Name: determining activation energy from temperature
Uploaded: 2020-09-24T15:06:50.000Z
Duration: 2 min 34 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.7

IPC.7.F

Arrhenius Plots

La velocità di una reazione chimica è altamente sensibile ai cambiamenti della temperatura. Questa dipendenza dalla temperatura, viene spiegata matematicamente utilizzando l'equazione di Arrhenius, che esprime la relazione fra la costante della velocità, la temperatura assoluta, il fattore di frequenza e l'energia di attivazione. L'energia di attivazione e il fattore di frequenza possono anche essere determinati graficamente, convertendo l'equazione di Arrhenius in una forma non esponenziale.

Utilizzando i logaritmi naturali su ambo i lati, viene generata un'equazione per una funzione lineare. Il valore della pendenza corrisponde al valore negativo dell'energia di attivazione sulla costante del gas, e l'intercetta sull'asse delle y corrisponde al log naturale del fattore di frequenza. Questa equazione può essere usata per generare un grafico chiamato diagramma di Arrhenius, in cui il log naturale della costante di velocità è indicato come una funzione dell'inverso della temperatura in kelvin.

I dati cinetici degli esperimenti e delle reazioni possono essere illustrati e analizzati usando questo diagramma di Arrhenius. In questo esempio, il grafico mostra una linea retta. Il valore della pendenza espresso in kelvin è posto uguale al valore negativo dell'energia di attivazione su R.Dopo aver assegnato il valore per la costante del gas e risolto per l'energia di attivazione, si ottiene un valore di 93, 1 kilojoule per mole.

Inoltre, l'intercetta dell'asse y di 26, 8 è uguale al log naturale del fattore di frequenza. Pertanto, risolvendo per A, si ottiene il valore di 4, 36 10¹¹, con l'unità uno su molarità per secondi la stessa unità della costante di velocità. In caso di dati cinetici limitati, o di difficoltà con la rappresentazione grafica, può essere utilizzata una forma a due punti dell'equazione di Arrhenius per calcolare l'energia di attivazione in maniera non grafica.

In tali casi, la forma non esponenziale dell'equazione di Arrhenius viene modificata, per includere costanti di velocità, a due diverse temperature. La successiva sottrazione e riarrangiamento dell'espressione, produce la forma a due punti dell'equazione di Arrhenius, che viene utilizzata per calcolare l'energia di attivazione delle costanti di velocità, generate sperimentalmente a due temperature differenti. Sostituendo i valori, si calcola che l'energia di attivazione per questa reazione sia di 145 kilojoule per mole.

emi-vita di 14, 7 ore, mostra un tasso di decadimento più rapido del cobalto-60, con un'emi-vita più lunga, di 5, 3 anni.

grafici di arrhenius

L'equazione di Arrhenius mette in relazione l'energia di attivazione e la costante di velocità, k, per le reazioni chimiche. Nell’equazione di Arrhenius, k = Ae^−Ea/RT, R è la costante dei gas ideali, che ha un valore di 8,314 J/mol·K, T è la temperatura sulla scala kelvin, E_a è l’energia di attivazione in J/mole, e è la costante 2.7183 e A è una costante chiamata fattore di frequenza, che è correlata alla frequenza delle collisioni e all'orientamento delle molecole reagenti.

L'equazione di Arrhenius può essere utilizzata per calcolare l'energia di attivazione di una reazione da dati cinetici sperimentali. Un approccio conveniente per determinare l'E_a di una reazione prevede la misurazione di k a due o più temperature diverse. Utilizza una versione modificata dell'equazione di Arrhenius che assume la forma di un'equazione lineare:

 <img alt="Eq1" src="/files/ftp_upload/11380/11380_Equation_1.png" style="height: 200px;" />

Un grafico di ln k rispetto a 1/T è lineare con una pendenza pari a −E_a/R e intercetta y pari a ln A.

Consideriamo la seguente reazione:

 <img alt="Eq2" src="/files/ftp_upload/11380/11380_Equation_2.png" style="height: 50px;" />

L'energia di attivazione di questa reazione può essere determinata se la variazione della costante di velocità con la temperatura è nota dai dati cinetici della reazione, come mostrato.

