Name: how reactant concentration sets reaction rate
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 33 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.12

SC.912.P.12.12

Rate Law and Reaction Order

Dla każdej reakcji zależność między szybkością reakcji a stężeniami reagentów można wyrazić matematycznie za pomocą prawa szybkości lub równania szybkości. 
W prawie szybkości "k" jest stałą proporcjonalności lub stałą szybkości, a "n" jest kolejnością reakcji w odniesieniu do pojedynczego reagenta, którego wartość jest często liczbą całkowitą. W prawach szybkości dla reakcji wieloreagentowych ogólna kolejność reakcji jest sumą wszystkich rzędów reagentów.
Dla każdego reagenta szybkość reakcji, stała szybkości, stężenie i kolejność reakcji są określane eksperymentalnie.
Poszczególne rzędy reagentów zwykle przyjmują wartości 0, 1 lub 2, a na podstawie ogólnej kolejności reakcji reakcje chemiczne można sklasyfikować jako reakcje zerowego rzędu, pierwszego lub drugiego rzędu.
Reakcja chemiczna z pojedynczym reagentem lub jednocząsteczkowa, dla której szybkość reakcji pozostaje stała przez cały czas jej trwania, jest reakcją zerowego rzędu. Kolejność reagentów w reakcji zerowego rzędu wynosi zero, a stężenie reagenta jest podnoszone do potęgi zerowej.
Ponieważ wartość dowolnej liczby podniesiona do potęgi zerowej wynosi jeden, szybkość reakcji reakcji zerowego rzędu jest równa stałej szybkości, a zatem niezależna od stężenia reagenta.
Jednocząsteczkowa reakcja chemiczna, w której szybkość reakcji jest wprost proporcjonalna do stężenia reagenta, jest reakcją pierwszego rzędu. Kolejność reagentów dla reakcji pierwszego rzędu wynosi jeden, a zgodnie z prawem szybkości stężenie reagenta jest podnoszone do pierwszej potęgi.
Ponieważ wartość dowolnej liczby podniesionej do potęgi jednego pozostaje taka sama, szybkość reakcji pierwszego rzędu zależy bezpośrednio od stężenia reagenta. Wraz ze spadkiem stężenia reagenta szybkość reakcji zmniejsza się proporcjonalnie w sposób liniowy.
Jednocząsteczkowa reakcja chemiczna, w której szybkość reakcji zależy od kwadratu stężenia reagenta, jest reakcją drugiego rzędu. Wraz ze spadkiem stężenia reagenta szybkość reakcji maleje wykładniczo w sposób kwadratowy.

równanie szybkości i kolejność reakcji

Na szybkość reakcji wpływa stężenie reagentów. Prawa szybkości (prawa szybkości różnicowej) lub równania szybkości to wyrażenia matematyczne opisujące zależność między szybkością reakcji chemicznej a stężeniem jej reagentów.

Na przykład w ogólnej reakcji aA + bB ⟶ produkty, gdzie a i b są współczynnikami stechiometrycznymi, równanie szybkości można zapisać jako:

szybkość = k[A]^m[B]^n

 [A] i [B] reprezentują stężenia molowe reagentów, a k jest stałą szybkości, która jest specyficzna dla konkretnej reakcji w określonej temperaturze.

Wykładniki m i n są rzędami reakcji i zazwyczaj są dodatnimi liczbami całkowitymi, chociaż mogą to być ułamki zwykłe, wartości ujemne lub zero.

Stałą szybkości k i rzędy reakcji m i n wyznacza się eksperymentalnie, obserwując, jak zmienia się szybkość reakcji wraz ze zmianą stężeń reagentów. Stała szybkości k jest niezależna od stężenia reagentów, ale zmienia się wraz z temperaturą.

Rzędy reakcji w prawie szybkości opisują matematyczną zależność szybkości od stężeń reagentów. Odnosząc się do ogólnego prawa szybkości (szybkość = k[A]^m[B]^n), reakcja jest m-tego rzędu w odniesieniu do A i n-tego rzędu w odniesieniu do B. Na przykład, jeśli m = 1 i n = 2, reakcja jest pierwszego rzędu w A i drugiego rzędu w B. Ogólny porządek reakcji to po prostu suma rzędów dla każdego reagenta. W przypadku przykładowego prawa szybkości reakcja jest ogólnie trzeciego rzędu (1 + 2 = 3).

