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

المحتوى الحراري ، المختصر H ، يساوي مجموع الطاقة الداخلية ، والاختصار E أو U ، وحاصل ضرب الضغط والحجم.
يتم التعبير عن التغير في المحتوى الحراري ، ΔH ، على أنه الفرق بين المحتوى الحراري للمنتجات والمواد المتفاعلة. عند درجة حرارة وضغط ثابتين ، ΔH يساوي كمية الطاقة الحرارية المتبادلة بين النظام والمناطق المحيطة ، أو حرارة التفاعل.
عندما يكون ΔH موجبا ، تمتص التفاعلات الحرارة وتكون ماصاة للحرارة. إذا كانت ΔH سالبة ، فإن التفاعلات تطلق الحرارة وتكون طاردة للحرارة.
الاحتراق هو مثال على عملية طاردة للحرارة ، حيث تحترق مادة في وجود مؤكسد مثل الأكسجين في الغلاف الجوي لإطلاق الطاقة على شكل حرارة.
يتم قياس الحرارة المنبعثة على أنها الحرارة المولية للاحتراق ، وهي كمية الطاقة الحرارية المنبعثة عند حرق مول واحد من المادة.
عندما يحترق الهيدروكربون ، يتحد الكربون والهيدروجين من الوقود مع الأكسجين الجزيئي لإنتاج الماء وثاني أكسيد الكربون ، إلى جانب إطلاق الطاقة.
تزداد قيمة حرارة الاحتراق للهيدروكربون مع عدد ذرات الكربون في السلسلة نظرا لتوفر المزيد من الكربون للحرق والمزيد من الروابط تخضع للتغييرات.
على سبيل المثال ، يولد احتراق الميثان ، وهو مركب أحادي الكربون ، طاقة حرارية أقل من احتراق البيوتان ، الذي يحتوي على أربع ذرات كربون.
تعتبر حرارة الاحتراق طريقة حاسمة لتحديد الاستقرار النسبي للهيدروكربونات بنفس الصيغة الجزيئية ولكن بهياكل مختلفة.
ضع في اعتبارك حرارة احتراق الأوكتان و 2-ميثيل هيبتان و 2،2-ثنائي ميثيل هكسان.
تحتوي هذه المركبات على نفس عدد ذرات الكربون ، لكن مجموعات الميثيل مرتبطة في مواضع مختلفة في كل جزيء.
الأوكتان لديه أكبر حرارة احتراق. مع زيادة التفرع ، ينخفض ΔH ، مما يشير إلى أن التفرع يزيد من استقرار الهيدروكربون.

المحتى الحراري وحرارة التفاعل

<div style="direction:rtl">الاحتراق، المعروف أيضًا بالاشتعال، هو تفاعل تتفاعل فيه المادة مع عامل مؤكسد، يكون في معظم الحالات الأكسجين الجزيئي، لتحرير الطاقة في شكل حرارة أو ضوء أو صوت. تُعرف حرارة الاحتراق أيضًا باسم المحتوى الحراري للاحتراق. تسمى الطاقة المنطلقة عندما يتعرض مول واحد من المادة للاحتراق الكامل عند ضغط ثابت بالحرارة المولية للاحتراق. تفاعلات الاحتراق طاردة للحرارة؛ أي أنها تطلق طاقة، وتكون إشارة ΔH الخاصة بها سالبة. في عام 1772، اكتشف الكيميائي الفرنسي أنطوان لافوازييه أن نواتج الكبريت المحترق تزن أكثر من الكتلة الأولية للمادة المتفاعلة. وافترض أن الكبريت يتحد مع الهواء مما يؤدي إلى زيادة الوزن. لاحقًا، أدى اكتشاف جوزيف بريستلي لـ "الأكسجين" عام 1774، كأحد مكونات الهواء، إلى اعتقاد لافوازييه أن الكبريت يتحد مع الأكسجين الموجود في الهواء، مما يؤدي إلى زيادة كتلته. وخلص إلى أن الاحتراق يعني الاتحاد مع الأكسجين. وبعبارة أخرى، خضع الكبريت للاحتراق. تشمل أمثلة تفاعلات الاحتراق حرق الوقود الهيدروكربوني مثل الغاز الطبيعي والفحم. في حالة تفاعلات الاحتراق التي تشتمل على الهيدروكربونات، تختلف كمية الطاقة المنطلقة اعتمادًا على نوع الوقود الجاري احتراقه. على سبيل المثال، احتراق الغاز الطبيعي، الميثان (CH4)، الناتج عن التفاعل: <img alt="Eq2" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Methane.svg" style="text-align: center; height: 18px;"> يولد طاقة حرارية أقل من تلك التي يولدها البيوتان (C4H10)، وذلك من خلال التفاعل: <img alt="Eq1" src="/files/ftp_upload/11692/11692_Reaction%20Equation-Combustion%20of%20Butane.svg" style="text-align: center; height: 17.5px;"> وبالتالي فإن عدد جزيئات الأكسجين اللازمة لاحتراق الهيدروكربون وعدد جزيئات كل منتج يعتمد على تركيبة الهيدروكربون. تتحكم حرارة الاحتراق في الاستقرار النسبي للهيدروكربونات المتفرعة التي لها نفس الصيغة الجزيئية. ينشأ الاختلاف في البنية بسبب مجموعات الميثيل المرتبطة في مواقع مختلفة على طول السلسلة الهيدروكربونية. تتناقص كمية الطاقة الحرارية المنطلقة مع زيادة التفرع، حيث يولد ثنائي ميثيل هكسان 2,2 عالي التفرع طاقة منخفضة مقارنة بالأوكتان. وبالتالي، فإن الأوكتان غير المتفرع أقل استقرارًا من نظيره المتفرع.</div>

الاحتراق ، المعروف باسم الاحتراق ، هو تفاعل تتفاعل فيه المادة مع عامل مؤكسد ، وهو في معظم الحالات أكسجين جزيئي ، إلى ...

Education

JoVE Core

Organic Chemistry

الكيمياء العضوية

Unit 1

الوحدة الأولى

Thermodynamics and Chemical Kinetics

الديناميكا الحرارية والحركية الكيميائية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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

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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

energy-diagrams-transition-states-and-intermediates

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 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, commonly known as burning, is a reaction in which a substance reacts with an oxidizing agent, which in most cases is molecular oxygen, to ...

Oxidation-Reduction Reactions

Oxidation&ndash;Reduction Reactions

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Oxidation Numbers

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Acids, Bases and Neutralization Reactions

An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to ...

combustion-energy-a-measure-of-stability-in-alkanes-and-cycloalkanes

Combustion Energy: A Measure of Stability in Alkanes and Cycloalkanes

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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

The activity coefficient of an ion depends on the ionic strength of the solution.
As indicated by the Debye&#8211;H&#252;ckel theory, when the ionic ...

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 ...

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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 ...

Nuclear Transmutation

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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

Throughout its ~4.5 billion year history, the Earth has experienced periods of warming and cooling. However, the current drastic increase in global ...

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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