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Temperature (K)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Rate constant (L/mol/s)</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">555</td>
			<td style="color: black; border: 1px solid black; text-align: center;">3.52 × 10–7</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">575</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1.22 × 10–6</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">645</td>
			<td style="color: black; border: 1px solid black; text-align: center;">8.59 × 10–5</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">700</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1.16 × 10–3</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">781</td>
			<td style="color: black; border: 1px solid black; text-align: center;">3.95 × 10–2</td>
		</tr>
	</tbody>
</table>

I dati forniti possono essere utilizzati per ricavare i valori dell'inverso della temperatura (1/T) e del logaritmo naturale di k (ln k).

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">1/T (K–1)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">ln k</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">   1.80 × 10–3    </td>
			<td style="color: black; border: 1px solid black; text-align: center;">     –14.860     </td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">1.74 × 10–3</td>
			<td style="color: black; border: 1px solid black; text-align: center;">–13.617</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">1.55 × 10–3</td>
			<td style="color: black; border: 1px solid black; text-align: center;">–9.362</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">1.43 × 10–3</td>
			<td style="color: black; border: 1px solid black; text-align: center;">–6.759</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">1.28 × 10–3</td>
			<td style="color: black; border: 1px solid black; text-align: center;">–3.231</td>
		</tr>
	</tbody>
</table>

 <img alt="Image1" src="/files/ftp_upload/11380/11380_Image 1.png" style="height: 350px;" />

Tracciando i punti dati derivati con ln k rispetto a 1/T, viene generato un grafico a linee che mostra una relazione lineare tra ln k e 1/T, come mostrato.

La pendenza della linea, che corrisponde all'energia di attivazione, può essere stimata utilizzando due qualsiasi delle coppie di dati sperimentali.

 <img alt="Eq3" src="/files/ftp_upload/11380/11380_Equation_3.png" style="height: 350px;" />

Un approccio alternativo per ricavare l'energia di attivazione prevede l'utilizzo della costante di velocità a due diverse temperature. In questo approccio, l'equazione di Arrhenius viene riorganizzata in una comoda forma a due punti:

 <img alt="Eq4" src="/files/ftp_upload/11380/11380_Equation_4v2.png" style="height: 75px;" />

Riorganizzando l'equazione, viene generata un'espressione per l'energia di attivazione.

 <img alt="Eq5" src="/files/ftp_upload/11380/11380_Equation_5.png" style="height: 75px;" />

Sostituendo due coppie di dati qualsiasi e un ulteriore calcolo si ottiene il valore dell'energia di attivazione in joule per mole o kilojoule per mole.

 <img alt="Eq6" src="/files/ftp_upload/11380/11380_Equation_6.png" style="height: 135px;" />

Questo approccio alternativo a due punti produce lo stesso risultato dell'approccio grafico. Tuttavia, in pratica, l’approccio grafico fornisce in genere risultati più affidabili mentre si lavora con dati sperimentali reali.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/12-5-collision-theory">Openstax, Chemistry 2e, Section 12.5: Collision Theory</a>

Grafici di Arrhenius

L'equazione di Arrhenius mette in relazione l'energia di attivazione e la costante di velocità, k, per le reazioni chimiche. Nell'equazione di Arrhenius, k = Ae−Ea/RT, R è ...

Education

Didattica

JoVE Core

Chemistry

Chimica

Chemical Kinetics

Cinetica Chimica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

determining activation energy from temperature

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 energy of a reaction. One practical method is to measure k at two or more temperatures. The Arrhenius equation can then be rewritten in a linear form. A plot of ln k versus 1/T gives a straight line. The slope equals −Ea/R, and the y-intercept equals ln A.

For example, a reaction with known rate constants at 555 K, 575 K, 645 K, 700 K, and 781 K can be analyzed this way. The rate constants are 3.52 × 10−7, 1.22 × 10−6, 8.59 × 10−5, 1.16 × 10−3, and 3.95 × 10−2 L/mol/s, respectively. These values can be converted to 1/T and ln k. The resulting data set can then be plotted to show the linear relationship between ln k and 1/T.

The slope of the line gives the activation energy. It can be estimated from any two experimental data pairs on the graph. Another option is the two-point form of the Arrhenius equation. This rearranged equation uses rate constants at two temperatures to calculate Ea directly. Both methods give the same result. In practice, the graphical method is often more reliable when working with real experimental data.

