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

Dans un processus chimique, la différence entre les énergies internes des réactifs et des produits, représentée par ΔU, est utilisée pour déterminer si le système a gagné ou perdu de l'énergie pendant la réaction. Si ΔU est supérieur à zéro, l'énergie interne finale est supérieure à l'énergie interne initiale et le système gagne de l'énergie pendant la réaction. Si ΔU est inférieur à zéro, l'énergie interne finale est inférieure à l'énergie interne initiale, ce qui signifie que le système a perdu de l'énergie.Selon la première loi de la thermodynamique, tout changement dans l'énergie d'un système doit être équilibré par un changement égal et opposé dans son environnement. Ainsi, le changement de l'énergie interne d'un système est égal à l'énergie transférée sous forme de chaleur, symbolisée par"q"plus l'énergie transférée sous forme d'action, symbolisée par"w"au cours du processus. En chimie, les signes de chaleur et d'action dépendent du fait que le système gagne ou perd de l'énergie.Considérons la transformation du dioxyde de carbone en carbone élémentaire et en oxygène. Le réactif a une énergie interne inférieure à celle des produits, ce qui signifie que ΔU est positif. L'énergie est transférée de l'environnement au système augmentant son énergie interne.Alternativement, au cours de la réaction entre le soufre et l'oxygène gazeux pour produire du dioxyde de soufre, l'énergie est transférée à l'environnement. Ici, les produits ont une énergie interne inférieure à celle des réactifs, et ΔU est négatif. Par conséquent, ΔU ne dépend que des états énergétiques internes initiaux et finaux du système et de l'ampleur de la chaleur et d'action échangés avec l'environnement.

Énergie interne

La somme de tous les types d'énergie possibles qui sont présents dans une substance est appelée énergie interne (U), parfois symbolisée par E. Supposons qu'un système avec l'énergie interne initiale, Uinitial, subit un changement d'énergie (échange de travail ou de chaleur), et l'énergie interne finale du système est Ufinal. La variation d'énergie interne est égale à la différence entre Ufinal et Uinitial.  
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/793/793_Equations-01.png" style="height: 40px;" />
 
 Bien que les valeurs pour Ufinal et Uinitial ne puissent pas être déterminées pour un système, la première loi de la thermodynamique exige seulement la valeur de ΔU, qui peut être déterminée même sans connaître les valeurs de Ufinal  et Uinitial. Une valeur positive de ΔU est obtenue lorsqueUfinal > Uinitial, et indique que le système a reçu de l'énergie de l'extérieur. Une valeur négative de ΔU est obtenue lorsque Ufinal < Uinitial, et indique que le système a perdu de l'énergie dans l'environnement extérieur. 
 
 La chaleur (énergie thermique) et le travail (énergie mécanique) sont les deux façons différentes dont un système peut échanger de l'énergie avec son environnement extérieur. L'énergie est transférée dans un système lorsqu'il absorbe la chaleur (q) à partir de l'environnement ou lorsque l'environnement extérieur fourni un travail (w) au système.  
 
 Par exemple, l'énergie est transférée dans un fil métallique à température ambiante s'il est immergé dans l'eau chaude (le fil absorbe la chaleur de l'eau), ou lorsque le fil est plié rapidement d'avant en arrière (le fil devient plus chaud en raison du travail effectué sur celui-ci). Les deux processus augmentent l’énergie interne du fil, ce qui se traduit par une élévation de la température du fil. À l'inverse, l'énergie est transférée hors d'un système lorsque la chaleur est cédée du système ou lorsque le système effectue un travail sur l'environnement extérieur. Par exemple, la combustion de carburant de fusée dégage une énorme quantité de chaleur et effectue également un travail sur l'environnement en appliquant une force sur une distance (faisant soulever une navette spatiale du sol). Les deux processus diminuent l'énergie interne du système. 
 
 La relation entre l'énergie interne, la chaleur et le travail peut être représentée par l'équation : 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/793/793_Equations-02.png" style="height: 40px;" />
 
 Il s'agit d'une version de la première loi de la thermodynamique, et elle montre que l'énergie interne d'un système varie selon le flux de chaleur vers l'intérieur ou vers l'extérieur du système, ou qu'un travail est effectué sur ou par le système. Les signes de la chaleur et du travail dépendent du fait que le système gagne ou perd de l'énergie. Un q positif est un flux de chaleur vers l'intérieur d'un système depuis l'extérieur, tandis qu'un q négatif est un flux de chaleur sortant du système. Le travail, w, est positif s'il est effectué sur le système et négatif s'il est effectué par le système. 
 
 Lorsque q et w sont tous deux positifs (> 0), ΔU est toujours positif (> 0) et l'énergie interne du système augmente. Lorsque q et w sont tous deux négatifs (< 0), ΔU est toujours négatif (< 0) et l'énergie interne du système diminue. Si q et w ont des signes différents, alors le signe de ΔU dépend des intensités relatives de q et w.  
 
 L'unité SI de l'énergie, de la chaleur et du travail est le joule (J). 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">OpenStax Chemistry 2e, Section 5.3: Enthalpie</a>.

L’ensemble de tous les types d’énergie possibles présents dans une substance est appelé l’énergie interne (U), parfois symbolisée par E. Supposons un système avec ...

Education

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Chimie

Thermochemistry

Thermochimie

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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

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 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

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

Explore how internal energy changes through heat and work, plus the sign rules for q, w, and ΔU in the first law of thermodynamics.

Internal Energy, Heat, and Work | JoVE K12

chemistry

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Matter and its Interactions

PS.Chm.4.1

Louisiana Student Standards for Science

High School

HS-PS3-1

HS-PS3-3

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.a

Texas Essential Knowledge and Skills

CHEM.13

CHEM.13.A

CHEM.13.C

Next Generation Science Standards

Physical Science

Florida Department of Education

SC.912.P.10

SC.912.P.10.1

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South Carolina College and Career Ready Science Standards

Physics

P-PS3-1

P-PS3-2

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Energy

HS-PS3-1.

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PS.Chm.7.1

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First Law of Thermodynamics

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Enthalpy

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Thermochemical Equations

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Hess's Law

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PS.Chm.7.3

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

Internal Energy, Heat, and Work

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