Name: combustion heat and hydrocarbon stability
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 12 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.A

Enthalpy and Heat of Reaction

A entalpia, abreviada como H, é igual à soma da energia interna, abreviada como E ou U, e o produto da pressão e do volume.
A mudança na entalpia, ΔH, é expressa como a diferença entre as entalpias dos produtos e os reagentes. Em temperatura e pressão constantes, ΔH é igual à quantidade de energia térmica trocada entre o sistema e o ambiente, ou o calor da reação.
Quando ΔH é positivo, as reações absorvem calor e são endotérmicas. Se ΔH for negativo, as reações liberam calor e são exotérmicas.
A combustão é um exemplo de processo exotérmico, em que uma substância queima na presença de um oxidante como o oxigênio atmosférico para liberar energia na forma de calor.
O calor liberado é quantificado como o calor molar de combustão, que é a quantidade de energia térmica liberada na queima de um mol de uma substância.
Quando um hidrocarboneto queima, o carbono e o hidrogênio do combustível se combinam com o oxigênio molecular para produzir água e dióxido de carbono, juntamente com a liberação de energia.
O valor do calor de combustão de um hidrocarboneto aumenta com o número de átomos de carbono na cadeia, pois mais carbono está disponível para queima e mais ligações sofrem alterações.
Por exemplo, a combustão do metano, um composto de carbono único, gera menos energia térmica do que a do butano, que possui quatro átomos de carbono.
O calor de combustão é uma maneira crítica de determinar a estabilidade relativa de hidrocarbonetos com a mesma fórmula molecular, mas estruturas diferentes.
Considere os calores de combustão de octano, 2-metilheptano e 2,2-dimetilhexano.
Esses compostos têm o mesmo número de átomos de carbono, mas os grupos metil estão ligados em posições diferentes em cada molécula.
A octanagem tem o maior calor de combustão. À medida que a ramificação aumenta, o ΔH diminui, sugerindo que a ramificação aumenta a estabilidade de um hidrocarboneto.

entalpia e calor de reação

A combustão, comumente conhecida como queima, é uma reação na qual uma substância reage com um agente oxidante, que na maioria dos casos é o oxigênio molecular, para liberar energia na forma de calor, luz ou som. O calor de combustão também é conhecido como entalpia de combustão. A energia liberada quando um mol de uma substância sofre combustão completa a pressão constante é chamada de calor molar de combustão. As reações de combustão são exotérmicas; isto é, eles liberam energia e sua convenção de sinal ΔH é negativa.

Em 1772, o químico francês Antoine Lavoisier descobriu que os produtos da queima do enxofre pesavam mais que a massa inicial do reagente. Ele postulou que o enxofre se combinava com o ar, resultando no aumento do peso. Mais tarde, a descoberta do "oxigênio" por Joseph Priestley em 1774, como um componente do ar, levou Lavoisier a acreditar que o enxofre se combinava com o oxigênio do ar, levando a um aumento em sua massa. Ele concluiu que combustão significa combinação com oxigênio. Em outras palavras, o enxofre sofreu combustão.

Exemplos de reações de combustão incluem a queima de combustíveis de hidrocarbonetos, como gás natural e carvão. No caso de reações de combustão envolvendo hidrocarbonetos, a quantidade de energia liberada varia dependendo do tipo de combustível em combustão.

Por exemplo, a combustão do gás natural, metano (CH4), dada pela reação:

<img alt="Eq2" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Methane.svg" style="text-align: center; height: 18px;">

gera menos energia térmica que a do butano (C4H10), dada pela reação:

<img alt="Eq1" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Butane.svg" style="text-align: center; height: 17.5px;">

Assim, o número de moléculas de oxigênio necessárias para queimar o hidrocarboneto e o número de moléculas de cada produto formado dependem da composição do hidrocarboneto.

O calor de combustão governa a estabilidade relativa de hidrocarbonetos ramificados com a mesma fórmula molecular. A diferença na estrutura surge devido aos grupos metil ligados em diferentes posições ao longo da cadeia de hidrocarbonetos. A quantidade de energia térmica liberada diminui com o aumento da ramificação, onde o altamente ramificado 2,2-dimetil-hexano gera baixa energia em comparação com o octano. Portanto, o octano não ramificado é menos estável do que sua contraparte ramificada.

Entalpia e Calor de Reação

A combustão, comumente conhecida como queima, é uma reação na qual uma substância reage com um agente oxidante, que na maioria dos casos é o oxigênio molecular, para ...

Education

JoVE Core

Organic Chemistry

Química Orgânica

Unit 1

Unidade 1

Thermodynamics and Chemical Kinetics

Termodinâmica e Cinética Química

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

combustion heat and hydrocarbon stability

Combustion is a burning reaction that releases energy as heat, light, or sound. It happens when a substance reacts with an oxidizing agent, usually molecular oxygen. The heat of combustion is also called the enthalpy of combustion. The energy released when one mole of a substance undergoes complete combustion at constant pressure is the molar heat of combustion.

