Name: reaction order from concentration
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 3 min 3 s
Channel: JoVE

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IPC.7

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Concentration and Rate Law

Szybkość reakcji często zależy od stężeń reagentów. 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. Prawo stawek wyraża zależność między wszystkimi tymi parametrami. 
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 zgodnie z prawem szybkości 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. Dlatego w reakcji zerowego rzędu, nawet gdy stężenie reagenta spada, szybkość reakcji nie zwalnia.
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. Kolejność reagentów wynosi dwa, a stężenie reagenta jest podnoszone do drugiej potęgi.
W związku z tym szybkość reakcji w reakcji drugiego rzędu zależy bezpośrednio od kwadratu stężenia reagenta. Wraz ze spadkiem stężenia reagenta szybkość reakcji maleje wykładniczo w sposób kwadratowy.

prawo koncentracji i szybkości 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, prawo szybkości można zapisać jako:

<img alt="Eq1" src="/files/ftp_upload/11375/11375_Equation_1.png" style="height: 50px;">

[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 są zazwyczaj dodatnimi liczbami całkowitymi, chociaż mogą być ułamkami, wartościami ujemnymi lub zerem.

Stałą szybkości k i rzędy reakcji m i n wyznacza się eksperymentalnie poprzez obserwację, 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 stopy procentowej (stopa = k[A]m[B]n), reakcja jest rzędu m w odniesieniu do A i rzędu n 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 stopy procentowej 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>

Prawo koncentracji i szybkości reakcji

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

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reaction order from concentration

Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k.

In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n are the reaction orders. They are usually positive integers, but they can also be fractions, negative numbers, or zero. The values of k, m, and n are found experimentally by measuring how the rate changes when reactant concentrations change.

The rate constant does not depend on the reactant concentrations, but it can change with temperature. The reaction orders show the mathematical link between concentration and rate. In the expression rate = k [A]m [B]n, the reaction is m order with respect to A and n order with respect to B. For example, 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 orders for each reactant. In the example above, the reaction is third order overall because 1 + 2 = 3. A common way to find a rate law is the method of initial rates. In this method, reaction rates are measured in several trials that start with different initial reactant concentrations. Comparing those rates makes it possible to find the reaction orders and then the rate constant.

Rate laws can also include fractional orders. In some cases, a reactant may have a negative order. That means increasing its concentration can lower the reaction rate. Rate laws are determined by experiment only, and they cannot be reliably predicted from reaction stoichiometry. The reaction order also describes how the rate changes as reactant or product concentration changes.

A zero-order reaction has a rate that does not change with reactant concentration. The rate stays constant throughout the reaction. In a first-order reaction, the rate is directly proportional to the change in reactant concentration. As the reactant concentration drops, the rate drops by the same proportion. In second-order or higher-order reactions, the rate depends on the reactants raised to an exponent. As the reaction continues and reactant concentration decreases, the rate falls exponentially.

Reaction Order from Concentration

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Reaction order from concentration shows how reactant amounts affect reaction rate. Rate laws, also called differential rate laws or rate equations, are mathematical expressions that connect reaction rate with the concentrations of reactants. For a generic reaction a A + b B → products, the rate law can be written using the reactant concentrations [A] and [B] and a rate constant k. In a rate law, k is the rate constant for one specific reaction at one specific temperature. The exponents m and n...

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Reaction Order from Concentration | JoVE K12

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Reaction Order from Concentration

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