Name: pressure-volume work in chemistry
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 30 s
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PS.Chm.7.2

Quantifying Work

ניזכר שהאנרגיה ההתחלתית של המערכת מוגדרת לפי סכום החום והעבודה. חום נמדד בעזרת קלורימטריה, אבל איך מכמתים עבודה? עבודה היא תוצאה של כוח המופעל על עצם מסוים, כגון משיכה, דחיפה או הרמה, לאורך מרחק.לכן עבודה שווה לכוח כפול מרחק. אם אלת גולף מכה בכדור למרחק של 100 רגל, היא ביצעה עבודה. אנרגיה מועברת מהאלה אל הכדור.במקרה הזה, הכדור הוא המערכת עליה הסביבה ביצעה עבודה. בתגובות כימיות, עבודה יכולה להתבטא בשינויים פיזקליים או כימיים מרובים במערכת. סוג שכיח של עבודה הוא עבודה של לחץ-נפח.חשבו על בערה בתוך צילינדר של מנוע. לא רק שהבערה מייצרת חום, היא גם יוצרת גז, שמבצע עבודה. כשנפח הגז בצילינדר מתרחב, הלחץ דוחף את הבוכנה למטה ופועל כנגד כוחות חיצוניים בסביבה.לחץ מוגדר ככוח הפועל על פני שטח. כאן, הכוח מהגזים המתרחבים מחולק על פני אזור של בסיס הבוכנה. במילים אחרות, הכוח הוא לחץ כפול שטח.לכן, עבודה, שהיא הכוח כפול מרחק, יכולה להיכתב מחדש כלחץ כפול השטח והמרחק שבו הוא פועל. בצילינדר של המנוע, זה גובה המרחק שהבוכנה עוברת. בהתחשב בכך שהשטח כפול גובה הוא נפח הצילינדר, המשוואה שמתארת את העבודה יכולה להיות מוגדרת כלחץ כפול שינוי הנפח במערכת.יחד עם זאת, מכיוון שהנפח גדל והבוכנה נדחפת למטה, המערכת למעשה מבצעת עבודה על סביבתה, שלפי המוסכמה היא בעלת ערך שלילי. לכן, העבודה מוגדרת כערך השלילי של הלחץ P כפול הדלתא של הנפח V או השינוי בין הנפח ההתחלתי לסופי, שמתרחש תוך כדי ההתרחבות. יחידת העבדות לחץ נפח מוגדרת בדרך כלל כליטר אטמוספריה.יחידה זו יכולה להיות מומרת ליחידה המוסכמת של אנרגיה, ג'ולים, על ידי שימוש ביחס ההמרה של 1 ליטר אטמוספרה שווה ל-101.3 ג'ולים.

<ol>
	<li>שמידט-רוהר, קלאוס. "התפשטות פועלת ללא לחץ חיצוני ותרמודינמיקה במונחים של תהליכים כמו-סטטיים בלתי הפיכים". כתב עת לחינוך כימי 91, מס '3 (2014): 402-409.</li>
	<li>גיזלסון, אריק א. ונורמן קרייג. "הגדרות כלליות של עבודה וחום בתהליכים תרמודינמיים". כתב עת לחינוך כימי 64, מס '8 (1987): 660.</li>
	<li>או'לואן, ג'יי קנת. "שינויים אדיאבטים: שינויים הפיכים ובלתי הפיכים הכוללים רק עבודה בנפח לחץ". כתב עת לחינוך כימי 30, מס '4 (1953): 190. 
	 </li>
</ol>

כימות עבודה

<div style="direction:rtl">כאשר מערכת עוברת שינוי, האנרגיה הפנימית שלה יכולה להשתנות, ואנרגיה יכולה לעבור מהמערכת לסביבה, או מהסביבה למערכת. העברת אנרגיה מתרחשת באמצעות חום ועבודה. הקשר בין אנרגיה פנימית, חום ועבודה מיוצג על ידי המשוואה: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-01.png" style="height: 40px;"> בעוד שחום הוא פונקציה של שינוי טמפרטורה שנצפה, עבודה היא פונקציה של שינוי נפח נצפה הנקרא עבודת לחץ-נפח. ניתן להגדיר עבודה (w) ככוח (F) הפועל במרחק (D). <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/Equations-02.png" style="height: 40px;"> עבודה בלחץ-נפח (או עבודת הרחבה) מתרחשת כאשר מערכת דוחפת את הסביבה לאחור כנגד לחץ מרסן, או כאשר הסביבה דוחסת את המערכת. דוגמה לכך מתרחשת במהלך פעולתו של מנוע בעירה פנימי. תגובת הבעירה של בנזין וחמצן היא אקסותרמית. חלק מהאנרגיה הזו ניתנת כחום, וחלק מבוצע כעבודה על ידי הרחבת הגזים בצילינדר, ובכך דוחפת את הבוכנה החוצה. החומרים המעורבים בתגובה הם המערכת, המנוע ושאר היקום הם הסביבה. המערכת מאבדת אנרגיה הן על ידי חימום והן על ידי ביצוע עבודה על הסביבה, והאנרגיה הפנימית שלה פוחתת. כאשר נפח הגליל גדל (כלומר, הגז מתרחב), הוא דוחף כנגד כוח חיצוני, שהוא הלחץ המוגדר ככוח ליחידת שטח. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-03.png" style="height: 40px;"> ממשוואות 2 ו-3: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-04.png" style="height: 40px;"> המכפלה של שטח ומרחק (A × D) שווה לשינוי בנפח (ΔV) של הגז בגליל. לָכֵן, <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-05.png" style="height: 40px;"> מכיוון שהנפח גדל במהלך הרחבה, Vfinal > Viinitial, ו-ΔV חיובי. עם זאת, עבור הרחבה חיובית (כלומר, כאשר המערכת אכן עובדת על הסביבה), w צריך להיות שלילי, ולכן, סימן שלילי נוסף למשוואה. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-06.png" style="height: 40px;"> לפי משוואה זו, עבודת לחץ-נפח היא השלילי של הלחץ החיצוני (או הלחץ המנוגד) כפול השינוי בנפח. יחידת העבודה המבוססת על משוואה זו היא L·atm. כמה גורמי המרות שימושיים אחרים הם: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-07.png" style="height: 40px;"> <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-08.png" style="height: 40px;"> טקסט זה מותאם מ <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chemistry 2e, Section 5.3: Enthalpy</a>.</div>

כאשר מערכת עוברת שינוי, האנרגיה הפנימית שלה יכולה להשתנות, ואנרגיה יכולה להיות מועברת מהמערכת לסביבה, או מ...

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מונחים ומושגים מרכזיים

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

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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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