Name: pressure-volume work in chemistry
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 30 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Quantifying Work

لنتذكر أن الطاقة الداخلية للنظام محددة كمجموع الحرارة والشغل. يتم قياس الحرارة باستخدام مقياس السعرات الحرارية،ولكن كيف يتم قياس الشغل كمياًالشغل هو نتيجة القوة التي يمر بها الكائن عن طريق السحب أو الدفع أو الرفع لمسافة. وبالتالي الشغل يساوي القوة ضرب المسافة.إذا اصطدم مضرب الجولف بالكرة على ارتفاع 100 قدم،فهذا يعني أنه قد قام بشغل. تنتقل الطاقة من المضرب إلى الكرة. في هذه الحالة،الكرة هي النظام الذي تؤدي المناطق المحيطة عليه الشغل.في التفاعلات الكيميائية،يمكن أن يكون الشغل مرتبط بتغيرات فيزيائية أو كيميائية متعددة في النظام. من أنواع الشغل الشائعة،شغل الضغط-حجم. لنضع في اعتبارنا،الاحتراق في أسطوانة المحرك.لا يولد الاحتراق الحرارة فحسب،بل أنها أيضاًتنتج الغاز،الذي يؤدي الشغل. عندما يتمدد حجم الغاز في الاسطوانة،يدفع الضغط المكبس لأسفل ضد القوى الخارجية للمحيط. يُعرَف الضغط،بأنه قوة تعمل فوق منطقة.هنا القوة من الغازات المتمددة،يتم توزيعها على مساحة قاعدة المكبس. بمعنى آخر،القوة هي منطقة الضغط. وهكذا،الشغل،وهو القوة مضروبة في المسافة،يمكن إعادة كتابته بضرب الضغط في المساحة والمسافة التي يعمل عليها.بالنسبة لأسطوانة المحرك،هذا هو فرق الارتفاع لتحرك المكبس. باعتبار أنّالمساحة مضروبة في الارتفاع تمثل حجم الاسطوانة،فإنّمعادلة الشغل يمكن تعريفها على أنها،الضغط ضرب حجم التغيير في النظام. ومع ذلك،لأن الحجم يزيد والمكبس يتم دفعه إلى الأسفل،يبذل النظام في الواقع شغل علي المناطق المحيطة والذي نعتبر قيمتة سالبة اصطلاحاًلذلك يتم تعريف الشغل بأنه سالب الضغط P،مضروبا بدلتا V،أي التغيير في الحجم النهائي والأبتدائي،والذي حدث أثناء التمدد.وحدة شغل ضغط-الحجم عادة ما يتم تعريفها على أنها ضغط جوي-لتر. يمكن تحويل هذه الوحدة إلى الوحدة التقليدية للطاقة،جول،باستخدام عامل التحويل واحد لتر-ضغط جوي يساوي 101.3 جول.

<ol>
	<li>شميدت رور ، كلاوس. "أعمال التمدد بدون الضغط الخارجي والديناميكا الحرارية من حيث العمليات شبه الساكنة التي لا رجعة فيها". مجلة التربية الكيميائية 91 ، العدد 3 (2014): 402-409.</li>
	<li>جيسلاسون ، إريك أ. ، ونورمان سي كريج. "التعريفات العامة للعمل والحرارة في العمليات الديناميكية الحرارية". مجلة التربية الكيميائية 64 ، العدد 8 (1987): 660.</li>
	<li>O'Loane ، J. Kenneth. "التغيرات الثابتة: تغييرات قابلة للعكس ولا رجعة فيها تنطوي فقط على عمل حجم الضغط". مجلة التعليم الكيميائي 30 ، العدد 4 (1953): 190. 
	 </li>
</ol>

