Name: reaction order and rate constants
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 4 min 37 s
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

3.2.1 9C.3.1.1.2

Concentration Dependence

Miara szybkości reakcji nazywana jest szybkością reakcji. W przypadku reakcji jednoetapowej szybkość jest równa zmianie stężenia każdego reagenta lub produktu w czasie pomnożonej przez odwrotność odpowiedniego współczynnika stechiometrycznego. Możesz myśleć o zmianie stężenia w czasie jako o stężeniu w czasie t minus stężenie początkowe podzielone przez t. Szybkości reakcji są zawsze dodatnie, więc wyrażenia reagentów mają znaki ujemne. Wskaźniki te mogą nie być równe w reakcjach wieloetapowych, ale nadal możemy użyć tej zależności jako oszacowania całej reakcji.
Prawo szybkości lub równanie szybkości opisuje związek między szybkością reakcji a stężeniami reagentów. W tym równaniu k jest stałą szybkości, A i B to dwa reagenty, a m i n to ich odpowiednie rzędy reakcji. Kolejność reakcji opisuje zależność między stężeniem reagenta a szybkością i nie jest związana ze stechiometrią. Należy pamiętać, że kolejność reakcji nie jest taka sama jak współczynnik reagenta w równaniu zrównoważonym.
Gdy reakcja obejmuje dwa lub więcej reagentów, musimy wziąć pod uwagę kolejność reakcji dla każdego reagenta. Ogólna kolejność reakcji jest równa sumie kolejności reakcji reagentów. Na przykład, jeśli reagent A jest pierwszego rzędu, a reagent B jest rzędem zerowym, ogólna kolejność reakcji wynosi jeden. Najczęstsze kolejność reakcji w prostych reakcjach to porządek zerowy, pierwszy i drugi porządek. Przejdźmy przez nie na przykładzie reakcji jednocząsteczkowej.
Jeśli reakcja jest rzędu zerowego, stężenie reagenta nie ma wpływu na szybkość reakcji. Zatem szybkość reakcji jest równa k, stałej szybkości. Wykres szybkości reakcji w odniesieniu do stężenia jest linią poziomą.
W relacjach pierwszego rzędu stężenie reagenta jest liniowo związane z szybkością reakcji. Zatem szybkość jest równa stałej szybkości pomnożonej przez stężenie reagenta. Wykres szybkości reakcji w odniesieniu do stężenia będzie liniowy, z k jako nachyleniem.
Jeśli reakcja jest drugiego rzędu, istnieje kwadratowa zależność między stężeniem a szybkością. Zatem szybkość jest równa stałej szybkości pomnożonej przez stężenie do kwadratu. Wykres szybkości w odniesieniu do stężenia będzie paraboliczny, z k jako nachyleniem.
Stała szybkości, k, jest wartością zależną od temperatury, która wiąże energię aktywacji reakcji z szybkością reakcji. Zbadasz to w następnym eksperymencie laboratoryjnym.
Szybkość reakcji jest zwykle podawana w molach na litr na sekundę. Szybkość ma te same jednostki, niezależnie od kolejności reakcji. W związku z tym stała szybkości ma różne jednostki, w zależności od kolejności reakcji. Na przykład, jeśli A jest pierwszym porządkiem, a B jest rzędem zerowym, to mamy jeden przypadek moli na litr w równaniu stawki. W związku z tym stała szybkości musi być wyrażona w sekundach odwrotnych.
Jak więc ustalić kolejność reakcji? Kolejność reakcji musi być określona doświadczalnie przy użyciu serii testów. Jeśli masz dwa reagenty, jedną z metod jest utrzymanie stałego stężenia jednego reagenta, zmiana drugiego i czas, jaki zajmuje wytworzenie określonej ilości produktu. Ten sam proces powtarza się dla drugiego reagenta. Następnie możesz oszacować kolejność dla każdego reagenta, wykreślając szybkość w stosunku do zmiennego stężenia reagentów i sprawdzając, czy wygląda to jak wykres zerowy, pierwszego czy drugiego rzędu. Dopasowanie danych do odpowiedniego równania stawki pozwoli również obliczyć k.
W tym laboratorium określisz kolejność reakcji dwóch reagentów, zmieniając ich stężenia i czas potrzebny na postęp reakcji do przekształcenia roztworu w nieprzezroczysty.

