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

L’enthalpie, abrégée H, est égale à la somme de l’énergie interne, abrégée E ou U, et du produit de la pression et du volume.
La variation d’enthalpie, ΔH, est exprimée comme la différence entre les enthalpies des produits et les réactifs. À température et pression constantes, ΔH est égal à la quantité d’énergie thermique échangée entre le système et l’environnement, ou à la chaleur de la réaction.
Lorsque ΔH est positif, les réactions absorbent la chaleur et sont endothermiques. Si ΔH est négatif, les réactions libèrent de la chaleur et sont exothermiques.
La combustion est un exemple de processus exothermique, où une substance brûle en présence d’un oxydant comme l’oxygène atmosphérique pour libérer de l’énergie sous forme de chaleur.
La chaleur dégagée est quantifiée comme la chaleur molaire de combustion, qui est la quantité d’énergie thermique libérée lors de la combustion d’une mole d’une substance.
Lorsqu’un hydrocarbure brûle, le carbone et l’hydrogène du carburant se combinent avec de l’oxygène moléculaire pour produire de l’eau et du dioxyde de carbone, ainsi que la libération d’énergie.
La valeur de la chaleur de combustion d’un hydrocarbure augmente avec le nombre d’atomes de carbone dans la chaîne, car plus de carbone est disponible pour la combustion et plus de liaisons subissent des changements.
Par exemple, la combustion du méthane, un composé à un seul atome de carbone, génère moins d’énergie thermique que celle du butane, qui a quatre atomes de carbone.
La chaleur de combustion est un moyen essentiel de déterminer la stabilité relative des hydrocarbures ayant la même formule moléculaire mais des structures différentes.
Considérez les chaleurs de combustion de l’octane, du 2-méthylheptane et du 2,2-diméthylhexane.
Ces composés ont le même nombre d’atomes de carbone, mais les groupes méthyle sont attachés à des positions différentes dans chaque molécule.
L’octane a la plus grande chaleur de combustion. Au fur et à mesure que la ramification augmente, le ΔH diminue, ce qui suggère que la ramification augmente la stabilité d’un hydrocarbure.

enthalpie et chaleur de réaction

La combustion, communément appelée combustion, est une réaction dans laquelle une substance réagit avec un agent oxydant, qui est dans la plupart des cas de l'oxygène moléculaire, pour libérer de l'énergie sous forme de chaleur, de lumière ou de son. La chaleur de combustion est également connue sous le nom d’enthalpie de combustion. L’énergie libérée lorsqu’une mole d’une substance subit une combustion complète à pression constante est appelée chaleur molaire de combustion. Les réactions de combustion sont exothermiques ; c'est-à-dire qu'ils libèrent de l'énergie et leur convention de signe ΔH est négative.

En 1772, le chimiste français Antoine Lavoisier découvrit que les produits du soufre brûlé pesaient plus que la masse initiale du réactif. Il a postulé que le soufre se combinait avec l’air, ce qui entraînait une augmentation du poids. Plus tard, la découverte de « l'oxygène » par Joseph Priestley en 1774, en tant que composant de l'air, a amené Lavoisier à croire que le soufre se combinait avec l'oxygène de l'air, entraînant une augmentation de sa masse. Il a conclu que la combustion signifie la combinaison avec l'oxygène. En d’autres termes, le soufre subit une combustion.

Des exemples de réactions de combustion incluent la combustion d’hydrocarbures comme le gaz naturel et le charbon. Dans le cas de réactions de combustion impliquant des hydrocarbures, la quantité d'énergie libérée varie en fonction du type de combustible en cours de combustion.

Par exemple, la combustion du gaz naturel, le méthane (CH4), donnée par la réaction :

<img alt="Eq2" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Methane.svg" style="text-align: center; height: 18px;">

génère moins d'énergie thermique que celle du butane (C4H10), donnée par la réaction :

<img alt="Eq1" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Butane.svg" style="text-align: center; height: 17.5px;">

Ainsi, le nombre de molécules d'oxygène nécessaires à la combustion de l'hydrocarbure et le nombre de molécules de chaque produit formé dépendent de la composition de l'hydrocarbure.

La chaleur de combustion régit la stabilité relative des hydrocarbures ramifiés ayant la même formule moléculaire. La différence de structure est due aux groupes méthyle attachés à différentes positions le long de la chaîne hydrocarbonée. La quantité d'énergie thermique libérée diminue avec l'augmentation de la ramification, le 2,2-diméthylhexane hautement ramifié générant une faible énergie par rapport à l'octane. Par conséquent, l’octane non ramifié est moins stable que son homologue ramifié.

Enthalpie et chaleur de réaction

La combustion, communément appelée combustion, est une réaction dans laquelle une substance réagit avec un agent oxydant, qui dans la plupart des cas est de l’oxygène moléculaire, pour ...

Education

Enseignement

JoVE Core

Organic Chemistry

Chimie organique

Unit 1

Unité 1

Thermodynamics and Chemical Kinetics

Thermodynamique et cinétique chimique

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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 Chimie Organique explique les concepts de base de la stéréochimie, des réactions et des mécanismes à l'aide de leçons vidéo animées concises et faciles à comprendre. De plus, les vidéos de scientifiques à l'action présentent l'application de ces concepts grâce à des expériences de recherche classiques et originales réalisées dans des laboratoires modernes.

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

Earth&rsquo;s atmosphere contains about 20% molecular oxygen, O2, a chemically reactive gas that plays an essential ...

Oxidation Numbers

In redox reactions, the transfer of electrons occurs between reacting species. Electron transfer is described by a hypothetical number called the ...

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

Description

Transcript

Tags

More videos by Standards