Name: heat changes in chemical equations
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 55 s
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K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Thermochemical Equations

מרבית התגובות הכימיות מתרחשות בלחץ אטמוספרי. בתנאים של לחץ קבוע, שינוי החום המקושר לתגובה, דלתא Q, שווה לשינוי באנתלפיה, דלתא H, שנקרא גם האנתלפיה או החום של תגובה. אנתלפיית התגובה היא ההפרש בין אנתלפיית התוצרים והמגיבים.כשאנתלפיית התוצרים גדולה יותר מאנתלפיית המגיבים, דלתא H היא חיובית. תגבות כאלו סופגות חום והן אנדותרמיות. לעומת זאת, אם אנתלפיית המגיבים גדולה מאנתלפיית התוצרים, דלתא H היא שלילית.תגובות כאלה פולטות חום והן אקסותרמיות. בכל תגובה כימית, עוצמת השינוי הנלווה באנתלפיה תלויה בכמות הסטויכיומטרית של מגיבים ותוצרים, שמצוינת באמצעות המקדמים של המשוואה המאוזנת. משוואה כימית מאוזנת שכוללת את סימון מצבי הצבירה ואנתלפיית התגובה, דלתא H, נקראת משוואה תרמוכימית.חשבו על הבערה של מתאן, מקור דלק מרכזי. שרפת מתאן משחררת חום לסביבה. הטבע האקסותרמי של התגובה מצוין על ידי השינוי השלילי של האנתלפיה במשוואה התרמוכימית.משוואת הבערה מראה שכשמול אחד של גז מתאן מגיב עם שני מולים של גז חמצן ליצירת מול אחד של גז פחמן דו חמצני ושני מולים של מים במצב נוזל, 890.8 קילוג'ול של חום משוחררים לסביבה. היחס המולי בין המגיבים או התוצרים וחופ התגבוה יכולים לשמש כפקטורי המרה כדי לחשב את מעבר החום בזמן התגובה. אם צילינדר של גז מכיל 25.5 קילוגרמים של מתאן וכל המתאן בצילינדר עובר בערה, כמה חום ייווצר?באופן כללי, הרעיון הוא להמיר מסה למולים ואז מולים לחום התגובה. תחילה, 25.5 קילוגרמים מוכפלים ב-1, 000 כדי למצוא את המסה בגרמים. ואז המסה של המתאן מחולקת במסה המולרית 16.0 גרם למול, והתוצאה היא 1, 594 מולים של מתאן.לבסוף, בעזרת פקטור המרה בין המולים של המתאן לחום התגובה, 1, 594 מולים של מתאן משחררים חום בכמות של 14.2 כפול עשר בחזקת 6 קילוג'ול של חום, וזהו חום התגובה. התשובה היא שלילית, מכיוון שהתגובה היא אקסותרמית כיוון שמתפתח חום במהלך תגובה.

<ol>
	<li>Canagaratna, סבסטיאן ג '"סיוע חזותי בחישובי אנתלפיה". כתב עת לחינוך כימי 77, מס '9 (2000): 1178.</li>
	<li>קיפר, דוד. "אנתלפיה והחוק השני של התרמודינמיקה". כתב עת לחינוך כימי 96, מס '7 (2019): 1407-1411.</li>
	<li>חליל, מוטסים א. "חישוב אנתלפיה של תגובה בשיטת מטריצה". כתב עת לחינוך כימי 77, מס '2 (2000): 185. 
	 </li>
</ol>

