Name: measuring reaction rates
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 53 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Reaction Rate

Химическая реакция включает химическое превращение исходных веществ в продукты реакции. По мере течения реакции концентрация исходных веществ уменьшается, а концентрация продуктов увеличивается. Это изменение концентраций исходных веществ и продуктов реакции может быть отображено на графике как функция времени.Скорость, с которой протекает реакция, называется скоростью реакции. Она выражается в скорости исчезновения исходных веществ или скорости появления продуктов и выражается в единицах молярности в секунду. Среднюю скорость реакции можно рассчитать по изменению концентраций исходных веществ или продуктов реакции в течение определенного периода времени.Значение молярной концентрации указано в скобках, t обозначает период времени, а символ дельта обозначает изменение"Поскольку во время химической реакции количество исходных веществ уменьшается, величина изменения концентрации исходных веществ всегда отрицательна. Следовательно, скорость реакции, рассчитанная с точки зрения исходных веществ, дополняется знаком минус, чтобы общее значение было положительным. Скорость реакции неоднородна на протяжении всей реакции.Скорость в начале реакции, в нулевой момент времени"называется начальной скоростью реакции. По мере уменьшения концентрации исходных веществ скорость реакции снижается, или реакция замедляется. Скорость реакции в данный момент времени или мгновенная скорость реакции измеряется путем вычисления наклона касательной к кривой реакции для исходных веществ или продуктов реакции в интересующий момент времени.Величина наклона или мгновенная скорость в определенный момент времени одинаковы для всех исходных веществ и продуктов реакции. Скорость химической реакции также отражает фактические стехиометрические коэффициенты исходных веществ и продуктов реакции. Таким образом, для любой сбалансированной реакции, где a"моль вещества A реагируют с b"моль вещества B, образуя c"моль вещества C и d"моль вещества D, скорость реакции может быть выражена и рассчитана с использованием общей формулы:минус 1/a умножить на изменение концентрации исходного вещества A со временем, равная минус 1/b умножить на изменение концентрации исходного вещества B со временем.Эти значения такие же, как изменение концентрации продукта C со временем, кратное плюс 1/c, что совпадает с изменением концентрации продукта D со временем, кратное плюс 1/d. Определение скорости реакции имеет фундаментальное значение для изучения химической кинетики, которая помогает понять, насколько быстро действует лекарство, катализатор или синтетическая реакция, чтобы их можно было лучше контролировать и оптимизировать в их функциях.

Скорость реакции

Скорость реакции — это изменение количества реагент или продукта за единицу времени. Таким образом, скорость реакции определяется путем измерения зависимости времени от некоторого имущества, которое может быть связано с реагент или количеством продукта. Например, скорость реакций, которые потребляют или производят газообразные вещества, удобно определяется путем измерения изменений объема или давления. 
 Математическое представление изменения концентрации реагенты и продуктов со временем является выражением скорости реакции. В скобках указаны молярные концентрации, а в символе дельта (Δ) – “изменение”. 
 Например, концентрация перекиси водорода, H2O2, в водном растворе раствор со временем меняется медленно, так как разлагается в соответствии с уравнением: 
  <img alt=""Eq1"" src=""/files/ftp_upload/11373/11373_Equation_1.png"" style=""height: 50px;"" />
 Скорость разложения перекиси водорода может быть выражена с точки зрения скорости изменения ее концентрации: 
  <img alt=""Eq2"" src=""/files/ftp_upload/11373/11373_Equation_2.png"" style=""height: 70px;"" />
 Таким образом, [H2O2]t1 представляет собой молярную концентрацию перекиси водорода во время t1; аналогично, [H2O2]t2 представляет молярную концентрацию перекиси водорода в более позднее время t2; и Δ[H2O2] представляет изменение молярной концентрации перекиси водорода в течение временного интервала Δt (то есть t2 – t1). Поскольку концентрация реагент снижается по мере реакции, Δ[H2O2] является отрицательным количеством. Скорость реакции, по условным данным, является положительной, поэтому это отрицательное изменение концентрации умножается на -1. 
 Средняя скорость реакции и мгновенная скорость реакции 
 Скорость реакции меняется со временем и уменьшается по мере поступления реакции. Среднюю скорость реакции за временной интервал можно рассчитать с помощью концентраций в начале и конце этого периода, в течение которых скорость реакции изменяется. В любой конкретный момент скорость, с которой происходит реакция, называется мгновенной скоростью. Мгновенная скорость реакции при “нулевом времени”, когда начинается реакция, является ее начальной скоростью. 
 Мгновенная скорость реакции может быть определена одним из двух способов. Если экспериментальные условия позволяют измерять изменения концентрации за короткие промежутки времени, то вычисление средних значений обеспечивает достаточно хорошие приближения мгновенных значений. В качестве альтернативы можно использовать графическую процедуру. Например, в примере разложения перекиси водорода путем построения графика концентрации перекиси водорода в зависимости от времени, мгновенная скорость разложения H2O2 может быть вычислена в любое время "t" от уклона касательной, нарисованного на кривой в то время. 
  <img alt=""Image1"" src=""/files/ftp_upload/11373/11373_Image_1.png"" style=""height: 300px;"" />
 На этом графике показана зависимость концентрации от времени для 1.000 M раствор H2O2. Скорость в любое время равна отрицательному уклону линии, касательной к кривой в то время. Касательные показаны на t = 0 ч (“начальный расход”) и на t = 12 ч (“мгновенный расход” при 12 ч). 
 Относительные скорости реакции 
 Скорость реакции может быть выражена как изменение концентрации любого реагент или продукта. Для любой заданной реакции эти выражения скорости связаны друг с другом в соответствии со стехиометрией реакции. Скорость общей реакции AA ⟶ BB может вырасти с точки зрения снижения концентрации A или увеличения концентрации B. эти два выражения скорости связаны со стехиометрией реакции, где: 
  <img alt=""Eq3"" src=""/files/ftp_upload/11373/11373_Equation_3.png"" style=""height: 70px;"" />
 Обратите внимание, что знак «минус» был включен в качестве фактора для учета противоположных признаков двух изменений суммы (сумма реагент уменьшается при увеличении объема продукта). 
 Этот текст был адаптирован к Openstax<a href=""https://openstax.org/books/chemistry-2e/pages/12-introduction"">, Химия 2е изд., Chapter 12 Introadtion</a> и <a href=""https://openstax.org/books/chemistry-2e/pages/12-1-chemical-reaction-rates#12163""> Openstax, Химия 2е изд., раздел 12.1: Chemical Reaction Rates. </a>

