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

Для любой реакции зависимость между скоростью реакции и концентрацией реагента может быть выражена математически с помощью закона скорости или уравнения скорости. 
В законе скорости «k» — это константа пропорциональности, или константа скорости, а «n» — порядок реакции относительно одного реагента, значение которого часто является целым числом. В законах скоростей для многореагентных реакций общий порядок реакции является суммой всех порядков реагентов.
Для каждого реагента скорость реакции, константа скорости, концентрация и порядок реакции определяются экспериментально.
Отдельные порядки реагентов обычно принимают значения 0, 1 или 2, и в зависимости от общего порядка реакций химические реакции могут быть классифицированы как реакции нулевого, первого или второго порядка.
Однореагентная, или одномолекулярная, химическая реакция, скорость которой остается постоянной на протяжении всей ее продолжительности, называется реакцией нулевого порядка. Порядок реагентов в реакции нулевого порядка равен нулю, а концентрация реагента повышается до нулевой степени.
Поскольку значение любого числа, возведенного в нулевую степень, равно единице, скорость реакции нулевого порядка равна константе скорости и, следовательно, не зависит от концентрации реагента.
Одномолекулярная химическая реакция, в которой скорость реакции прямо пропорциональна концентрации реагента, является реакцией первого порядка. Порядок реагента для реакции первого порядка равен единице, и в соответствии с законом скорости, концентрация реагента повышается до первой степени.
Так как величина любого числа, возведенного в степень единицы, остается неизменной, то скорость реакции первого порядка напрямую зависит от концентрации реагента. По мере снижения концентрации реагента скорость реакции уменьшается пропорционально линейным образом.
Одномолекулярная химическая реакция, в которой скорость реакции зависит от квадрата концентрации реагента, является реакцией второго порядка. По мере уменьшения концентрации реагента скорость реакции уменьшается экспоненциально квадратичным образом.

Закон скорости и порядок реакции

На скорость реакции влияет концентрация реагирующих веществ. Законы скорости (дифференциальные законы скорости) или уравнения скорости — это математические выражения, описывающие связь между скоростью химической реакции и концентрацией ее реагентов.

Например, в общей реакции продуктов aA + bB ⟶, где a и b — стехиометрические коэффициенты, закон скорости можно записать как:

 скорость = k[A]m[B]n

Здесь [A] и [B] представляют собой молярные концентрации реагентов, а k — константа скорости, специфичная для конкретной реакции при определенной температуре.

Показатели степени m и n представляют собой порядки реакции и обычно являются положительными целыми числами, хотя они могут быть дробными, отрицательными значениями или нулем.

Константу скорости k и порядки реакции m и n определяют экспериментально, наблюдая за изменением скорости реакции при изменении концентрации реагирующих веществ. Константа скорости k не зависит от концентрации реагентов, но меняется в зависимости от температуры.

Порядки реакции в законе скорости описывают математическую зависимость скорости от концентрации реагентов. Ссылаясь на общий закон скорости (скорость = k[A]m[B]n), реакция имеет m-й порядок по отношению к A и n-й порядок по отношению к B. Например, если m = 1 и n = 2, реакция имеет первый порядок по A и второй порядок по B. Общий порядок реакции представляет собой просто сумму порядков для каждого реагента. Для приведенного здесь примера закона скорости реакция в целом имеет третий порядок (1 + 2 = 3).

Распространенным экспериментальным подходом к определению законов скорости является метод начальных скоростей. Этот метод включает измерение скорости реакции для нескольких экспериментальных испытаний, проведенных с использованием различных начальных концентраций реагентов. Сравнение измеренных скоростей в этих испытаниях позволяет определить порядки реакций и, следовательно, константу скорости, которые вместе используются для формулирования закона скорости.

Законы скорости могут иметь дробные порядки для некоторых реагентов, а отрицательные порядки реакции иногда наблюдаются, когда увеличение концентрации одного реагента вызывает уменьшение скорости реакции. Важно отметить, что законы скорости определяются только экспериментом и не могут быть надежно предсказаны стехиометрией реакции.

Порядок реакции определяет связь между скоростью реакции и концентрацией реагентов или продуктов. 

<ul style="list-style-type: disc; display: flex; flex-direction: column; gap: 10px; padding: 0 0 0 40px;">
 <li>В реакции нулевого порядка концентрация реагентов не оказывает никакого влияния на скорость реакции, которая остается постоянной на протяжении всей реакции.
 </li>
 <li>В реакции первого порядка скорость реакции прямо и линейно пропорциональна изменению концентрации реагента. С уменьшением концентрации реагента пропорционально уменьшается и скорость реакции.
 </li>
 <li>В реакциях второго и более высокого порядка скорость реакции пропорциональна экспоненциальному значению реагентов. Следовательно, по мере развития реакции и уменьшения концентрации реагентов скорость реакции снижается экспоненциально.
 </li>
</ul>

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

Education

JoVE Core

Organic Chemistry

Органическая химия

Unit 1

Раздел 1

Thermodynamics and Chemical Kinetics

Термодинамика и химическая кинетика

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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.

How Reactant Concentration Sets Reaction Rate

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Learn how rate laws connect reactant concentration to reaction speed, how reaction orders work, and how the method of initial rates is used to find them.

How Reactant Concentration Sets Reaction Rate | JoVE K12

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B. Momentum is conserved under well-defined conditions. A change in momentum occurs when a net force is applied to an object over a time interval.

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How Reactant Concentration Sets Reaction Rate

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