משוואות תרמוכימיות

<div style="direction:rtl">עבור תגובה כימית (המערכת) המתבצעת בלחץ קבוע - כשהעבודה היחידה שנעשתה כתוצאה מהתפשטות או התכווצות - האנתלפיה של התגובה (נקראת גם חום התגובה, ΔHrxn) שווה לחום המוחלף עם הסביבה (qp ). <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;"> השינוי באנתלפיה היא תכונה נרחבת, והיא תלויה בכמויות המגיבים המשתתפים בתגובה (או במספר המולות של מגיבים). השינוי באנתלפיה הוא ספציפי לתגובה, והמצבים הפיזיקליים של המגיב ומין התוצר הם חשובים. תגובה אקסותרמית מאופיינת בערך −ΔHrxn, בעוד של תגובה אנדותרמית יש ערך +ΔHrxn. מכיוון שכמות החום המשתחררת או נספגת בתגובה מתאימה לכמות של כל חומר שנצרך או מיוצר בתגובה, נוח להשתמש במשוואה תרמוכימית כדי לייצג את השינויים בחומר ובאנרגיה כאחד. במשוואה תרמוכימית, השינוי באנתלפיה של התגובה מוצג כ-ΔHrxn, והוא מסופק בדרך כלל בעקבות המשוואה לתגובה. הגודל של ΔHrxn מציין את כמות החום הקשור לתגובה המוצגת במשוואה הכימית. הסימן של ΔHrxn מציין אם התגובה היא אקסותרמית או אנדותרמית, כפי שנכתב. במשוואה הבאה, 1 מול גז מימן ו 1/2 מול גז חמצן (בטמפרטורה ולחץ מסויים) מגיבים ליצירת מול 1 של מים נוזליים (באותה טמפרטורה ולחץ). <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;"> משוואה זו מצביעה על כך ש-286 קילו-ג'ול של חום משתחרר לסביבה. במילים אחרות, 286 קילו-ג'ול של חום משתחרר (התגובה היא אקסותרמית) עבור כל מול מימן שנצרך או עבור כל מול מים שנוצר. לכן, האנתלפיה של התגובה היא גורם המרה שניתן להשתמש בו כדי לחשב את כמות החום המשתחררת או הנספגת במהלך תגובות המערבות כמויות ספציפיות של מגיבים ותוצרים. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;"> אם המקדמים של המשוואה הכימית מוכפלים בגורם כלשהו (כלומר, אם כמות החומר משתנה), יש להכפיל את השינוי באנתלפיה באותו גורם. (עלייה של פי שניים בכמויות) <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;"> (ירידה של פי שניים בכמויות) <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;"> כדי להמחיש ששינוי האנתלפיה של תגובה תלוי במצבים הפיזיקליים של המגיבים והתוצרים, שקול את היווצרותם של מים גזים (או אדי מים). כאשר מול 1 של גז מימן ו-½ מול של גז חמצן מגיבים ליצירת מול 1 של מים גזים, רק 242 קילו-ג'ול של חום משתחררים, בניגוד ל-286 קילו-ג'ול של חום, אשר משתחררים כאשר נוצרים מים נוזליים. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;"></div>

עבור תגובה כימית (המערכת) המתבצעת בלחץ קבוע – כאשר העבודה היחידה שנעשית כתוצאה מהתרחבות או התכווצות – ה ...

Education

JoVE Core

כימיה

אחד

Thermochemistry

תרמוכימיה

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

heat changes in chemical equations

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property.

The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a positive one. For that reason, a thermochemical equation is useful because it shows both the chemical change and the energy change together.

In a thermochemical equation, ΔHrxn is written after the chemical equation. Its magnitude tells how much heat is linked to the reaction as written. Its sign shows whether the reaction is exothermic or endothermic in that form.

For example, 1 mole of hydrogen gas and 1/2 mole of oxygen gas can react to form 1 mole of liquid water. This reaction releases 286 kJ of heat to the surroundings. That means 286 kJ is released for every mole of hydrogen used or every mole of water made, so ΔHrxn can act like a conversion factor.

If the coefficients in the chemical equation are multiplied, the enthalpy change must be multiplied by the same factor. A two-fold increase in the amounts gives a two-fold increase in ΔHrxn, and a two-fold decrease gives a two-fold decrease. The physical state matters too. When the same reaction makes gaseous water instead of liquid water, only 242 kJ of heat are released.

Heat Changes in Chemical Equations

energy basics

How Energy Changes in Chemical Reactions

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

Energy Conservation and Internal Energy

Energy conservation describes how energy changes form during chemical and physical changes. Energy can be converted from one form into another, but the energy present before a change still exists in some form after the change is complete. This is the law of conservation of energy. It says energy can neither be created nor destroyed. The law of conservation of matter also applies during chemical change. In most chemical reactions, the total amount of matter does not show a detectable change.

internal energy

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...

quantifying heat

Heat Transfer and Specific Heat

Heat transfer and specific heat explain how thermal energy moves and how materials respond to it. Thermal energy is the kinetic energy linked to the random motion of atoms and molecules. Temperature is a measure of how hot or cold a substance is, and it depends on how much thermal energy those particles have. When atoms and molecules move or vibrate faster, their average kinetic energy is higher. That means the object has more thermal energy and a higher temperature. When they move more...

