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

K12 Standards

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Chemistry and Physics

IPC.8

IPC.8.A

Enthalpy and Heat of Reaction

Die Enthalpie, abgekürzt H, entspricht der Summe der inneren Energie, abgekürzt E oder U, und dem Produkt aus Druck und Volumen.
Die Änderung der Enthalpie, ΔH, wird ausgedrückt als Differenz zwischen den Enthalpien der Produkte und den Reaktanten. Bei konstanter Temperatur und konstantem Druck entspricht ΔH der Menge an Wärmeenergie, die zwischen dem System und der Umgebung ausgetauscht wird, oder der Wärme der Reaktion.
Wenn ΔH positiv ist, absorbieren Reaktionen Wärme und sind endotherm. Ist ΔH negativ, setzen die Reaktionen Wärme frei und sind exotherm.
Die Verbrennung ist ein Beispiel für einen exothermen Prozess, bei dem eine Substanz in Gegenwart eines Oxidationsmittels wie Luftsauerstoff verbrennt, um Energie in Form von Wärme freizusetzen.
Die freigesetzte Wärme wird als molare Verbrennungswärme quantifiziert, d. h. die Menge an Wärmeenergie, die beim Verbrennen eines Mols einer Substanz freigesetzt wird.
Wenn ein Kohlenwasserstoff verbrennt, verbinden sich der Kohlenstoff und der Wasserstoff aus dem Kraftstoff mit molekularem Sauerstoff zu Wasser und Kohlendioxid, zusammen mit der Freisetzung von Energie.
Der Wert der Verbrennungswärme für einen Kohlenwasserstoff steigt mit der Anzahl der Kohlenstoffatome in der Kette, da mehr Kohlenstoff für die Verbrennung zur Verfügung steht und sich mehr Bindungen ändern.
So entsteht bei der Verbrennung von Methan, einer Einzelkohlenstoffverbindung, weniger Wärmeenergie als bei Butan, das aus vier Kohlenstoffatomen besteht.
Die Verbrennungswärme ist ein kritisches Mittel, um die relative Stabilität von Kohlenwasserstoffen mit der gleichen Summenformel, aber unterschiedlichen Strukturen zu bestimmen.
Betrachten Sie die Verbrennungswärme von Oktan, 2-Methylheptan und 2,2-Dimethylhexan.
Diese Verbindungen haben die gleiche Anzahl von Kohlenstoffatomen, aber die Methylgruppen sind in jedem Molekül an unterschiedlichen Positionen gebunden.
Oktan hat die größte Verbrennungswärme. Mit zunehmender Verzweigung nimmt das ΔH ab, was darauf hindeutet, dass die Verzweigung die Stabilität eines Kohlenwasserstoffs erhöht.

enthalpie und reaktionswärme

Verbrennung ist eine Reaktion, bei der eine Substanz mit einem Oxidationsmittel, in den meisten Fällen molekularem Sauersubstanz, reagiert, um Energie in Form von Wärme, Licht oder Schall freizusetzen. Die Verbrennungswärme wird auch als Verbrennungsenthalpie bezeichnet. Die Energie, die freigesetzt wird, wenn ein Mol einer Substanz bei konstantem Druck vollständig verbrannt wird, wird als molare Verbrennungswärme bezeichnet. Verbrennungsreaktionen sind exotherm; das heißt, sie setzen Energie frei und ihre ΔH-Vorzeichenkonvention ist negativ.

Im Jahr 1772 entdeckte der französische Chemiker Antoine Lavoisier, dass die Produkte des verbrannten Schwefels mehr wogen als die ursprüngliche Masse des Reaktanten. Er postulierte, dass sich Schwefel mit Luft verband, was zu einer Gewichtszunahme führte. Später führte Joseph Priestleys Entdeckung von „Sauersubstanz“ als Bestandteil der Luft im Jahr 1774 Lavoisier zu der Annahme, dass sich Schwefel mit dem Sauersubstanz in der Luft verbindet, was zu einer Zunahme seiner Masse führt. Er kam zu dem Schluss, dass Verbrennung die Verbindung mit Sauersubstanz bedeutet. Mit anderen Worten: Schwefel wurde verbrannt.

Beispiele für Verbrennungsreaktionen sind die Verbrennung von Kohlenwassersubstanzbrennsubstanzen wie Erdgas und Kohle. Bei Verbrennungsreaktionen von Kohlenwassersubstanzen variiert die freigesetzte Energiemenge je nach Art des zu verbrennenden Brennsubstanzen.

Zum Beispiel die Verbrennung von Erdgas, Methan (CH4), gegeben durch die Reaktion:

<img alt="Eq2" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Methane.svg" style="text-align: center; height: 18px;">

erzeugt weniger Wärmeenergie als Butan (C4H10), gegeben durch die Reaktion:

<img alt="Eq1" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Butane.svg" style="text-align: center; height: 17.5px;">

Somit hängen die Anzahl der Sauersubstanzmoleküle, die zur Verbrennung des Kohlenwassersubstanzen erforderlich sind, und die Anzahl der Moleküle jedes gebildeten Produkts von der Kohlenwassersubstanzzusammensetzung ab.

Die Verbrennungswärme bestimmt die relative Stabilität verzweigter Kohlenwassersubstanze mit derselben Summenformel. Der Strukturunterschied entsteht durch Methylgruppen, die an unterschiedlichen Positionen entlang der Kohlenwassersubstanzkette angebracht sind. Die Menge der freigesetzten Wärmeenergie nimmt mit zunehmender Verzweigung ab, wobei das hochverzweigte 2,2-Dimethylhexan im Vergleich zu Oktan eine geringe Energie erzeugt. Daher ist unverzweigtes Oktan weniger stabil als sein verzweigtes Gegenstück.

Enthalpie und Reaktionswärme

Die Verbrennung, allgemein bekannt als Verbrennung, ist eine Reaktion, bei der eine Substanz mit einem Oxidationsmittel, bei dem es sich in den meisten Fällen um molekularen Sauerstoff handelt, reagiert, um ...

Education

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Unit 1

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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.

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Combustion Heat and Hydrocarbon Stability

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