reazione.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/4-3-reaction-stoichiometry">OpenStax Chemistry 2e, Section 4.3: Reaction Stoichiometry</a>; <a href="https://openstax.org/books/chemistry-2e/pages/4-4-reaction-yields">Section 4.4: Reaction Yield</a>; <a href="https://openstax.org/books/chemistry-2e/pages/12-1-chemical-reaction-rates#12163">Section 12.1: Chemical Reaction Rates</a>; <a href="https://openstax.org/books/chemistry-2e/pages/13-1-chemical-equilibria">Section 13.1 Chemical Equilibria</a>, <a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">Section 13.2 Equilibrium Constants</a><a href="https://openstax.org/books/chemistry-2e/pages/13-2-equilibrium-constants">.</a>

Reazioni chimiche

Un'equazione chimica bilanciata fornisce le informazioni sulle formule chimiche dei reagenti e dei prodotti coinvolti nel cambiamento chimico. Un...

Education

Didattica

JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Thermodynamics and Chemical Kinetics

Termodinamica e Cinetica Chimica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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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IPC.8.B

Louisiana Student Standards for Science

Chemistry

Matter And Its Interactions

HS-PS1-6

Tennessee Academic Standards

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CHEM1.PS1.3

CHEM1.PS1.4

Virginia  Standards of Learning

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CH.3

CH.3.f

Florida Department of Education

Physical Science

SC.912.P.10

SC.912.P.10.6

SC.912.P.10.12

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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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