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'entalpia, abbreviata H, è uguale alla somma dell'energia interna, abbreviata E o U, e al prodotto di pressione e volume.
La variazione di entalpia, ΔH, è espressa come differenza tra le entalpie dei prodotti e i reagenti. A temperatura e pressione costanti, ΔH è uguale alla quantità di energia termica scambiata tra il sistema e l'ambiente circostante, o al calore della reazione.
Quando ΔH è positivo, le reazioni assorbono calore e sono endotermiche. Se ΔH è negativo, le reazioni rilasciano calore e sono esotermiche.
La combustione è un esempio di processo esotermico, in cui una sostanza brucia in presenza di un ossidante come l'ossigeno atmosferico per rilasciare energia sotto forma di calore.
Il calore rilasciato è quantificato come il calore molare di combustione, che è la quantità di energia termica rilasciata bruciando una mole di una sostanza.
Quando un idrocarburo brucia, il carbonio e l'idrogeno del combustibile si combinano con l'ossigeno molecolare per produrre acqua e anidride carbonica, insieme al rilascio di energia.
Il valore del calore di combustione per un idrocarburo aumenta con il numero di atomi di carbonio nella catena, poiché più carbonio è disponibile per la combustione e più legami subiscono cambiamenti.
Ad esempio, la combustione del metano, un composto a carbonio singolo, genera meno energia termica rispetto a quella del butano, che ha quattro atomi di carbonio.
Il calore di combustione è un modo critico per determinare la stabilità relativa degli idrocarburi con la stessa formula molecolare ma strutture diverse.
Si considerino i calori di combustione di ottano, 2-metileptano e 2,2-dimetilesano.
Questi composti hanno lo stesso numero di atomi di carbonio, ma i gruppi metilici sono attaccati in posizioni diverse in ciascuna molecola.
Il numero di ottano ha il più grande calore di combustione. All'aumentare della ramificazione, il ΔH diminuisce, suggerendo che la ramificazione aumenta la stabilità di un idrocarburo.

entalpia e calore di reazione

La combustione è una reazione in cui una sostanza reagisce con un agente ossidante, che nella maggior parte dei casi è l'ossigeno molecolare, per liberare energia sotto forma di calore, luce o suono. Il calore di combustione è anche noto come entalpia di combustione. L'energia rilasciata quando una mole di una sostanza subisce la combustione completa a pressione costante è chiamata calore molare di combustione. Le reazioni di combustione sono esotermiche; cioè rilasciano energia e la loro convenzione di segno ΔH è negativa.

Nel 1772, il chimico francese Antoine Lavoisier, scoprì che i prodotti dello zolfo bruciato pesavano più della massa iniziale del reagente. Postulò quindi che lo zolfo si combinasse con l'aria, provocando un aumento di peso. Successivamente, la scoperta dell' "ossigeno" da parte di Joseph Priestley nel 1774, come componente dell'aria, portò Lavoisier a credere che lo zolfo si combinasse con l'ossigeno nell'aria, portando ad un aumento della sua massa. Concluse che combustione significasse combinarsi con l'ossigeno. In altre parole, lo zolfo subiva la combustione.

Esempi di reazioni di combustione includono la combustione di combustibili idrocarburici come gas naturale e carbone. Nel caso delle reazioni di combustione che coinvolgono idrocarburi, la quantità di energia liberata varia a seconda del tipo di combustibile sottoposto a combustione.

Ad esempio la combustione del gas naturale, metano (CH4), data dalla reazione:

<img alt="Eq2" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Methane.svg" style="text-align: center; height: 18px;">

genera meno energia termica di quella del butano (C4H10), data dalla reazione:

<img alt="Eq1" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Butane.svg" style="text-align: center; height: 17.5px;">

Pertanto il numero di molecole di ossigeno necessarie per bruciare l'idrocarburo e il numero di molecole di ciascun prodotto formato dipendono dalla composizione dell'idrocarburo.

Il calore di combustione governa la stabilità relativa degli idrocarburi ramificati con la stessa formula molecolare. La differenza nella struttura è dovuta ai gruppi metilici attaccati in diverse posizioni lungo la catena idrocarburica. La quantità di energia termica rilasciata diminuisce con l'aumentare della ramificazione, dove il 2,2-dimetilesano altamente ramificato genera una bassa energia rispetto all'ottano. Quindi, l’ottano non ramificato è meno stabile della sua controparte ramificata.

Entalpia e calore di reazione

La combustione, comunemente nota come combustione, è una reazione in cui una sostanza reagisce con un agente ossidante, che nella maggior parte dei casi è l'ossigeno molecolare, per ...

Education

Didattica

JoVE Core

Organic Chemistry

Chimica Organica

Unit 1

Unità 1

Thermodynamics and Chemical Kinetics

Termodinamica e Cinetica Chimica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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

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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

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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

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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

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Arrhenius Equation in Reaction Speed

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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

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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 Organic Chemistry spiega i concetti di base della stereochimica, delle reazioni e dei meccanismi usando video-lezioni concise e facili da capire. Inoltre, i video scientist-in-action dimostrano l'applicazione di tali concetti in esperimenti di ricerca classici e originali eseguiti al giorno d'oggi nei laboratori in tutto il mondo.

Alabama Course of Study Science

9-12

Environmental Science

Earth and Human Activity

9-12.EVS.4.6

9-12.EVS.4.11.b

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Chemical Equilibria: Redefining Equilibrium Constant

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IPC.8.B

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Nuclear Power

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

IPC.8.C

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Climate Change

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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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