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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High School

Chemistry and Physics

IPC.8

IPC.8.A

Enthalpy and Heat of Reaction

Enthalpy, abbreviated H, equals the sum of internal energy, abbreviated E or U, and the product of pressure and volume.
The change in enthalpy, &#916;H, is expressed as the difference between the enthalpies of the products and the reactants. At constant temperature and pressure, &#916;H is equal to the amount of heat energy exchanged between the system and the surroundings, or the heat of the reaction.
When &#916;H is positive, reactions absorb heat and are endothermic. If &#916;H is negative, reactions release heat and are exothermic.
Combustion is an example of an exothermic process, where a substance burns in the presence of an oxidant like atmospheric oxygen to release energy in the form of heat.
The heat released is quantified as the molar heat of combustion, which is the amount of heat energy released on burning one mole of a substance.
When a hydrocarbon burns, the carbon and hydrogen from the fuel combine with molecular oxygen to produce water and carbon dioxide, along with the release of energy.
The value of the heat of combustion for a hydrocarbon increases with the number of carbon atoms in the chain since more carbon is available for burning and more bonds undergo changes.
For example, the combustion of methane, a single-carbon compound, generates less heat energy than that of butane, which has four carbon atoms.
The heat of combustion is a critical way to determine the relative stability of hydrocarbons with the same molecular formula but different structures.
Consider the heats of combustion of octane, 2-methylheptane, and 2,2-dimethylhexane.
These compounds have the same number of carbon atoms, but the methyl groups are attached at different positions in each molecule.
Octane has the largest heat of combustion. As the branching increases the &#916;H decreases, suggesting that branching increases the stability of a hydrocarbon.

enthalpy and heat of reaction

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 liberate energy in the form of heat, light, or sound. The heat of combustion is also known as the enthalpy of combustion. The energy released when one mole of a substance undergoes complete combustion at constant pressure is called molar heat of combustion. Combustion reactions are exothermic; that is, they release energy, and their &#916;H sign convention is negative.
In 1772, French chemist Antoine Lavoisier, discovered that the products of burnt sulfur weighed more than the initial mass of the reactant. He postulated that sulfur combined with air, which resulted in the increased weight. Later, Joseph Priestley's discovery of "oxygen" in 1774, as a component of air, led Lavoisier to believe that sulfur combined with oxygen in the air, leading to an increase in its mass. He concluded that combustion means combining with oxygen. In other words, sulfur underwent combustion.
Examples of combustion reactions include the burning of hydrocarbon fuels like natural gas and coal. In the case of combustion reactions involving hydrocarbons, the amount of energy released varies depending on the type of fuel undergoing combustion.
For example, the combustion of natural gas, methane (CH4), given by the reaction:
<img alt="Chemical equation diagram; methane combustion: CH&#8324; + 2O&#8322; &#8594; CO&#8322; + 2H&#8322;O; stoichiometry concept." src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Methane.svg" style="text-align: center; height: 18px;" data-altupdatedat="1754904545000" data-alt="Chemical reaction equation: CH&#8324; + 2 O&#8322; &#8594; CO&#8322; + 2 H&#8322;O; combustion, stoichiometry concept." />
generates less heat energy than that of butane (C4H10), given by the reaction:
<img alt="Butane combustion equation, balanced formula, chemical reaction diagram." src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Butane.svg" style="text-align: center; height: 17.5px;" data-altupdatedat="1754904545000" data-alt="Butane combustion reaction equation, C4H10 + 6.5O2 &#8594; 4CO2 + 5H2O, chemical process." />
Thus the number of oxygen molecules required to combust the hydrocarbon and the number of molecules of each product formed depend on the hydrocarbon composition.
The heat of combustion governs the relative stability of branched hydrocarbons with the same molecular formula. The difference in structure arises due to methyl groups attached at different positions along the hydrocarbon chain. The amount of heat energy released decreases with increasing branching, where the highly branched 2,2-dimethylhexane generates low energy compared to octane. Hence, unbranched octane is less stable than its branched counterpart.

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 ...

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JoVE Core

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Unit 1

Thermodynamics and Chemical Kinetics

KEY TERMS AND CONCEPTS

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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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...

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

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IPC.8.D

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Combustion Heat and Hydrocarbon Stability

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