Combustion reactions are exothermic, so they give off energy. For that reason, the sign of ΔH is negative. In 1772, Antoine Lavoisier found that burned sulfur products weighed more than the original sulfur. He suggested that sulfur combined with air. After Joseph Priestley identified oxygen in 1774, Lavoisier concluded that sulfur combined with oxygen in air. He therefore defined combustion as combining with oxygen.

Hydrocarbon fuels such as natural gas and coal also undergo combustion. The amount of energy released can change with the fuel. For example, methane (CH4) releases less heat energy than butane (C4H10). The number of oxygen molecules needed and the number of product molecules formed depend on the hydrocarbon’s composition.

The heat of combustion also helps explain the stability of branched hydrocarbons with the same molecular formula. Branching comes from methyl groups attached at different positions along the hydrocarbon chain. As branching increases, the amount of heat energy released decreases. For example, highly branched 2,2-dimethylhexane releases less energy than octane. This means unbranched octane is less stable than its branched form.

Combustion Heat and Hydrocarbon Stability

combustion-heat-and-hydrocarbon-stability

chemical reactions

Stoichiometry, Rates, and Equilibrium

Stoichiometry, reaction rates, and equilibrium explain how chemical reactions are measured and predicted. A balanced chemical equation shows the chemical formulas of the reactants and products in a chemical change. It also gives the relative amounts of each substance involved. Reaction stoichiometry helps predict how much reactant is needed to make a desired amount of product. It can also show how much product forms from a given amount of reactant. The amounts shown by a balanced equation are...

enthalpy and heat of reaction

Combustion is a burning reaction that releases energy as heat, light, or sound. It happens when a substance reacts with an oxidizing agent, usually molecular oxygen. The heat of combustion is also called the enthalpy of combustion. The energy released when one mole of a substance undergoes complete combustion at constant pressure is the molar heat of combustion. Combustion reactions are exothermic, so they give off energy. For that reason, the sign of ΔH is negative. In 1772, Antoine Lavoisier...

energetics of solution formation

Why Some Solutions Form Spontaneously

Solution formation can happen spontaneously under the right conditions. A spontaneous process is one that occurs without energy from an outside source. During dissolution, solute and solvent particles separate and then attract each other in a new way. The energy change depends on the balance of intermolecular forces, which are the attractions between particles. If the solute-solute and solvent-solvent forces are about the same as the new solute-solvent forces, the solution forms with no energy...

entropy and solvation

Hydrophobic Effects in Solvation

Solvation is the process of surrounding a solute with solvent. It spreads the solute evenly through the solvent. A useful rule is that like dissolves like. A good solvent has molecular features similar to the substance being dissolved. Polar solvents dissolve polar solutes, and apolar solvents dissolve apolar solutes. A polar solvent has a high dielectric constant, with ε greater than or equal to 15. An apolar solvent has a low dielectric constant. The dielectric constant is defined by the...

gibbs free energy and thermodynamic favorability

gibbs-free-energy-and-thermodynamic-favorability

Temperature Effects on Gibbs Free Energy

Temperature changes can decide whether a process is spontaneous. This idea applies to phase changes and to some chemical reactions. The key link is Gibbs free energy, which connects free energy change with enthalpy and entropy. The sign of the Gibbs free energy change, or ΔG, depends on the signs of enthalpy change (ΔH) and entropy change (ΔS), and sometimes on the absolute temperature. Because temperature is measured in kelvin, it is always a positive value. That means there are four main...

chemical and solubility equilibria

How Solubility Depends on Free Energy

Solubility depends on free energy changes when a solute dissolves in a solvent. The free energy change for this process is called the free energy of solution, ΔGsolution. When ΔGsolution is less than zero, solution formation is favorable. When it is greater than zero, solution formation is unfavorable. For complete dissolution to happen, the Gibbs energy change must be negative. This means the balance of forces in the mixture must favor solution formation. The amount a solute dissolves in a...

rate law and reaction order

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...

effect of temperature change on reaction rate

effect-of-temperature-change-on-reaction-rate

Arrhenius Equation in Reaction Speed

The Arrhenius equation connects activation energy, temperature, and the rate constant, k, for many chemical reactions. It helps explain why some reactions move slowly while others happen faster. In this equation, R is the ideal gas constant, with a value of 8.314 J/mol·K. T is the temperature in kelvin, Ea is the activation energy in joules per mole, e is the constant 2.7183, and A is the frequency factor. The frequency factor, A, describes how often reacting molecules collide and how well...

multi-step reactions

Reaction Mechanisms and Rate-Determining Steps

Chemical reactions can happen in a sequence of steps. A balanced chemical equation shows the reactants and products, but it does not show how the reaction occurs at the molecular level. The reaction mechanism, or reaction path, gives that step-by-step detail. Each step in a mechanism is an elementary reaction. These elementary reactions happen in order, and together they add up to the balanced equation for the overall reaction. In a multistep mechanism, one step is often much slower than the...