Скорость реакции – это изменение количества реагента или продукта в единицу времени. Таким образом, скорость реакции определяется путем измерения времени ...

Education

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Один

Chemical Kinetics

Химическая кинетика

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

measuring reaction rates

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts.

For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes over time. In this expression, brackets mean molar concentration, and the delta symbol means change in amount.

Hydrogen peroxide provides a clear example. As H2O2 decomposes in water, its concentration slowly decreases with time. The reaction rate can be written as the rate of change of the H2O2 concentration. If the concentration at time t1 is compared with the concentration at a later time t2, the change in concentration over the time interval Δt is found.

Because the reactant concentration goes down, Δ[H2O2] is negative. Reaction rates are reported as positive values, so the negative sign is reversed by multiplying by -1. This keeps the reaction rate positive even when the reactant amount is falling.

Reaction rates are not constant. They usually change as the reaction continues, so an average reaction rate is calculated over a time interval using the starting and ending concentrations. The rate at one exact moment is the instantaneous rate, and the instantaneous rate at time zero is the initial rate.

Instantaneous rate can be estimated in two ways. If concentration changes can be measured over very short time intervals, the average rate gives a good approximation. A graph can also be used. When concentration is plotted against time, the instantaneous rate at a given time equals the negative slope of a tangent line to the curve. In the hydrogen peroxide example, tangents can be drawn at t = 0 h for the initial rate and at t = 12 h for the instantaneous rate at 12 h.

The rate of a reaction can be written using either a reactant or a product. For a general reaction aA → bB, the decrease in A and the increase in B are linked by the reaction stoichiometry. The two rate expressions are related by that stoichiometry, and a negative sign is included to account for the reactant decreasing while the product increases.

Measuring Reaction Rates

reaction rate

Measuring reaction rates shows how fast reactants are used up or products are formed over time. The rate of reaction is the change in the amount of a reactant or product per unit time. Because of that, scientists measure how a property changes as the reaction proceeds and connect that change to reactant or product amounts. For reactions that make or use a gas, the rate can be followed by changes in volume or pressure. A rate expression is the mathematical way to write how concentration changes...

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concentration and rate law

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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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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,...

the integrated rate law: the dependence of concentration on time

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Reading Reaction Progress with Integrated Laws

Integrated rate laws connect reaction concentration to time. They help show how much reactant or product is present after a certain period. They can also help estimate how long a reaction must run to reach a given extent. A common use is finding how long radioactive material must be stored before its radioactivity drops to a safe level. These equations come from the differential rate law after it is integrated with respect to time. For a first-order reaction, where rate = k[A], the integrated...

half-life of a reaction

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Reaction half-life is the time needed for half of a reactant to be consumed. After each half-life, half of the remaining reactant is used up. A hydrogen peroxide decomposition example shows this pattern clearly. The concentration drops from 1.000 M to 0.500 M in the first 6.00 hours, then to 0.250 M in the next 6.00 hours, and then to 0.125 M in the third period. Each later 6.00-hour interval removes half of what is left. For first-order reactions, the half-life does not depend on the reactant...

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arrhenius plots

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The Arrhenius equation links activation energy and the rate constant, k, for chemical reactions. It is written as k = Ae−Ea/RT. In this equation, R is the ideal gas constant with a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor depends on collision frequency and the orientation of reacting molecules. Experimental kinetic data can be used to find the activation...

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Chemical reactions can happen in steps, and a balanced equation does not show the full path at the molecular level. A reaction mechanism, also called a reaction path, gives the step-by-step process that explains how reactants become products. The decomposition of ozone is one example of a reaction that can be described by a sequence of steps. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and when they are added together, they give the balanced...

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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...

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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 reaction rates are measured using concentration, volume, pressure, average rate, and instantaneous rate with the hydrogen peroxide example.

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Measuring Reaction Rates

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