قياس العمل

مع خضوع النظام للتغيير، فإن طاقته الداخلية قد تتغير، ويمكن نقل الطاقة من النظام إلى المحيط، أو من المحيط إلى النظام.  يحدث انتقال الطاقة من خلال الحرارة والعمل. يتم تمثيل العلاقة بين الطاقة الداخلية والحرارة والعمل بالمعادلة: <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-01.png" style="height: 40px;" />على الرغم من أن السخونة هي دالّة على التغير الملحوظ في درجة الحرارة، إلا أن العمل عبارة عن دالّة على التغيير الملحوظ في الحجم ويسمى عمل الحجم والضغط. يمكن تعريف العمل (w) على أنه قوة (F) تعمل من خلال المسافة (D). <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/Equations-02.png" style="height: 40px;" />يحدث عمل الحجم والضغط (أو عمل التوسيع) عندما يقوم النظام بدفع المحيط للخلف مقابل ضغط التقييد، أو عندما تضغط المناطق المحيطة على النظام. مثال على ذلك يحدث أثناء تشغيل محرك الاحتراق الداخلي. تفاعل احتراق البنزين والأكسجين هو طارد للحرارة. يتم إطلاق بعض هذه الطاقة كحرارة، ويتم تنفيذ البعض الآخر كعمل من خلال توسيع الغازات الموجودة في الأسطوانة، وبالتالي دفع المكبس للخارج. فالمواد الداخلة في التفاعل هي النظام، والمحرك وبقية الكون هي البيئة المحيطة. يفقد النظام الطاقة عن طريق كلا التسخين والعمل على البيئة المحيطة، وتنخفض طاقته الداخلية. عندما يزيد حجم الأسطوانة (أي يتمدد الغاز)، فإنه يندفع ضد قوة خارجية، وهو الضغط المحدد على أنه القوة لكل وحدة مساحة. <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-03.png" style="height: 40px;" />من المعادلتين 2 و3: <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-04.png" style="height: 40px;" />ناتج المساحة والمسافة (A × D) يساوي التغير في حجم(ΔV) الغاز في الأسطوانة. لذلك، <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-05.png" style="height: 40px;" />بما أن الحجم يزداد أثناء التمدد، فإن Vنهائي > Vأولي, و ΔV موجب. ومع ذلك، بالنسبة للتوسع الإيجابي (أي عندما يعمل النظام على المحيط) فإن w يجب أن يكون سالباً، وبالتالي تتم إضافة علامة سالبة إلى المعادلة.  <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-06.png" style="height: 40px;" />وفقاً لهذه المعادلة، يكون حجم الضغط هو القيمة السالبة للضغط الخارجي (أو الضغط المعارض) مضروبًا في التغير في الحجم.  وحدة العمل القائمة على هذه المعادلة هي L·atm. بعض عوامل التحويل الناجحة الأخرى المفيدة هي:  <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-07.png" style="height: 40px;" /><img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/795/795_Equations-08.png" style="height: 40px;" />هذا النص مقتبس من <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chemistry 2e, Section 5.3: Enthalpy</a>.

عندما يخضع النظام لتغيير ، يمكن أن تتغير طاقته الداخلية ، ويمكن نقل الطاقة من النظام إلى المناطق المحيطة ، أو من ...

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Thermochemistry

الكيمياء الحرارية

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

pressure-volume work in chemistry

Pressure-volume work shows how gas volume changes can transfer energy in chemistry. As a system changes, its internal energy may change too. Energy can move between the system and the surroundings through heat and work.

Internal energy, heat, and work are related by an energy equation. Heat depends on a measured temperature change. Work depends on a measured change in volume called pressure-volume work.

Work can be defined as a force acting through a distance. Pressure-volume work, also called expansion work, happens when a system pushes against a restraining pressure. It can also happen when the surroundings compress the system.

A common example is an internal combustion engine. The combustion of gasoline and oxygen is exothermic, so it gives off energy. Some of that energy is released as heat, and some is used as work that expands the gases in the cylinder and pushes the piston outward. In this situation, the reacting substances are the system, and the engine and the rest of the universe are the surroundings. The system loses energy by heating and by doing work on the surroundings, so its internal energy decreases.

When a gas expands in a cylinder, it pushes against an external force. That force is pressure, which is force per unit area. The area of the piston times the distance it moves equals the change in gas volume, or ΔV. Since the volume increases during expansion, ΔV is positive.

For expansion, the work is written with a negative sign because the system is doing work on the surroundings. Pressure-volume work is the negative of the external pressure multiplied by the change in volume. The unit of this work is L·atm, and it can be converted using other common unit factors.

Pressure-Volume Work in Chemistry

energy basics

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quantifying work

Pressure-volume work shows how gas volume changes can transfer energy in chemistry. As a system changes, its internal energy may change too. Energy can move between the system and the surroundings through heat and work. Internal energy, heat, and work are related by an energy equation. Heat depends on a measured temperature change. Work depends on a measured change in volume called pressure-volume work. Work can be defined as a force acting through a distance. Pressure-volume work, also...

enthalpy

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key terms and concepts

Learn how pressure-volume work links gas expansion, internal energy, and sign conventions in chemistry, with an engine example and the L·atm unit.

Pressure-Volume Work in Chemistry | JoVE K12

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Pressure-Volume Work in Chemistry

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