Name: internal energy, heat, and work
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Internal Energy

בתהליך כימי, ההפרש בין אנרגיות פנימיות של המגיבים והתוצרים, מיוצג על ידי ΔU ומשמש כדי להגדיר האם המערכת הרוויחה או הפסידה אנרגיה במהלך ההתגובה. אם ΔU גדול מאפס, סך האנרגיה הפנימית הייתה גבוהה מהאנרגיה הפנימית ההתחלתית והמערכת הרוויחה אנרגיה תוך כדי התגובה. אם ΔU קטנה מאפס, האנרגיה הפנימית הסופית נמוכה מסך האנרגיה הפנימית ההתחלתית, כלומר, המערכת הפסידה אנרגיה.לפי החוק הראשון של התרמודינמיקה, כל שינוי באנרגיה של המערכת חייב להיות מאוזן על ידי שינוי נגדי שווה ערך בסביבתו. בהתאם לכך, השינוי באנרגיה הפנימית של המערכת שווה לאנרגיה המועברת בצורת חום, המסומלת על ידי Q, ועוד האנרגיה המועברת בצורת עבודה, המסומלת על ידי W, במהלך התהליך. בכימיה, הסימנים של חום ועבודה תלויים בהאם המערכת הפסידה או הרוויחה אנרגיה.חשבו על השינוי של פחמן דו חמצני ליסודות פחמן וחמצן. המגיב הוא בעל אנרגיה נמוכה מהתוצרים, כלומר, דלתא U חיובית. האנרגיה מועברת מהסביבה למערכת ומגבירה את האנרגיה הפנימית שלה.לחלופין, במהלך התגובה בין גופרית לגז חמצן ביצירה של גופרית דו חמצנית, אנרגיה מועברת אל הסביבה. כאן התוצרים הם בעלי אנרגיה פנימית נמוכה מהמגיבים וΔU שלילית. לכן, ΔU תלויה רק במצב האנרגיה הפנימית ההתחלתית והסופית של המערכת ובעוצמת החום והעבודה שמוחלפים עם הסביבה.

אנרגיה פנימית

<div style="direction:rtl">

 סכום של כל סוגי האנרגיה האפשריים הקיימים בחומר נקרא האנרגיה הפנימית (U), לפעמים מסומלת כ-E. נניח שמערכת עם אנרגיה פנימית ראשונית, Uinitial, עוברת שינוי באנרגיה (העברת עבודה או חום), והאנרגיה הפנימית הסופית של המערכת היא Ufinal. שינוי באנרגיה הפנימית שווה להפרש בין Ufinal ל-Uinitial.
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/793/793_Equations-01.png" style="height: 40px;"> 

 למרות שלא ניתן לקבוע את הערכים עבור Ufinal ו-Uinitial עבור מערכת, החוק הראשון של התרמודינמיקה דורש רק את הערך של ΔU, אותו ניתן לקבוע גם מבלי לדעת את הערכים של Ufinal ו-Uinitial. ערך חיובי של ΔU נוצר כאשר Ufinal > Uinitial, ומציין שהמערכת צברה אנרגיה מהסביבה. ערך שלילי של ΔU מתקבל כאשר Ufinal < Uinitial, ומציין שהמערכת איבדה אנרגיה לסביבה.


 חום (אנרגיה תרמית) ועבודה (אנרגיה מכנית) הן שתי הדרכים השונות שבהן מערכת יכולה להחליף אנרגיה עם סביבתה. אנרגיה מועברת למערכת כאשר היא סופגת חום (q) מהסביבה או כאשר הסביבה עושה עבודה (w) על המערכת. 

 לדוגמה, אנרגיה מועברת לחוט מתכת בטמפרטורת החדר אם הוא טובל במים חמים (החוט סופג חום מהמים), או כאשר החוט מתכופף במהירות קדימה ואחורה (החוט מתחמם בגלל העבודה שנעשתה עליו זה). שני התהליכים מגבירים את האנרגיה הפנימית של החוט, המתבטאת בעלייה בטמפרטורה של החוט. לעומת זאת, אנרגיה מועברת ממערכת כאשר חום אובד מהמערכת או כאשר המערכת אכן עובדת על הסביבה. למשל, שריפת דלק רקטי משחררת כמות אדירה של חום וגם מבצעת עבודה על הסביבה על ידי הפעלת כוח על פני מרחק (הגורם לחללית להתרומם מהקרקע). שני התהליכים מפחיתים את האנרגיה הפנימית של המערכת. 