bond dissociation energy and activation energy

bond-dissociation-energy-and-activation-energy

How Bond Strength Shapes Reaction Speed

Bond strength and activation energy help explain why organic reactions happen. Bond energy is the energy needed to break a bond homolytically, which means each atom takes one of the shared electrons. When a bond value is given for one specific bond, it is called bond dissociation energy. When it is given for a bond type across many compounds, it is called average bond energy. Bond dissociation energy usually follows a clear pattern. A single bond is weaker than a double bond, and a double bond...

energy diagrams, transition states, and intermediates

energy-diagrams-transition-states-and-intermediates

Reaction Coordinate Diagrams in Chemical Reactions

Reaction coordinate diagrams show how energy changes during a chemical reaction. These free-energy graphs use the horizontal axis to show reaction progress. Points near the reactants look like the reactants, while points near the products look like the products. The peaks on the diagram represent stable structures with measurable lifetimes. Other points on the curve are unstable and cannot be isolated. The high-energy, unstable peak is called the transition state, or activated complex. At the...

predicting reaction outcomes

How Temperature Changes Reaction Rate

Reaction rate and reaction pathway are key ideas in chemical kinetics. Thermodynamics, by contrast, focuses on the state of a system. It describes the properties, behavior, and components of reactants and products. It does not describe the path of the process or the speed of the reaction. Kinetics gives information on the atomic or molecular level, while thermodynamics focuses on the energetics of the reactants and products. Thermodynamics can show what may happen in a reaction, but it does...

key terms and concepts

Learn how combustion releases energy, why heat of combustion is negative, and how hydrocarbon structure affects stability and heat release.

Combustion Heat and Hydrocarbon Stability | JoVE K12

organic chemistry

JoVE Core - Química Orgânica explica conceitos básicos de estereoquímica, reações e mecanismos usando vídeos de animação concisos e fáceis de entender. Além disso, os vídeos de Cientistas em ação demonstram a aplicação de conceitos relacionados em experimentos de pesquisa clássicos e originais realizados nos laboratórios atuais em todo o mundo.

Alabama Course of Study Science

9-12

Environmental Science

Earth and Human Activity

9-12.EVS.4.6

9-12.EVS.4.11.b

Chemical Reactions

A balanced chemical equation provides the information of chemical formulas of the reactants and products involved in the chemical change. A ...

Combustion, commonly known as burning, is a reaction in which a substance reacts with an oxidizing agent, which in most cases is molecular oxygen, to ...

Oxidation-Reduction Reactions

Oxidation&ndash;Reduction Reactions

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Oxidation Numbers

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Acids, Bases and Neutralization Reactions

An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to ...

combustion-energy-a-measure-of-stability-in-alkanes-and-cycloalkanes

Combustion Energy: A Measure of Stability in Alkanes and Cycloalkanes

The low reactivity in alkanes can be attributed to the non-polar nature of C&#8211;C and C&#8211;H &#963; bonds. Alkanes, therefore, were&#160; initially ...

Investigate how changes in properties are indicative of chemical reactions such as hydrochloric acid with a metal, oxidation of metal, combustion, and neutralizing an acid with a base.

Balancing Redox Equations

Electrochemistry is the science involved in the interconversion of electrical and chemical reactions. Such reactions are called reduction-oxidation, or ...

chemical-equilibria-redefining-equilibrium-constant

Chemical Equilibria: Redefining Equilibrium Constant

Recall that the solubility of a sparingly soluble salt increases with the addition of an inert salt in a phenomenon known as the salt effect.
The extent ...

Factors Affecting Activity Coefficient

The activity coefficient of an ion depends on the ionic strength of the solution.
As indicated by the Debye&#8211;H&#252;ckel theory, when the ionic ...

Thermochemical Equations

For a chemical reaction (the system) carried out at constant pressure &ndash; with the only work done caused by expansion or contraction &ndash; the ...

Develop and use models to balance chemical equations and support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

IPC.8.B

Nuclear Fission

Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger ...

Nuclear Power

Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

Nuclear Fusion

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process ...

Nuclear Transmutation

Nuclear transmutation is the conversion of one nuclide into another. It can occur by the radioactive decay of a nucleus, or the reaction of a nucleus with ...

Biological Effects of Radiation

All radioactive nuclides emit high-energy particles or electromagnetic waves. When this radiation encounters living cells, it can cause heating, break ...

Research and communicate the uses, advantages, and disadvantages of nuclear reactions in current technologies.

IPC.8.C

What is Climate?

Climate refers to the prevailing weather conditions in a specific area over an extended period. As the saying goes, &#8220;Climate is what you expect. ...

Global Climate Change

Throughout its ~4.5 billion year history, the Earth has experienced periods of warming and cooling. However, the current drastic increase in global ...

Climate Change

The certainty of climate change remains a public controversy despite the consensus among approximately 97% of active climate researchers, who not only ...

Sustainable Development

As the human population continues to grow and use resources, we must be mindful of our planet&rsquo;s natural limits. Sustainable development provides a ...

Construct and communicate an evidence-based explanation of the environmental impact of the end-products of chemical reactions such as those that may result in degradation of water, soil, air quality, and global climate change.

IPC.8.D

Science concepts. The student knows that changes in matter affect everyday life. The student is expected to:

Combustion Heat and Hydrocarbon Stability

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