zależność od koncentracji

Kinetyka chemiczna i prawo szybkości reakcji
Kinetyka chemiczna odnosi się do szybkości lub szybkości reakcji chemicznej. Szybkość zależy od mechanizmu, złożoności i liczby reagentów w reakcji. Stężenie reagentów również odgrywa znaczącą rolę w szybkości reakcji.
Prawo stawki określa ilościowo tę zależność poprzez eksperymenty. Każdy reagent przyczynia się do szybkości reakcji za pomocą czynnika znanego jako kolejność reakcji. Czynnik ten może wynosić od zera do dwóch i zależy od związku tego reagenta z szybkością reakcji.
r = k[A]m[B]n dla aA + bB + ... → cC
W tym równaniu r jest szybkością reakcji, k jest stałą szybkością reakcji, [A] i [B] są stężeniami reagentów A i B, a m i n są odpowiednio rzędami reakcji reagentów A i B. Szybkość reakcji, r, jest definiowana jako zmiana stężenia produktu w czasie i ma jednostki moli na litr na sekundę.
<h4>Kolejność reakcji</h4>
Kolejność reakcji opisuje zależność mocy stężenia reagentów od szybkości reakcji. Ogólna kolejność reakcji jest sumą kolejności reakcji dla każdego z reagentów. Ważne jest, aby pamiętać, że kolejność reakcji NIE jest związana z czynnikami stechiometrycznymi zrównoważonego równania chemicznego. Innymi słowy, stechiometria nie może być używana do określania kolejności reakcji dla reakcji chemicznej. Kolejność reakcji musi być wyznaczona doświadczalnie
Aby określić kolejność reakcji, stężenie drugiego reagenta jest utrzymywane na stałym poziomie, podczas gdy stężenie pierwszego reagenta jest zmieniane. Czas potrzebny do zajścia reakcji mierzy się w sekundach; Jednak czas ten nie odpowiada czasowi, w którym reakcja jest potrzebna do ukończenia. Zamiast tego jest to czas potrzebny na rozpoczęcie reakcji. Czasy reakcji dla różnych stężeń reagentów są następnie porównywane w celu określenia kolejności. Ta sama seria reakcji jest przeprowadzana w celu określenia kolejności drugiego reagenta, gdzie stężenie pierwszego reagenta jest utrzymywane na stałym poziomie, a stężenie drugiego reagenta jest zmieniane.
Porównując czasy reakcji, można określić kolejność reakcji. Na przykład, jeśli czas reakcji pozostaje stały pomimo zmian stężenia reagenta, to reagent jest rzędu zerowego. Oznacza to, że szybkość reakcji jest równa współczynnikowi szybkości reakcji, k, który musi mieć jednostki M/s. Jeśli czas reakcji zmienia się liniowo wraz ze zmianami stężenia (tj. podwojenie stężenia wpływa na czas reakcji o współczynnik 2), to reagent jest pierwszego rzędu. Oznacza to, że szybkość reakcji jest równa iloczynowi współczynnika szybkości reakcji i stężenia reagenta. W tym przypadku k ma jednostki 1/s. Wreszcie, jeśli na czas reakcji wpływa współczynnik 4, gdy stężenie reagenta jest podwajane, reagent jest drugiego rzędu. Oznacza to, że szybkość reakcji jest równa stałej szybkości pomnożonej przez kwadrat stężenia reagenta, tworząc zależność kwadratową. W takim przypadku jednostkami k musi być 1/M⋅s.
Ogólna kolejność reakcji jest sumą poszczególnych porządków reakcji. Na przykład, jeśli reakcja jest pierwszego rzędu względem A, to m = 1. A jeśli reakcja jest rzędu zerowego względem B, to n = 0. Ogólna kolejność reakcji to pierwszy rząd, ponieważ m + n = 1.
<h4>Stała szybkości reakcji</h4>
Stała szybkości, k, jest specyficzna dla reakcji i temperatury, w której przeprowadzana jest reakcja. Stała szybkości jest określana na podstawie serii eksperymentów. Korzystając ze zmierzonej szybkości i początkowych stężeń reagentów, równanie szybkości służy do obliczenia stałej szybkości. Stała szybkości jest zależna od temperatury i jest zdefiniowana równaniem Arrheniusa. Równanie to opisuje zależność między k a energiąaktywacji, E a, temperaturą, T i stałą gazu doskonałego, R. Stała A jest stałą proporcjonalności i nie należy jej mylić z reagentem A.
<img src="/CDNSource/lm/labs/50/50_Concepts_2.jpg">
<h4>Odwołania</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chemia i reaktywność chemiczna. Belmont, Kalifornia: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Chemia: molekularna natura materii i zmiana. Boston, Massachusetts: McGraw Hill.</li>
</ol>

Zależność od koncentracji

Kinetyka chemiczna i prawo szybkości reakcji
Kinetyka chemiczna odnosi się do szybkości lub szybkości reakcji chemicznej. Stawka zależy od mechanizmu, ...

Education

JoVE Lab Manual

Chemia

reaction order and rate constants

Chemical kinetics describes the speed of a chemical reaction. The rate depends on the reaction mechanism, the complexity of the reaction, and the number of reactants involved. Reactant concentration also affects how fast a reaction happens. The rate law is the experimental relationship that connects these factors to reaction rate.

In the rate law, each reactant contributes through a reaction order. The general form is r = k[A]^m[B]^n for aA + bB + ... → cC. Here, r is the reaction rate, k is the rate constant, and [A] and [B] are the reactant concentrations. The exponents m and n show the reaction order for each reactant.

The reaction rate is the change in product concentration over time. Its units are moles per liter per second. The overall reaction order is the sum of the individual orders. Reaction order is not the same as the numbers in the balanced chemical equation, so stoichiometric coefficients cannot be used to find it. Reaction orders must be measured experimentally.