 הקשר בין אנרגיה פנימית, חום ועבודה יכול להיות מיוצג על ידי המשוואה: 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/793/793_Equations-02.png" style="height: 40px;"> 

 זוהי גרסה אחת של החוק הראשון של התרמודינמיקה, והיא מראה שהאנרגיה הפנימית של המערכת משתנה באמצעות זרימת חום לתוך המערכת או מחוץ למערכת או שעבודה נעשית על המערכת או על ידה. הסימנים לחום ולעבודה תלויים בשאלה האם המערכת צוברת או מאבדת אנרגיה. q חיובי הוא זרימת חום לתוך המערכת מהסביבה, בעוד ש-q שלילי הוא זרימת חום אל מחוץ למערכת. העבודה, w, היא חיובית אם היא נעשית על המערכת ושלילית אם היא נעשית על ידי המערכת.

 כאשר q ו-w שניהם חיוביים (>0), ΔU תמיד חיובי (>0), והאנרגיה הפנימית של המערכת עולה. כאשר q ו-w שניהם שליליים (<0), ΔU תמיד שלילי (<0), והאנרגיה הפנימית של המערכת פוחתת. אם ל-q ול-w יש מוסכמות לסימנים שונים, אז הסימן של ΔU תלוי בגדלים היחסיים של q ו-w.

 יחידת ה-SI של אנרגיה, חום ועבודה היא ג'ול (J). 

 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>

סך כל סוגי האנרגיה האפשריים הקיימים בחומר נקרא האנרגיה הפנימית (U), המסומלת לעתים כ- E. נניח מערכת עם ...

Education

JoVE Core

כימיה

אחד

Thermochemistry

תרמוכימיה

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

internal energy, heat, and work

Internal energy is the total of all possible kinds of energy in a substance. It is often written as U, and sometimes as E. For a system that changes energy, the important value is the change in internal energy, or ΔU.

ΔU is the difference between the final internal energy and the initial internal energy. The exact values of U final and U initial are usually not known, but the first law of thermodynamics only requires ΔU. A positive ΔU means the system gained energy from the surroundings. A negative ΔU means the system lost energy to the surroundings.

Heat and work are the two ways a system can exchange energy with its surroundings. A system gains energy when it absorbs heat, q, or when the surroundings do work on it, w. For example, a room-temperature metal wire gains energy if it is placed in hot water. It also gains energy if it is bent back and forth quickly, because work is done on it. In both cases, the wire’s internal energy increases, and its temperature rises.

A system loses energy when heat leaves it or when it does work on the surroundings. Burning rocket fuel is one example. It releases a large amount of heat and also does work by pushing on the surroundings over a distance. That is why the system’s internal energy decreases.

The first law of thermodynamics can be written as a relationship between internal energy, heat, and work. It shows that internal energy changes when heat flows into or out of a system or when work is done on or by the system. The sign of q is positive when heat enters the system and negative when heat leaves it. The sign of w is positive when work is done on the system and negative when work is done by the system. When q and w are both positive, ΔU is positive. When q and w are both negative, ΔU is negative. If q and w have different signs, the sign of ΔU depends on which one is larger in magnitude.

The SI unit for energy, heat, and work is the joule, J.

Internal Energy, Heat, and Work

energy basics

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Chemical reactions change matter and energy at the same time. A match lighting is a simple example. These energy changes are part of everyday life and appear in food, fuels, and industry. Food provides energy through metabolic reactions. The macronutrients in food break down and help keep the body functioning. Fuels such as gasoline, natural gas, and coal are burned to produce energy for transportation, heating, and electricity generation. Industrial chemical reactions also use large amounts...

first law of thermodynamics

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internal energy

Internal energy is the total of all possible kinds of energy in a substance. It is often written as U, and sometimes as E. For a system that changes energy, the important value is the change in internal energy, or ΔU. ΔU is the difference between the final internal energy and the initial internal energy. The exact values of U final and U initial are usually not known, but the first law of thermodynamics only requires ΔU. A positive ΔU means the system gained energy from the surroundings. A...

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Internal Energy, Heat, and Work | JoVE K12

chemistry

JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

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PS.Chm.4.1

Louisiana Student Standards for Science

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HS-PS3-1

HS-PS3-3

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CHEM.13.A

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P-PS3-2

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PS.Chm.7.3

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Internal Energy, Heat, and Work

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