Powszechnym eksperymentalnym podejściem do wyznaczania praw szybkości jest metoda stóp początkowych. Metoda ta polega na pomiarze szybkości reakcji dla wielu prób eksperymentalnych prowadzonych przy różnych początkowych stężeniach reagentów. Porównanie zmierzonych szybkości w tych próbach pozwala na określenie rzędów reakcji, a następnie stałej szybkości, które łącznie służą do sformułowania prawa szybkości.

W przypadku niektórych reagentów prawa szybkości mogą wykazywać rzędy ułamkowe, a czasami obserwuje się ujemne rzędy reakcji, gdy wzrost stężenia jednego reagenta powoduje zmniejszenie szybkości reakcji. Należy zauważyć, że prawa szybkości są określane wyłącznie eksperymentalnie i nie można ich wiarygodnie przewidzieć na podstawie stechiometrii reakcji.

Kolejność reakcji określa zależność między szybkością reakcji a stężeniem reagentów lub produktów.

<ul style="list-style-type: disc; display: flex; flex-direction: column; gap: 10px; padding: 0 0 0 40px;">
<li>W reakcji zerowego rzędu stężenie reagentów nie ma żadnego wpływu na szybkość reakcji, która pozostaje stała przez cały czas.</li>
<li>W reakcji pierwszego rzędu szybkość reakcji jest bezpośrednio i liniowo proporcjonalna do zmiany stężenia reagenta. Wraz ze spadkiem stężenia reagenta, szybkość reakcji również maleje proporcjonalnie.</li>
<li>W reakcjach drugiego lub wyższego rzędu szybkość reakcji jest proporcjonalna do wykładniczej wartości reagentów. Dlatego w miarę postępu reakcji i zmniejszania się stężenia reagentów, szybkość reakcji maleje wykładniczo.</li>
</ul>

Ten tekst jest adaptacją<a href="https://openstax.org/books/chemistry-2e/pages/12-3-rate-laws#57033">Openstax, Chemistry 2e, Section 12.3: Rate Laws</a>.

Równanie szybkości i kolejność reakcji

Na szybkość reakcji mają wpływ stężenia reagentów. Prawa szybkości (prawa różniczkowe) lub równania szybkości są wyrażeniami matematycznymi ...

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

how reactant concentration sets reaction rate

Reaction rate depends on reactant concentration, and rate laws show that relationship with a mathematical equation. A rate law, also called a differential rate law or rate equation, links the rate of a chemical reaction to the concentrations of its reactants.

For a generic reaction, a A + b B  products, the rate law can be written as rate = k [A]^m [B]^n. Here, [A] and [B] are the molar concentrations of the reactants. The constant k is the rate constant, and it is specific to one reaction at a specific temperature.

The exponents m and n are the reaction orders. They are usually positive integers, but they can also be fractions, negative values, or zero. If m = 1 and n = 2, the reaction is first order in A and second order in B. The overall reaction order is the sum of the separate orders, so this example is third order overall.

Rate law values are found by experiment, not by stoichiometry. Scientists change the initial reactant concentrations in several trials and measure the reaction rate. This method, called the method of initial rates, helps determine the reaction orders and then the rate constant.

Some reactions show zero-order behavior. In a zero-order reaction, changes in reactant concentration do not change the rate, so the rate stays constant. In a first-order reaction, the rate changes in direct proportion to the reactant concentration. In second-order and higher-order reactions, the rate depends on the concentration raised to an exponent, so the rate drops more sharply as reactants are used up.

Rate laws can also show fractional orders, and some can even show negative orders. A negative order means that increasing one reactant can decrease the reaction rate. These patterns are useful for describing how reaction rate changes in real experiments. They are not reliably predicted from the reaction equation alone, so the rate law must be determined experimentally.

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How Reactant Concentration Sets Reaction Rate

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