quantifying work

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...

enthalpy

Enthalpy and Heat at Constant Pressure

Enthalpy describes heat changes in chemical and physical processes. Chemists use the symbol H for enthalpy. It is defined as a system’s internal energy, E, plus the product of pressure, P, and volume, V. Enthalpy is a state function, so only changes in enthalpy can be measured, not the enthalpy of a substance by itself. For many reactions, the pressure stays constant. Under those conditions, the enthalpy change, ΔH, can be written in a way that includes pressure-volume work. The term PΔV...

thermochemical equations

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property. The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a...

constant pressure calorimetry

Coffee Cup Calorimeter Heat Changes

Coffee cup calorimetry measures heat changes in reactions and physical changes using an insulated calorimeter. The calorimeter is calibrated and designed to limit heat exchange with the outside environment. That makes it possible to measure the heat taken in or released by a substance more accurately, including the energy content of foods. In calorimetry, the system is the substance or substances undergoing change. The surroundings are everything else inside the calorimeter, including the...

constant volume calorimetry

Bomb Calorimeter Heat Measurement

Bomb calorimetry measures the heat released by reactions that produce a lot of energy, especially combustion reactions. A coffee cup calorimeter works at constant pressure and is useful for reactions in solution. A bomb calorimeter works at constant volume and is used for reactions that make large amounts of heat and gaseous products. The word "bomb" refers to the strong, explosive force some reactions can have. In a bomb calorimeter, the volume does not change, so 94V = 0. That means the...

hess's law

Calculating Reaction Heat with Hess's Law

Calculating reaction heat with Hess's law lets chemists find enthalpy changes without measuring every reaction directly. Some reactions are hard or impossible to test in the lab. Even when a reaction can be measured, it is often easier to calculate the heat change from other known enthalpy values. Hess's law says that if a process can be written as the sum of several stepwise processes, the enthalpy change for the total process equals the sum of the enthalpy changes for the steps. This works...

standard enthalpy of formation

Using Standard Enthalpy Values in Chemistry

Enthalpy changes are often listed for reactions that start and end under the same conditions. Chemists use a standard state as a reference set of conditions for comparing properties. In IUPAC terms, the standard state means 1 bar of pressure and 1 M solutions, but it does not set a temperature. Many thermochemical tables still use 1 atm as the reference pressure. Because the difference between 1 bar and 1 atm is very small, enthalpy change usually stays almost the same. A superscripted o in...

enthalpies of reaction

Calculating reaction heat with Hess's law uses standard enthalpies of formation to find the enthalpy change of a reaction. When the formation values for the reactants and products are known, the overall reaction can be broken into simpler stepwise reactions. This approach helps show how the heat change for the full reaction is built from smaller parts. First, the reactants are thought of as being decomposed into their component elements. The enthalpy change for this step is proportional to the...

key terms and concepts

Learn how ΔHrxn shows heat released or absorbed in reactions, why equation coefficients matter, and how physical state changes the enthalpy value.

Heat Changes in Chemical Equations | 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.

Matter and its Interactions

PS.Chm.4.1

Texas Essential Knowledge and Skills

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Chemistry and Physics

IPC.8

IPC.8.B

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

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CHEM1.PS3.3

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HIgh School

CH.7

CH.7.a

CH.7.c

CHEM.13

CHEM.13.A

CHEM.13.C

Florida Department of Education

SC.912.P.10

SC.912.P.10.7

Georgia Standards of Excellence

SC3.

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PS.Chm.7.1

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The ideal gas law describes the behavior ...

Derivation of the Ideal Gas Law
Gases are a fundamental state of matter. A gas is a collection of molecules that have a significant distance between their ...

Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Preparation of the LaboratoryExpand
	Here, we show the laboratory preparation for 10 students ...

Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Determining the Mass of the Unknown LiquidExpand
	According to the ideal gas law for a given ...

Use mathematics and computational thinking to explain the relationships among pressure, temperature, volume, and quantity of gas, both qualitatively and quantitatively.

PS.Chm.7.3

Understand the relationship among pressure, temperature, volume, and phase.

Heat Changes in Chemical Equations

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