Determining Activation Energy from Temperature

determining-activation-energy-from-temperature

reaction rate

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...

measuring reaction rates

Tracking Reaction Kinetics with Light and Pressure

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...

concentration and rate law

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...

determining order of reaction

Finding Rate Law from Initial Rates

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

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

Reaction Half-Life in Kinetics

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...

temperature dependence on reaction rate

Collision Theory and Reaction Speed

Collision theory explains why reaction speed depends on how particles collide. Atoms, molecules, or ions must come close together before they can react. The theory says the reaction rate depends on the collision rate, the orientation of the colliding species, and whether the collision has enough energy for bonds to change. A gas-phase reaction between carbon monoxide and oxygen shows why orientation matters. The molecules can collide in more than one way. If the oxygen side of carbon monoxide...

arrhenius plots

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...

reaction mechanisms

Elementary Steps in Reaction Mechanisms

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...

rate-determining steps

Reaction Bottlenecks in Chemical Kinetics

Reaction bottlenecks help explain rate-determining steps in multistep reaction mechanisms. In a mechanism with several elementary steps, one step is often much slower than the others. That slowest step is called the rate-limiting step, or rate-determining step. A reaction cannot move faster than its slowest step, so this step controls the overall reaction rate. The idea is like traffic on a four-lane freeway with one lane closed. Cars can only move as fast as the bottleneck allows. In the same...

catalysis

Catalyst Action in Reaction Rates

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

Enzyme Action and Inhibition

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 the Arrhenius equation uses temperature and rate constants to find activation energy with a graph or a two-point calculation.

Determining Activation Energy from Temperature | JoVE K12

chemistry

JoVE Core: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Matter and its Interactions

PS.Chm.5.1

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

3.2.9-12.E

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.e

Next Generation Science Standards

Matter and Its Interactions

HS-PS1-4

HS-PS1-5

Florida Department of Education

SC.912.P.12

SC.912.P.12.12

south-carolina-college-and-career-ready-science-standards

South Carolina College and Career Ready Science Standards

C-PS1-4

C-PS1-5

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-5.

Georgia Standards of Excellence

SC4.

Collision and Transition State Theory

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

Elements: Chemical Symbols and Isotopes

A chemical symbol is an abbreviation used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. The same symbol is ...

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

The Born-Haber Cycle

Lattice Energy&#xA0;

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a ...

trends-in-lattice-energy-ion-size-and-charge

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Electronegativity

Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

bond-polarity-dipole-moment-and-percent-ionic-character

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Bonding in Metals

Metallic bonds are formed between two metal atoms. A simplified model to describe metallic bonding has been developed by Paul Dr&#xFC;de called the ...

Model basic atomic structure and relate an element's atomic structure to its bonding, reactivity, and placement on the Periodic Table.

IPC.7.A

physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

Use patterns within the Periodic Table to predict the relative physical and chemical properties of elements.

IPC.7.B

Classifying Matter by State

Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

Classifying Matter by Composition

Matter: Pure Substances and Mixtures
According to its composition, the matter can be classified into two broad categories &mdash; pure substances and ...

Phase Transitions

Whether solid, liquid, or gas, a substance&#39;s state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the ...

phase-transitions-vaporization-and-condensation

Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize ...

Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy ...

phase-transitions-sublimation-and-deposition

Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and ...

Melting Points

Melting Points in Organic Chemistry
The melting point of a compound is the temperature at which the solid phase transitions into the liquid phase at a ...

Boiling Points

Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA

	Preparation of the LaboratoryExpand
	Here, we show the laboratory preparation ...

Explain how physical and chemical properties of substances are related to their usage in everyday life such as in sunscreen, cookware, industrial applications, and fuels.

IPC.7.C

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

Metal Flame Emission

Metal Flame Emission
Flame testing is an analytical technique where a sample is placed in a flame, and the characteristic flame color is used to identify ...

Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution ...

The Pauli Exclusion Principle

The arrangement of electrons in the orbitals of an atom is called its electron configuration. We describe an electron configuration with a symbol that ...

The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

Explain how electrons can transition from a high energy level to a low energy state, emitting photons at different frequencies for different energy transitions.

IPC.7.D

The de Broglie Wavelength

In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

Werner Heisenberg considered the limits of how accurately one can measure properties of an electron or other microscopic particles. He determined that ...

The Quantum-Mechanical Model of an Atom

Shortly after de Broglie published his ideas that the electron in a hydrogen atom could be better thought of as being a circular standing wave instead of ...

Explain how atomic energy levels and emission spectra present evidence for the wave particle duality.

IPC.7.E

Science concepts. The student knows that relationships exist between the structure and properties of matter. The student is expected to:

Determining Activation Energy from Temperature

Description

Transcript

Tags

More videos by Standards