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

Энтальпия, сокращенно H, равна сумме внутренней энергии, сокращенно E или U, и произведению давления на объем.
Изменение энтальпии ΔH выражается как разница между энтальпиями продуктов и реагентов. При постоянной температуре и давлении ΔH равна количеству тепловой энергии, которой обмениваются система и окружающая среда, или теплоте реакции.
Когда ΔH положительна, реакции поглощают тепло и являются эндотермическими. Если ΔH отрицательный, реакции выделяют тепло и являются экзотермическими.
Горение является примером экзотермического процесса, при котором вещество сгорает в присутствии окислителя, такого как кислород воздуха, чтобы высвободить энергию в виде тепла.
Выделяемая теплота измеряется как молярная теплота сгорания, которая представляет собой количество тепловой энергии, выделяемой при сжигании одного моля вещества.
Когда углеводород сгорает, углерод и водород из топлива соединяются с молекулярным кислородом, образуя воду и углекислый газ, а также выделяя энергию.
Величина теплоты сгорания для углеводорода увеличивается с увеличением количества атомов углерода в цепи, так как больше углерода доступно для горения и большее количество связей претерпевает изменения.
Например, при сгорании метана, одноуглеродного соединения, вырабатывается меньше тепловой энергии, чем при сжигании бутана, который имеет четыре атома углерода.
Теплота сгорания является критически важным способом определения относительной стабильности углеводородов с той же молекулярной формулой, но разной структурой.
Рассмотрим теплоту сгорания октана, 2-метилгептана и 2,2-диметилгексана.
Эти соединения имеют одинаковое количество атомов углерода, но метильные группы присоединены в разных положениях каждой молекулы.
Октановое число обладает самой большой теплотой сгорания. По мере увеличения разветвления ΔH уменьшается, что позволяет предположить, что разветвление увеличивает стабильность углеводорода.

Энтальпия и теплота реакции

Сгорание, широко известное как горение, представляет собой реакцию, в которой вещество реагирует с окислителем, которым в большинстве случаев является молекулярный кислород, с высвобождением энергии в форме тепла, света или звука. Теплоту сгорания также называют энтальпией сгорания. Энергия, выделяющаяся при полном сгорании одного моля вещества при постоянном давлении, называется молярной теплотой сгорания. Реакции горения экзотермичны; то есть в процессе этих реакций выделяется энергия, и по соглашению о знаках, ΔH для них отрицательно.

В 1772 году французский химик Антуан Лавуазье обнаружил, что продукты сгорания серы весят больше, чем первоначальная масса реагирующего вещества. Он предположил, что сера соединяется с воздухом, что приводит к увеличению веса. Позднее открытие Джозефом Пристли в 1774 году «кислорода» как компонента воздуха привело Лавуазье к мысли, что сера соединяется с кислородом воздуха, что приводит к увеличению его массы. Он пришел к выводу, что горение означает соединение с кислородом. Другими словами, сера сгорела.

Примеры реакций сгорания включают сжигание углеводородного топлива, такого как природный газ и уголь. В случае реакций сгорания с участием углеводородов количество выделяемой энергии варьируется в зависимости от типа горящего топлива.

Например, горение природного газа метана (CH4) определяется реакцией:

<img alt="Eq2" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Methane.svg" style="text-align: center; height: 18px;">

и генерирует меньше тепловой энергии, чем бутан (C4H10), что определяется реакцией:

<img alt="Eq1" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Butane.svg" style="text-align: center; height: 17.5px;">

Таким образом, количество молекул кислорода, необходимое для сгорания углеводорода, и количество молекул каждого образующегося продукта зависят от состава углеводорода.

Теплота сгорания определяет относительную стабильность разветвленных углеводородов с одинаковой молекулярной формулой. Разница в строении возникает из-за того, что метильные группы присоединены в разных положениях углеводородной цепи. Количество выделяемой тепловой энергии уменьшается с ростом ветвления, при этом сильно разветвленный 2,2-диметилгексан генерирует низкую энергию по сравнению с октаном. Следовательно, неразветвленный октан менее стабилен, чем его разветвленный аналог.

Горение, широко известное как горение, представляет собой реакцию, в которой вещество вступает в реакцию с окислителем, которым в большинстве случаев является молекулярный кислород, к...

Education

JoVE Core

Organic Chemistry

Органическая химия

Unit 1

Раздел 1

Thermodynamics and Chemical Kinetics

Термодинамика и химическая кинетика

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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

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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

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

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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

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predicting reaction outcomes

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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 explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

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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Combustion Energy: A Measure of Stability in Alkanes and Cycloalkanes

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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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IPC.8.B

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IPC.8.C

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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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