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

La entalpía, abreviada H, es igual a la suma de la energía interna, abreviada E o U, y el producto de la presión y el volumen.
El cambio en la entalpía, ΔH, se expresa como la diferencia entre las entalpías de los productos y los reactivos. A temperatura y presión constantes, ΔH es igual a la cantidad de energía térmica intercambiada entre el sistema y el entorno, o el calor de la reacción.
Cuando ΔH es positivo, las reacciones absorben calor y son endotérmicas. Si ΔH es negativo, las reacciones liberan calor y son exotérmicas.
La combustión es un ejemplo de proceso exotérmico, en el que una sustancia se quema en presencia de un oxidante como el oxígeno atmosférico para liberar energía en forma de calor.
El calor liberado se cuantifica como el calor molar de combustión, que es la cantidad de energía térmica liberada al quemar un mol de una sustancia.
Cuando un hidrocarburo se quema, el carbono y el hidrógeno del combustible se combinan con el oxígeno molecular para producir agua y dióxido de carbono, junto con la liberación de energía.
El valor del calor de combustión de un hidrocarburo aumenta con el número de átomos de carbono en la cadena, ya que hay más carbono disponible para quemar y más enlaces sufren cambios.
Por ejemplo, la combustión de metano, un compuesto de un solo carbono, genera menos energía térmica que la del butano, que tiene cuatro átomos de carbono.
El calor de combustión es una forma crítica de determinar la estabilidad relativa de hidrocarburos con la misma fórmula molecular pero diferentes estructuras.
Considere los calores de combustión de octano, 2-metilheptano y 2,2-dimetilhexano.
Estos compuestos tienen el mismo número de átomos de carbono, pero los grupos metilo están unidos en diferentes posiciones en cada molécula.
Octane tiene el mayor calor de combustión. A medida que aumenta la ramificación, el ΔH disminuye, lo que sugiere que la ramificación aumenta la estabilidad de un hidrocarburo.

entalpía y calor de reacción.

La combustión, comúnmente conocida como quemar, es una reacción en la que una sustancia reacciona con un agente oxidante, que en la mayoría de los casos es oxígeno molecular, para liberar energía en forma de calor, luz o sonido. El calor de combustión también se conoce como entalpía de combustión. La energía liberada cuando un mol de una sustancia se quema completamente a presión constante se llama calor molar de combustión. Las reacciones de combustión son exotérmicas; es decir, liberan energía y su convención de signos ΔH es negativa.


 En 1772, el químico francés Antoine Lavoisier descubrió que los productos del azufre quemado pesaban más que la masa inicial del reactivo. Postuló que el azufre se combinaba con el aire, lo que daba como resultado un aumento de peso. Posteriormente, el descubrimiento del "oxígeno" por Joseph Priestley en 1774, como componente del aire, llevó a Lavoisier a creer que el azufre se combinaba con el oxígeno del aire, provocando un aumento de su masa. Concluyó que la combustión significa combinarse con el oxígeno. En otras palabras, el azufre se quemó.


 Ejemplos de reacciones de combustión incluyen la quema de combustibles de hidrocarburos como el gas natural y el carbón. En el caso de reacciones de combustión que involucran hidrocarburos, la cantidad de energía liberada varía dependiendo del tipo de combustible que se quema.


 Por ejemplo, la combustión del gas natural, metano (CH4), dada por la reacción:


 <img alt="Eq2" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Methane.svg" style="text-align: center; height: 18px;">


 Genera menos energía térmica que la del butano (C4H10), dada por la reacción:


 <img alt="Eq1" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Butane.svg" style="text-align: center; height: 17.5px;">


 Por tanto, el número de moléculas de oxígeno necesarias para quemar el hidrocarburo y el número de moléculas de cada producto formado dependen de la composición del hidrocarburo.



 El calor de combustión gobierna la estabilidad relativa de los hidrocarburos ramificados con la misma fórmula molecular. La diferencia de estructura surge debido a los grupos metilo unidos en diferentes posiciones a lo largo de la cadena de hidrocarburos. La cantidad de energía térmica liberada disminuye al aumentar la ramificación, donde el 2,2-dimetilhexano altamente ramificado genera poca energía en comparación con el octano. Por tanto, el octano no ramificado es menos estable que su homólogo ramificado.

Entalpía y calor de reacción.

La combustión, comúnmente conocida como combustión, es una reacción en la que una sustancia reacciona con un agente oxidante, que en la mayoría de los casos es el oxígeno molecular, para ...

Education

Educación

JoVE Core

Organic Chemistry

Química Orgánica

Unit 1

Unidad 1

Thermodynamics and Chemical Kinetics

Termodinámica y cinética química

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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

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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

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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 Orgánica explica los conceptos básicos de la estereoquímica, las reacciones y sus mecanismos mediante lecciones de vídeo animadas, concisas y fáciles de entender. Además, los vídeos de científicos en acción muestran la aplicación de esos conceptos en experimentos de investigación clásicos y originales, desarrollados actualmente en laboratorios de todo el 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

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Balancing Redox Equations

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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.
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Factors Affecting Activity Coefficient

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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 ...

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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 ...

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

IPC.8.C

What is Climate?

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Global Climate Change

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Climate Change

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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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