Name: reaction rate, arrhenius, and catalysts
Uploaded: 2020-03-26T07:22:26.000Z
Duration: 3 min 17 s
Channel: JoVE

K12 Standards

Louisiana Student Standards for Science

High School

Chemistry

Matter And Its Interactions

HS-PS1-4

Temperature Dependence

A medida da rapidez com que uma reação ocorre é chamada de taxa de reação. A taxa de uma reação química é definida pela lei da taxa, que descreve a relação entre a velocidade da reação e as concentrações do reagente. Nesta equação, k é a constante de taxa, A e B são os dois reagentes e m e n são suas respectivas ordens de reação.
A constante de taxa converte a relação para as unidades adequadas de taxa, moles por litro por segundo. Assim, a constante de taxa tem unidades diferentes, dependendo da ordem geral da reação. No entanto, a constante de taxa tem mais significado do que simplesmente a conversão de unidades. A constante de taxa está relacionada à quantidade mínima de energia necessária para que uma reação química ocorra - chamada de energia de ativação.
Em uma reação, os reagentes estão em um estado inicial de energia potencial. À medida que a reação prossegue, ela deve superar uma certa energia potencial, a energia de ativação, antes de atingir seu estado final. A energia líquida da reação é a diferença entre os estados inicial e final. Essa diferença pode ser negativa, o que significa que a reação libera energia, ou positiva, o que significa que absorve energia.
Se não houver energia suficiente disponível para superar a energia de ativação, a reação não prosseguirá. Em alguns casos, a energia pode ser fornecida na forma de calor. Isso fornece energia adicional para superar a barreira à ativação, e a reação pode prosseguir. Um catalisador também pode ser adicionado, o que fornece uma via alternativa de menor energia de ativação entre os reagentes e os produtos.
Os catalisadores não são consumidos na reação e, portanto, não afetam a energia líquida da reação. A energia de ativação é determinada experimentalmente e está relacionada à constante de reação k pela equação de Arrhenius, onde A é o fator pré-exponencial ou de frequência, R é a constante universal do gás e T é a temperatura absoluta na qual a reação ocorre.
A partir dessa equação, sabemos que aumentar a temperatura da reação ou diminuir a energia de ativação aumenta a constante de taxa. Voltando à equação da lei de taxa, segue-se que uma constante de taxa mais alta resulta em uma taxa de reação mais alta. Isso faz sentido porque, à medida que a temperatura aumenta, as moléculas se movem mais rápido e colidem com mais frequência, resultando em uma fração aumentada de moléculas com maior energia do que a energia de ativação.
Neste laboratório, você aprenderá a medir a energia de ativação de uma reação experimentalmente usando a decomposição do peróxido de hidrogênio como reação modelo.

dependência da temperatura

Cinética Química 
A taxa de reação é a velocidade com que ocorre uma reação química. A taxa de reação é definida como a mudança na concentração de um componente na reação com o tempo. A velocidade de uma reação depende de vários fatores, incluindo a concentração de reagentes e a temperatura na qual a reação é realizada. Cada reagente contribui para a velocidade da reação por um fator específico. Essa relação é definida pela lei da taxa de reação.
<h4>Lei de taxas</h4>
A lei de velocidade é uma equação que descreve a relação entre a concentração de reagentes, A e B, e suas ordens de reação, m e n. A constante de taxa, k, relaciona as concentrações e ordens dos reagentes com a taxa de reação. Depende da reação como a temperatura na qual a reação é realizada.
r = k [A]m[B]n for aA + bB → cC
<h4>Equação de Arrhenius</h4>
A equação de Arrhenius relaciona a constante de taxa de reação com a energia de ativação de uma reação química. A energia de ativação é definida como a quantidade de energia que uma reação química precisa para prosseguir. Se uma reação não atender a esse requisito de energia de ativação, a reação não prosseguirá.
<img src="/CDNSource/lm/labs/51/51_Concepts_2.jpg">
A relação exponencial negativa entre k e a temperatura indica que, à medida que a temperatura aumenta, o valor de k também aumenta. Como a constante de taxa pode ser determinada experimentalmente em uma faixa de temperaturas, a energia de ativação pode ser calculada usando a equação de Arrhenius. Tomando o logaritmo natural de ambos os lados, a equação de Arrhenius é reescrita como uma equação linear.
<img src="/CDNSource/lm/labs/51/51_Concepts_3.jpg">
Um gráfico de ln k vs. 1/T produz uma linha reta com uma inclinação igual a -Ea/R e uma interceptação y de ln A. Como a constante de gás ideal, R, é conhecida, Ea pode ser determinada graficamente usando uma série de valores de k em diferentes temperaturas.
<h4>Catalisadores e Energia de Ativação</h4>
Algumas reações químicas têm uma energia de ativação suficientemente grande que faz com que a reação prossiga lentamente, se é que ocorre. A reação de decomposição do peróxido de hidrogênio em oxigênio e água ocorre espontaneamente, mas ocorre a uma taxa incrivelmente lenta. Uma maneira de superar essa barreira inicial é fornecer energia na forma de calor. No entanto, isso nem sempre é ideal, pois o calor excessivo pode afetar a estabilidade dos produtos ou reagentes ou pode facilitar reações colaterais.
A energia de ativação para reações químicas pode ser alterada usando catalisadores. Um catalisador reduz a energia de ativação de uma reação química, mas não é consumida pela reação. Em outras palavras, um catalisador facilita a reação química, facilitando a superação do requisito crítico de energia de ativação. Na decomposição do peróxido de hidrogênio, a adição de nitrato de ferro reduz a energia de ativação e permite que a reação prossiga em um ritmo mais rápido. No entanto, é importante observar que, embora um catalisador possa afetar a taxa de uma reação, um catalisador NÃO altera a quantidade de produto produzido pela reação.
<h4>Referências</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química e Reatividade Química. Belmont, CA: Brooks / Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009). Química: A Natureza Molecular da Matéria e da Mudança. Boston, MA: Colina McGraw.</li>
</ol>

