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

K12 Standards

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IPC.7

IPC.7.F

Reaction Rate

Reakcja chemiczna polega na chemicznej przemianie reagentów w produkty. W miarę postępu reakcji stężenie reagentów maleje, podczas gdy stężenie produktów wzrasta. Tę wariancję stężeń reagentów i produktów można wykreślić na wykresie w funkcji czasu.
Szybkość, z jaką postępuje reakcja, nazywana jest szybkością reakcji. Mierzy szybkość znikania reagentów lub szybkość pojawiania się produktów i jest wyrażona w jednostkach molowości na sekundę. 
Średnią szybkość reakcji można obliczyć na podstawie zmiany stężeń reagentów lub produktów w określonym czasie. Wartość stężenia molowego jest oznaczona w nawiasach, t oznacza okres czasu, a symbol delta oznacza "zmianę".
Ponieważ reagenty ulegają wyczerpaniu podczas reakcji chemicznej, wartość zmiany stężenia reagentów jest zawsze ujemna. Dlatego szybkość reakcji obliczona pod względem reagentów jest uzupełniona znakiem ujemnym, aby ogólna wartość była dodatnia.
Szybkość reakcji nie jest jednolita w całej reakcji. Szybkość na początku reakcji, w "czasie zero", nazywana jest początkową szybkością reakcji. Wraz ze spadkiem stężeń reagentów szybkość reakcji maleje lub reakcja zwalnia.
Szybkość reakcji w danym punkcie czasowym lub chwilową szybkość reakcji mierzy się, obliczając nachylenie stycznej narysowanej do krzywej reakcji, dla reagentów lub produktów, w czasie zainteresowania.
Wartość nachylenia lub szybkość chwilowa w określonym punkcie czasowym jest równa dla wszystkich reagentów i produktów.
Szybkość reakcji chemicznej odzwierciedla również rzeczywiste współczynniki stechiometryczne reagentów i produktów.
Tak więc dla dowolnej zrównoważonej reakcji, w której mol A reaguje z b molami B, wytwarzając c mole C i d moli D, szybkość reakcji można wyrazić i obliczyć za pomocą ogólnego wzoru: −1/a × Δ[A]/Δt = −1/b × Δ[B]/Δt. Wartości te są takie same jak +1/c × Δ[C]/Δt, co jest takie samo jak +1/d × Δ[D]/Δt.
Określenie szybkości reakcji ma fundamentalne znaczenie dla badania kinetyki chemicznej, co pomaga zrozumieć, jak szybko działa lek, katalizator lub reakcja syntetyczna, dzięki czemu można je lepiej kontrolować i optymalizować pod względem funkcji.

szybkość reakcji

Szybkość reakcji to zmiana ilości reagenta lub produktu w jednostce czasu. Szybkość reakcji określa się zatem poprzez pomiar zależności od czasu pewnej właściwości, która może być powiązana z ilością reagenta lub produktu. Na przykład szybkości reakcji, w których zużywane są lub wytwarzają substancje gazowe, dogodnie określa się poprzez pomiar zmian objętości lub ciśnienia.

Matematyczne przedstawienie zmiany stężenia reagentów i produktów w czasie jest wyrażeniem szybkości reakcji. Nawiasy wskazują stężenia molowe, a symbol delta (Δ) oznacza „zmianę w”.

Na przykład stężenie nadtlenku wodoru, H2O2, w roztworze wodnym zmienia się powoli w czasie, rozkładając się zgodnie z równaniem:

<img alt="Eq1" src="/files/ftp_upload/11373/11373_Equation_1.png" style="height: 50px;">

Szybkość rozkładu nadtlenku wodoru można wyrazić szybkością zmiany jego stężenia:

<img alt="Eq2" src="/files/ftp_upload/11373/11373_Equation_2.png" style="height: 70px;">

Zatem [H2O2]t1 oznacza stężenie molowe nadtlenku wodoru w czasie t1. Podobnie [H2O2]t2 oznacza stężenie molowe nadtlenku wodoru w późniejszym czasie t2 oraz Δ[H2O2] oznacza zmianę stężenia molowego nadtlenku wodoru w przedziale czasu Δt (tj. t2 – t1). Ponieważ stężenie reagenta maleje w miarę postępu reakcji, Δ[H2O2] jest wielkością ujemną. Szybkości reakcji są umownie wielkościami dodatnimi, zatem tę ujemną zmianę stężenia mnoży się przez -1.