To find a reaction order, one reactant concentration is changed while the other is held constant. The reaction time is measured in seconds, but this is the time for the reaction to start, not the time to completion. The times from different concentrations are compared to determine the order. The same process is repeated for the second reactant by switching which concentration is varied.

If the reaction time stays the same even when concentration changes, the reactant is zero-order. In that case, the rate equals k, and k has units of M/s. If the time changes linearly with concentration, the reactant is first order. Then the rate equals k times the reactant concentration, and k has units of 1/s. If doubling the concentration changes the time by a factor of 4, the reactant is second order. Then the rate equals k times the square of the concentration, and k has units of 1/M·s.

The overall order is the sum of the individual orders. For example, if A is first order and B is zero-order, the total order is first order because m + n = 1. The rate constant, k, is specific to the reaction and the temperature used. It is found from the experimental data by using the measured rate and initial concentrations in the rate equation.

The rate constant also depends on temperature through the Arrhenius equation. This equation relates k to activation energy, Ea, temperature, T, and the ideal gas constant, R. The constant A in the equation is a proportionality constant. It should not be confused with reactant A.

Reaction Order and Rate Constants

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Reaction Order and Rate Constants | JoVE K12

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Matter and Its Interactions: Energy

9-12.C.3.3.d

Next Generation Science Standards

High School

Physical Science

Matter and Its Interactions

HS-PS1-5

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-5.

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Molecular Orbital (MO) Theory

Source: Tamara M. Powers, Department of Chemistry, Texas A&#38;M University
This protocol serves as a guide in the synthesis of two metal complexes ...

Electron Behavior

Electrons are negatively charged subatomic particles that are attracted to an orbit around the positively-charged nucleus of an atom. They reside in ...

effect-of-temperature-change-on-reaction-rate

Effect of Temperature Change on Reaction Rate

The Arrhenius equation,

relates the activation energy and the rate constant, k, for many chemical reactions.
In this equation, R is the ideal gas ...

factors-influencing-the-rate-of-chemical-reactions

Factors Influencing the Rate of Chemical Reactions

A variety of factors influence the rate of chemical reactions. For a chemical reaction to happen, atoms must collide with enough energy to overcome the ...

Concentration and Rate Law

The rate of a reaction is affected by the concentrations of reactants. Rate laws (differential rate laws) or rate equations are mathematical expressions ...

Measuring Reaction Rates

Polarimetry finds application in chemical kinetics to measure the concentration and reaction kinetics of optically active substances during a chemical ...

Temperature Dependence

Chemical Kinetics 
The reaction rate is the speed at which a chemical reaction occurs. The reaction rate is defined as the change in concentration of a ...

Reaction Rate

The rate of reaction is the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time ...

Chemical Kinetics and the Reaction Rate Law
Chemical kinetics refers to the rate or speed of a chemical reaction. The rate depends on the mechanism, ...

Matter and Its Interactions: Develop a model based on evidence to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy. (P: 2, CC: 5, CI: PS1)

Solution Formation

There is no one solvent that can dissolve every type of solute. Some substances that readily dissolve in a certain solvent might be insoluble in a ...

Energetics of Solution Formation

The formation of a solution is an example of a spontaneous process, which is a process that occurs under specified conditions without energy from some ...

General Properties of Solutions

Many common substances around us exist as a solution, such as ocean water, air, and gasoline. All solutions are mixtures of substances that are composed ...

Solutions and Concentrations

Source: Laboratory of Dr. Michael Evans &#8212; Georgia Institute of Technology
A solution is a homogeneous mixture containing some components in small ...

Solubility

Solubility
Solubility describes how much of a solute can dissolve in a given volume of a specific solvent. Solubility is usually reported in terms of ...

Solution, Solubility, and Solubility Equilibrium
A solution is a homogeneous mixture&#160;composed of a solvent, the major component, and a solute, the ...

Solution Equilibrium and Saturation

Imagine adding a small amount of sugar to a glass of water, stirring until all the sugar has dissolved, and then adding a bit more. You can repeat this ...

Intermolecular Forces in Solutions

The formation of a solution is an example of a spontaneous process, a process that occurs under specified conditions without energy from some external ...

Matter and Its Interactions: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay. (P: 2, CC: 5, CI: PS1)

9C.3.1.1.3

Chemical and Solubility Equilibria

The free energy change associated with dissolving a solute in a liter of solvent is called the free energy of a solution, &#916;Gsolution. The overall ...

Matter and Its Interactions: Apply scientific principles and evidence to provide an explanation about the effects of changing the surface area, agitation, temperature, and concentration of the reacting particles on the rate at which the reaction occurs. (P: 6, CC: 1, CI: PS1)

9C.3.2.1.2

3 Developing possible explanations of phenomena or designing solutions to engineering problems
 3.2 Constructing explanations and designing solutions 
  3.2.1 Students will be able to apply scientific principles and empirical evidence (primary or secondary) to explain the causes of phenomena or identify weaknesses in explanations developed by the students or others.

Reaction Order and Rate Constants

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