Dependência da temperatura

Cinética Química 
A taxa de reação é a velocidade com que ocorre uma reação química. A taxa de reação é definida como a mudança na concentração de um ...

Education

JoVE Lab Manual

Química

reaction rate, arrhenius, and catalysts

Chemical kinetics explains how fast a reaction happens and why that speed changes. The reaction rate is the speed of a chemical reaction. It is defined as the change in concentration of a reactant or product over time.

Reaction speed depends on several factors, including the concentration of the reactants and the temperature of the reaction. Each reactant affects the rate in a specific way. That relationship is described by the rate law.

The rate law is an equation that connects the concentrations of reactants A and B, their reaction orders m and n, and the rate constant k. For a reaction written as aA + bB → cC, the rate law is r = k[A]^m[B]^n. The rate constant depends on the reaction and on the temperature at which it is performed.

The Arrhenius equation links the rate constant to the activation energy of a chemical reaction. Activation energy is the energy a reaction must have to proceed. If a reaction does not reach this energy requirement, it will not continue. The equation also shows that as temperature increases, k increases.

Because the rate constant can be measured at different temperatures, activation energy can be calculated from experimental data. Taking the natural log of both sides of the Arrhenius equation gives a linear form. A plot of ln k versus 1/T produces a straight line with slope -Ea/R and y-intercept ln A. Since R is known, Ea can be found graphically from a series of k values.

Some reactions have such a large activation energy that they are very slow. The decomposition of hydrogen peroxide into oxygen and water happens spontaneously, but at an incredibly slow rate. Heat can help overcome this barrier, but too much heat may affect the stability of the reactants or products or promote side reactions.

Catalysts offer another way to lower activation energy. A catalyst speeds up a reaction by making the energy barrier easier to cross, but it is not consumed. In the decomposition of hydrogen peroxide, iron nitrate lowers the activation energy and increases the reaction rate. A catalyst changes how fast the reaction occurs, but it does not change the amount of product formed.

Reaction Rate, Arrhenius, and Catalysts

lab techniques

Measuring with Accuracy in the Lab

Measuring with accuracy in the lab starts with careful records, the right tools, and correct reading of data. Scientists use lab notebooks to record how an experiment was done. These notes include the amounts of materials used and each step of the procedure. Without that record, an experiment cannot be repeated in the same way. Balances and glassware are used to measure solids and liquids for experiments. The precision of a measurement depends on the equipment used. Digital balances show the...

scientific measurement and lab skills

Graphs, Curve Fit, and Standard Deviation

Two-dimensional graphs help students compare an independent variable with a dependent variable in science data. In a chemistry example, temperature is the independent variable and volume is the dependent variable. After data are collected, the relationship between the two measurements must be quantified so it can be evaluated and compared with other relationships. A graph can also show when no relationship exists between the two variables. To make a two-dimensional graph, each data point needs...

stoichiometry, product yield, and limiting reactants

stoichiometry-product-yield-and-limiting-reactants

Balancing Equations and Reactant Limits

Chemical equations show how reactants change into products during a reaction. Stoichiometry is the study of the relative amounts of reactants and products. It is based on the Law of Conservation of Mass, which says matter is neither created nor destroyed. Because atoms are only rearranged, the number and identity of atoms must match on both sides of a reaction. That is why a chemical equation must be balanced. A balanced equation has the same number of atoms for each element in the reactants...