<h4>Średnia szybkość reakcji i chwilowa szybkość reakcji</h4>

Szybkość reakcji zmienia się w czasie i maleje wraz z postępem reakcji. Średnią szybkość reakcji w danym przedziale czasu można obliczyć na podstawie stężeń na początku i na końcu tego okresu, w którym zmienia się szybkość reakcji. W dowolnym momencie szybkość, z jaką przebiega reakcja, nazywana jest jej szybkością chwilową. Chwilowa szybkość reakcji w „czasie zerowym”, kiedy reakcja się rozpoczyna, jest jej szybkością początkową.

Chwilową szybkość reakcji można określić na jeden z dwóch sposobów. Jeżeli warunki eksperymentalne pozwalają na pomiar zmian stężeń w krótkich odstępach czasu, wówczas obliczenie średnich szybkości zapewnia dość dobre przybliżenie szybkości chwilowych. Alternatywnie można zastosować procedurę graficzną. Na przykład w przykładzie rozkładu nadtlenku wodoru, wykreślając stężenie nadtlenku wodoru w funkcji czasu, chwilową szybkość rozkładu H2O2 można obliczyć w dowolnym momencie „t” z nachylenia stycznej poprowadzonej do krzywej w tym czasie.

<img alt="Image1" src="/files/ftp_upload/11373/11373_Image_1.png" style="height: 300px;">

Ten wykres przedstawia wykres stężenia w funkcji czasu dla 1000 M roztworu H2O2. Szybkość w dowolnym momencie jest równa ujemnemu nachyleniu linii stycznej do krzywej w tym momencie. Styczne pokazano w t = 0 h („szybkość początkowa”) i w t = 12 h („szybkość chwilowa” w 12 h).

<h4>Względne szybkości reakcji</h4>

Szybkość reakcji można wyrazić jako zmianę stężenia dowolnego reagenta lub produktu. Dla dowolnej reakcji wszystkie te wyrażenia szybkości są ze sobą powiązane zgodnie ze stechiometrią reakcji. Szybkość ogólnej reakcji aA ⟶ bB można wyrazić w postaci spadku stężenia A lub wzrostu stężenia B. Te dwa wyrażenia szybkości są powiązane stechiometrią reakcji, gdzie:

<img alt="Eq3" src="/files/ftp_upload/11373/11373_Equation_3.png" style="height: 70px;">

Należy zauważyć, że jako współczynnik uwzględniający przeciwne znaki dwóch zmian ilości uwzględniono znak ujemny (ilość reagenta zmniejsza się, podczas gdy ilość produktu rośnie).

Szybkość reakcji

Szybkość reakcji to zmiana ilości reagenta lub produktu w jednostce czasu. Szybkość reakcji określa się zatem poprzez pomiar czasu ...

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KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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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Reaction kinetics can be tracked with polarimetry, spectrometry, and pressure measurements. These methods let scientists follow a reaction without disturbing the sample. They also help determine how reactant or product concentrations change over time. Polarimetry is used for optically active substances. These substances rotate the plane of polarized light, which is called optical rotation. The effect comes from molecular structure. Normal monochromatic light is unpolarized, but after it passes...

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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

the-integrated-rate-law-the-dependence-of-concentration-on-time

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

Measuring Reaction Rates | JoVE K12

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Measuring Reaction Rates

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