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

In un processo chimico, la differenza fra le energie interne dei reagenti e dei prodotti, rappresentata da ΔU, viene utilizzata per determinare se il sistema ha guadagnato o perso energia durante la reazione. Se ΔU è maggiore di zero, l'energia interna finale é maggiore dell'energia interna iniziale, e il sistema ha guadagnato energia durante la reazione. Se ΔU è inferiore a zero, l'energia interna finale è inferiore all'energia interna iniziale, il che significa che il sistema ha perso energia.

Secondo la prima legge della termodinamica, qualsiasi cambiamento nell'energia di un sistema deve essere bilanciato da un cambiamento uguale e opposto nei suoi dintorni. Pertanto, il cambiamento nell'energia interna di un sistema è uguale all'energia trasferita come calore, simboleggiata da q"più l'energia trasferita come lavoro, simboleggiata da w"durante il processo. In chimica, i segni del calore e del lavoro dipendono dal fatto che il sistema guadagni o perda energia.

Si consideri la trasformazione dell'anidride carbonica in carbonio elementare e ossigeno. Il reagente ha un'energia interna inferiore rispetto ai prodotti, il che significa che ΔU è positivo. L'energia viene trasferita dall'ambiente circostante al sistema aumentando la sua energia interna.

In alternativa, durante la reazione tra zolfo e ossigeno gassoso per produrre anidride solforosa, l'energia viene trasferita all'ambiente circostante. In questo caso, i prodotti hanno un'energia interna inferiore rispetto ai reagenti e ΔU è negativo. Quindi, ΔU dipende solo dagli stati energetici interni iniziali e finali del sistema e dall'entità del calore e del lavoro scambiati con l'ambiente circostante.

energia interna

Il totale di tutti i possibili tipi di energia presenti in una sostanza è chiamato energia interna (U), a volte simboleggiata come E. Supponiamo che un sistema con energia interna iniziale, Uinitial, subisca un cambiamento di energia (trasferimento di lavoro o calore), e l'energia finale interna del sistema è Ufinal. La variazione dell'energia interna è uguale alla differenza tra Ufinal e Uinitial.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/793/793_Equations-01.png" style="height: 40px;" />

Sebbene i valori di Ufinal e Uinitial non possano essere determinati per un sistema, la prima legge della termodinamica richiede solo il valore di ΔU, che può essere determinato anche senza conoscere i valori di Ufinal e Uinitial. Un valore positivo di ΔU risulta quando Ufinal > Uinitial e indica che il sistema ha guadagnato energia dall'ambiente circostante. Un valore negativo di ΔU si ottiene quando Ufinal < Uinitial e indica che il sistema ha perso energia verso l'ambiente circostante.

Calore (energia termica) e lavoro (energia meccanica) sono i due diversi modi in cui un sistema può scambiare energia con l'ambiente circostante. L'energia viene trasferita in un sistema quando assorbe calore (q) dall'ambiente circostante o quando l'ambiente circostante lavora (w) sul sistema.

Ad esempio, l'energia viene trasferita nel filo metallico a temperatura ambiente se viene immerso in acqua calda (il filo assorbe calore dall'acqua) o quando il filo viene piegato rapidamente avanti e indietro (il filo diventa più caldo a causa del lavoro svolto su Esso). Entrambi i processi aumentano l’energia interna del filo, che si riflette in un aumento della temperatura del filo. Al contrario, l'energia viene trasferita fuori da un sistema quando il calore viene perso dal sistema o quando il sistema lavora sull'ambiente circostante. Ad esempio, la combustione del carburante per missili rilascia un'enorme quantità di calore e svolge anche un lavoro sull'ambiente circostante applicando una forza a distanza (provocando il sollevamento di una navetta spaziale da terra). Entrambi i processi diminuiscono l'energia interna del sistema.

La relazione tra energia interna, calore e lavoro può essere rappresentata dall’equazione:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/793/793_Equations-02.png" style="height: 40px;" />

Questa è una versione della prima legge della termodinamica e mostra che l'energia interna di un sistema cambia attraverso il flusso di calore dentro o fuori dal sistema o che il lavoro viene svolto sul o dal sistema. I segni del calore e del lavoro dipendono dal fatto che il sistema guadagni o perda energia. Il q positivo indica il flusso di calore dall'ambiente circostante al sistema, mentre il q negativo rappresenta il flusso di calore fuori dal sistema. Il lavoro w è positivo se compiuto sul sistema e negativo se compiuto dal sistema.

Quando q e w sono entrambi positivi (>0), ΔU è sempre positivo (>0) e l'energia interna del sistema aumenta. Quando q e w sono entrambi negativi (<0), ΔU è sempre negativo (<0) e l'energia interna del sistema diminuisce. Se q e w hanno convenzioni sui segni diverse, allora il segno di ΔU dipende dalle grandezze relative di q e w.

L'unità SI di energia, calore e lavoro è il joule (J).

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">OpenStax Chemistry 2e, Section 5.3: Enthalpy</a>.

Energia interna

Il totale di tutti i possibili tipi di energia presenti in una sostanza è chiamato energia interna (U), a volte simboleggiato come E. Supponiamo un sistema con ...

Education

Didattica

JoVE Core

Chimica

Thermochemistry

Termochimica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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

JoVE Core: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

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

south-carolina-college-and-career-ready-science-standards

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.

Classifying Matter by State

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Phase Transitions

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Phase Diagrams

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PS.Chm.7.1

Energy Basics

Chemical reactions, such as those that occur when you light a match, involve changes in energy as well as matter.

Chemical changes and their accompanying ...

First Law of Thermodynamics

Energy Conservation

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Quantifying Heat

Thermal Energy&#160;

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Quantifying Work

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Enthalpy

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Thermochemical Equations

For a chemical reaction (the system) carried out at constant pressure &ndash; with the only work done caused by expansion or contraction &ndash; the ...

Constant Pressure Calorimetry

Calorimetry is a technique used to measure the amount of heat involved in a chemical or physical process or to measure the heat transferred to or from a ...

Constant Volume Calorimetry

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Using Differential Scanning Calorimetry to Measure Changes in Enthalpy

Source: Laboratory of Dr. Terry Tritt &#8212; Clemson University
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Hess's Law

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Standard Enthalpy of Formation

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Enthalpies of Reaction

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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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