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

IPC.7.F

Concentration and Rate Law

Скорость реакции часто зависит от концентраций исходных веществ. Для любой реакции взаимосвязь между скоростью реакции и концентрациями исходных веществ может быть выражена математически с использованием закона скорости или уравнения скорости. В законе скорости k константа пропорциональности или константа скорости, а n порядок реакции по отношению к одному исходному веществу, значение которого часто является целым числом.В законах скорости для реакций с множеством исходных веществ общий порядок реакции является суммой всех порядков исходных веществ. Для каждого исходного вещества скорость реакции, константа скорости, концентрация и порядок реакции определяются экспериментально. Закон скорости выражает взаимосвязь между всеми этими параметрами.Индивидуальные порядки исходных веществ обычно принимают значения 0, 1 или 2, и на основе общего порядка реакции химические реакции могут быть разделены на реакции нулевого, первого или второго порядка. Однореагентная или мономолекулярная химическая реакция, скорость которой остается постоянной на протяжении всего времени ее протекания, является реакцией нулевого порядка. Порядок исходных веществ в реакции нулевого порядка равен нулю, и в соответствии с законом скорости концентрация исходных веществ возводится в нулевую степень.Поскольку значение любого числа, возведенного в нулевую степень, равно единице, скорость реакции нулевого порядка равна константе скорости и, следовательно, не зависит от концентрации исходного вещества. Следовательно, в реакции нулевого порядка, даже если концентрация исходного вещества уменьшается, скорость реакции не замедляется. Мономолекулярная химическая реакция, скорость которой прямо пропорциональна концентрации исходного вещества, является реакцией первого порядка.Порядок исходных веществ для реакции первого порядка равен единице, и согласно закону скорости концентрация исходного вещества возводится в первую степень. Поскольку значение любого числа, возведенного в степень единицы, остается неизменным, скорость реакции первого порядка напрямую зависит от концентрации исходного вещества. По мере уменьшения концентрации исходного вещества скорость реакции пропорционально линейно уменьшается.Мономолекулярная химическая реакция, где скорость реакции зависит от квадрата концентрации исходного вещества, является реакцией второго порядка. Порядок исходных веществ равен двум, и концентрация исходных веществ возводится во вторую степень. Соответственно, скорость реакции второго порядка напрямую зависит от квадрата концентрации исходного вещества.По мере уменьшения концентрации исходного вещества скорость реакции уменьшается экспоненциально со степенью 2.

Закон о концентрации и скорости

На скорость реакции влияют концентрации реагентов. Законы нормы (законы о дифференциальной скорости) или уравнения ставок — это математические выражения, описывающие соотношение между скоростью химической реакции и концентрацией ее реагенты. 
 Например, в общей реакции AA + BB ⟶ продукты, где a и b являются стехиометрическими коэффициентами, закон о ставке можно записать как: 
  <img alt="Eq1" src="/files/ftp_upload/11375/11375_Equation_1.png" style="height: 50px;" />
 [A] и [B] представляют собой молярные концентрации реагенты, а k — константу скорости, которая характерна для конкретной реакции при определенной температуре. 
 Экспоненты m и n являются порядками реакции и обычно являются положительными целыми числами, хотя они могут быть дроби, отрицательными или нулевыми. 
 Константа скорости k и порядки реакции m и n определяются экспериментально путем наблюдения за изменением скорости реакции по мере изменения концентраций реагенты. Константа скорости k не зависит от концентраций реагент, но изменяется в зависимости от температуры. 
 В указаниях на реакцию в законе о ставке описывается математическая зависимость скорости от концентраций реагент. Ссылаясь на общий закон о ставке (Rate = k[A]m[B]n), реакция является порядком m относительно ПОРЯДКА A и n относительно B. например, если m = 1 и n = 2, Реакция является первой в порядке А и второй в B. Общий порядок реакции — это просто сумма ордеров для каждого реагент. Например, закон ставок здесь, реакция является третьим общим порядком (1 + 2 = 3). 
 Общий экспериментальный подход к определению норм внесения является методом начальных ставок. Этот метод включает в себя измерение скорости реакции для нескольких экспериментальных исследований, проведенных с использованием различных начальных концентраций реагент. Сравнение измеренных ставок для этих испытаний позволяет определить порядок действий и, впоследствии, константу ставок, которая вместе используется для формулирования закона ставок. 
 Законы о норме могут представлять дробные приказы для некоторых реагенты, а иногда при увеличении концентрации одного реагент происходит снижение скорости реакции. Важно отметить, что законы о частоте определяются только экспериментом и не могут быть надежно предсказаны стехиометрией реакции. 
 Порядок реакции определяет соотношение между скоростью реакции и концентрацией реагенты или продуктов. 
 • в реакции нулевого порядка концентрация реагенты не оказывает никакого влияния на скорость реакции, которая остается постоянной на протяжении всего процесса. 
 • в реакции первого порядка скорость реакции прямо и линейно пропорциональна изменению концентрации реагент. По мере снижения концентрации реагент скорость реакции также пропорционально уменьшается. 
 • в реакциях второго порядка или более высокого порядка скорость реакции пропорциональна экспоненциальному значению реагенты. Поэтому по мере развития реакции и снижения концентрации реагенты скорость реакции уменьшается экспоненциально. 
 Этот текст был адаптирован к<a href="https://openstax.org/books/chemistry-2e/pages/12-3-rate-laws#57033"> Openstax, Химия 2е изд., раздел 12.3: Законы о тарифе</a>.

На скорость реакции влияют концентрации реагентов. Законы скорости (дифференциальные законы скорости) или уравнения скорости — это математические выражения...

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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

reaction rate

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concentration and rate law

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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Rate laws connect the speed of a chemical reaction to the concentration of its reactants. The rate constant, k, and the reaction orders are not taken from the chemical equation. They are found experimentally by watching how the rate changes when the reactant concentrations change. One common way to do this is the method of initial rates. In this method, several trials are run with different starting concentrations. The measured rates are then compared to determine the reaction orders and,...

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catalysis

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Catalysts change the rate of a chemical reaction by giving it a faster path. A catalyst increases reaction rate without being consumed during the process. Its effect is often explained with reaction mechanisms and energy diagrams. For an endothermic reaction, the reactants and products start and end at the same energies in both cases. The uncatalyzed path is a one-step mechanism with one transition state. The catalyzed path follows a different mechanism with two transition states and a lower...

enzymes

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Enzymes are biological catalysts that speed up many reactions in living organisms. They help control cellular metabolism by lowering the activation energy needed for biochemical reactions. When activation energy drops, reaction rates can increase. Enzyme problems can also lead to serious disease. A genetic abnormality that causes G6PD, or glucose-6-phosphate dehydrogenase, deficiency can disrupt the pathway that supplies NADPH to cells. In red blood cells, this can reduce glutathione and...

key terms and concepts

Learn how concentration affects reaction rate, how rate laws are written, and how initial rates reveal reaction order, the rate constant, and overall order.

Reaction Order from Concentration | JoVE K12

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Reaction Order from Concentration

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