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

Rozważ zrównoważoną reakcję chemiczną, taką jak spalanie wodoru gazowego. Tutaj ilościowa zależność między reagentami i produktami - H2, O2 i H2O - polega na tym, że dwie cząsteczki H2 reagują z jedną cząsteczką O2 w celu wytworzenia dwóch cząsteczek H2O. Ta zależność ilościowa jest znana jako stechiometria.
W reakcji chemicznej reagenty oddziałują ze sobą, tworząc produkty. Reagent, który jest całkowicie zużyty, jest reagentem ograniczającym, a reagent obecny w ilości większej niż jest to konieczne do całkowitej reakcji z reagentem ograniczającym, to reagent nadmiarowy.
Reakcja chemiczna polega na chemicznej przemianie reagentów w produkty. W miarę postępu reakcji stężenie reagentów maleje, podczas gdy stężenie produktów wzrasta. Tę wariancję stężeń reagentów i produktów można wykreślić na wykresie w funkcji czasu.
Szybkość, z jaką postępuje reakcja, nazywana jest szybkością reakcji. Mierzy szybkość znikania reagentów lub szybkość pojawiania się produktów i jest wyrażona w jednostkach molowości na sekundę.
Odwracalna reakcja chemiczna to taka, w której konwersja reagentów w produkty i konwersja produktów z powrotem w reagenty zachodzą jednocześnie. Podwójna strzałka między reagentami a produktami oznacza jego odwracalny charakter.
Gdy szybkość tworzenia produktów lub szybkość reakcji postępowej jest równa szybkości powstawania reagentów lub szybkości reakcji odwrotnej, osiągana jest równowaga chemiczna.
Wyrażenie stałej równowagi jest zapisywane jako stężenia molowe produktów C i D nad reagentami A i B w stanie równowagi, z których każdy jest podniesiony do odpowiednich współczynników stechiometrycznych. Po rozwiązaniu wyrażenie jest równe stałej równowagi, Kc.
Wyrażenie w tej samej postaci można również zapisać dla reagentów i produktów o dowolnym stężeniu, a obliczona ilość jest znana jako iloraz reakcji, Qc.

reakcje chemiczne

Zrównoważone równanie chemiczne dostarcza informacji o wzorach chemicznych reagentów i produktów biorących udział w przemianie chemicznej. Stechiometria reakcji pomaga przewidzieć, jaka ilość reagenta jest potrzebna do wytworzenia pożądanej ilości produktu lub, w niektórych przypadkach, ile produktu powstanie z określonej ilości reagenta.

Względne ilości reagentów i produktów przedstawione w zrównoważonym równaniu chemicznym są często określane jako ilości stechiometryczne. Jednakże w rzeczywistości reagenty nie zawsze występują w ilościach stechiometrycznych wskazanych w zrównoważonym równaniu.

W reakcji chemicznej reagent, który ulega zużyciu jako pierwszy i ogranicza ilość powstającego produktu, jest reagentem ograniczającym, podczas gdy druga substancja staje się reagentem nadmiarowym. Często stosuje się nadmiar jednego lub większej liczby reagentów, aby zapewnić całkowitą konwersję drugiego reagenta w produkt.

Rozważmy reakcję tworzenia się wody przedstawioną równaniem:

<img alt="Eq1" src="/files/ftp_upload/11691/11691_Equation1.svg" style="text-align: center; height: 20px;">

Stechiometria wskazuje, że dwa mole wodoru i jeden mol tlenu reagują, tworząc dwa mole wody; to znaczy wodór i tlen łączą się w stosunku 2:1.

Wyobraź sobie, że obecnych jest 5 moli wodoru i 2 mole tlenu. Stosunek reagentów wynosi teraz 5:2 (lub 2,5:1), czyli jest większy niż stosunek stechiometryczny wynoszący 2:1. Wodór jest zatem obecny w nadmiarze, a tlen jest reagentem ograniczającym. Reakcja całego dostarczonego tlenu (2 mole) spowoduje zużycie 4 moli z 5 moli dostarczonego wodoru, pozostawiając 1 mol wodoru nieprzereagowanego. Obliczenie ilości molowych każdego dostarczonego reagenta i porównanie ich z ilościami stechiometrycznymi przedstawionymi w zrównoważonym równaniu chemicznym jest jednym ze sposobów identyfikacji reagentów ograniczających i nadmiarowych.

<img alt="Figure1" src="/files/ftp_upload/11691/11691_Figure1.png" style="text-align: center; height: 300px;">

 Szybkość reakcji to zmiana ilości reagenta lub produktu w jednostce czasu. Szybkość reakcji określa się zatem poprzez pomiar zależności od czasu pewnej właściwości, która może być powiązana z ilością reagenta lub produktu.

 Szybkość reakcji chemicznej można przedstawić na wykresie jako zmienność stężeń reagentów i produktów w funkcji czasu.

 Odwracalna reakcja chemiczna reprezentuje proces chemiczny, który przebiega zarówno w kierunku do przodu (od lewej do prawej), jak i do tyłu (od prawej do lewej). Stan odwracalnej reakcji dogodnie ocenia się poprzez ocenę jej współczynnika reakcji (Q).

<img alt="Eq2" src="/files/ftp_upload/11691/11691_Equation2.svg" style="text-align: center; height: 20px;">

 Dla odwracalnej reakcji opisanej przez iloraz reakcji wyprowadza się bezpośrednio ze stechiometrii zrównoważonego równania jako 

<img alt="Eq3" src="/files/ftp_upload/11691/11691_Equation3.svg" style="text-align: center; height: 44px;">

gdzie indeks dolny c oznacza użycie stężeń molowych w danym wyrażeniu.

<img alt="Figure2" src="/files/ftp_upload/11691/11691_Figure2.png" style="text-align: center; height: 300px;">

 Gdy szybkości reakcji do przodu i do tyłu są równe, stężenia reagentów i produktów pozostają stałe w czasie, a układ znajduje się w równowadze. Specjalna podwójna strzałka służy do podkreślenia odwracalnego charakteru reakcji.

 Ten tekst jest adaptacją;<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>

Reakcje chemiczne

Zrównoważone równanie chemiczne dostarcza informacji o wzorach chemicznych reagentów i produktów biorących udział w przemianie chemicznej. A ...

Education

JoVE Core

Organic Chemistry

Chemia organiczna

Unit 1

Jednostka 1

Thermodynamics and Chemical Kinetics

Termodynamika i kinetyka chemiczna

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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.

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IPC.8.D

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Stoichiometry, Rates, and Equilibrium

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