redox reactions

Oxidation States in Redox Reactions

Oxidation states help identify redox reactions. In a redox reaction, oxidation is the loss of one or more electrons, and reduction is the gain of one or more electrons. These reactions are often tracked by changes in oxidation state, also called oxidation number. Each atom in a molecule has an oxidation state. This number shows how oxidized an atom is compared with its free elemental form. It is written as the charge an atom would have if every bond were purely ionic. In that model, bonding...

ideal gas law

Gas Laws Behind the Ideal Gas Equation

The ideal gas equation brings together the main gas laws used in chemistry. Gases have molecules that are far apart, so their behavior is described with four measurable variables: pressure, volume, number of moles, and temperature. The ideal gas law links these variables with the gas constant, R, which equals 8.314 J·K−1 mol−1. Boyle’s law describes the link between pressure and volume. When temperature and the number of moles stay constant, pressure and volume change in opposite directions.

acid and base concentrations

Acids, Bases, and pH in Titration

Acids, bases, and pH explain how acidic or basic a solution is. An Arrhenius acid releases hydrogen ions when it dissolves in water, while an Arrhenius base releases hydroxide ions. These reactions also create conjugate partners: an acid forms a conjugate base after losing hydrogen, and a base forms a conjugate acid after losing hydroxyl. Every acid has a conjugate base, and every base has a conjugate acid. pH measures the amount of hydrogen ions in a solution. The pH scale runs from 0 to 14...

buffers

Buffer pH and the Common Ion Effect

Buffer solutions help keep pH steady when acids or bases are added. In water, an Arrhenius acid (HA) dissociates into hydrogen ions, H+, and a conjugate base, A-. More hydrogen ions make a solution more acidic and lower its pH. An Arrhenius base (BOH) dissociates into a conjugate acid, B+, and hydroxide ions, OH-. Strong acids and strong bases usually change pH a lot because they react completely in water and raise the amount of H+ or OH-. Buffers reduce those pH changes. A buffer usually...

enthalpy of reaction

Reaction Heat, Hess’s Law, and Calorimetry

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solubility

Solubility in Water and Other Solvents

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metal flame emission

Flame Test Metal Color Clues

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Hydrogen Emission Lines

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beer's law

Absorbance Spectra and Beer-Lambert Analysis

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Reaction Order and Rate Constants

Chemical kinetics describes the speed of a chemical reaction. The rate depends on the reaction mechanism, the complexity of the reaction, and the number of reactants involved. Reactant concentration also affects how fast a reaction happens. The rate law is the experimental relationship that connects these factors to reaction rate. In the rate law, each reactant contributes through a reaction order. The general form is r = k[A]^m[B]^n for aA + bB + ... → cC. Here, r is the reaction rate, k is...

temperature dependence

Chemical kinetics explains how fast a reaction happens and why that speed changes. The reaction rate is the speed of a chemical reaction. It is defined as the change in concentration of a reactant or product over time. Reaction speed depends on several factors, including the concentration of the reactants and the temperature of the reaction. Each reactant affects the rate in a specific way. That relationship is described by the rate law. The rate law is an equation that connects the...

galvanic cells

Battery Power from Redox Reactions

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electrolytic cells

Electroplating with Electrolytic Cells

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proper lab notebook keeping

Recording a Reproducible Lab Experiment

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basic organic chemistry techniques

Organic Lab Glassware and Safe Handling

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melting points

How Impurities Change Melting Points

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boiling points

Boiling Point and Vapor Pressure

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Purifying Crystals by Solubility

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extraction

Separating Mixtures with Solvent Extraction

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simple distillation

Separating Liquids by Boiling Point

Separating liquids by boiling point is the goal of distillation. A miscible mixture contains liquids that mix evenly, and it can be heated so that the more volatile liquid evaporates first. Volatility is a liquid’s tendency to vaporize, and boiling begins when bubbles form in the liquid phase. In a closed container, liquid and vapor can reach equilibrium, which means the rate of vaporization equals the rate of condensation. The pressure from the vapor above the liquid is called vapor pressure.

steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

TLC Separation by Polarity

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column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

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Soap Formation from Triglycerides

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Luminol Chemiluminescence in Forensics

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Ester Formation and Everyday Fragrances

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identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

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How Dyes Absorb Visible Light

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identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

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Minnesota Academic Standards in Science

9-12

3.2.1 9C.3.1.1.2

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New Jersey Student Learning Standards for Science

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HS-PS1-2

Reaction Rate, Arrhenius, and Catalysts

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