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

Num processo químico, a diferença entre as energias internas dos reagentes e produtos, representada por ΔU, é utilizada para determinar se o sistema ganhou ou perdeu energia durante a reação. Se ΔU for maior do que zero, a energia interna final era superior à energia interna inicial e o sistema ganhou energia durante a reação. Se ΔU for inferior a zero, a energia interna final é menor do que a energia interna inicial, o que significa o sistema perdeu energia.De acordo com a primeira lei da termodinâmica, qualquer alteração na energia de um sistema deve ser equilibrado por uma alteração igual e oposta nas suas áreas circundantes. Assim, a alteração na energia interna de um sistema é igual à energia transferida como calor, simbolizada por q"mais a energia transferida como trabalho, simbolizada por w"durante o processo. Na química, os sinais de calor e trabalho dependem do fato de o sistema ganhar ou perder energia.Consideremos a transformação do dióxido de carbono em carbono e oxigênio elementares. O reagente tem menos energia interna do que os produtos, o que significa que ΔU é positivo. A energia é transferida do meio envolvente para o sistema, aumentando a sua energia interna.Em alternativa, durante a reação entre o enxofre e o gás de oxigênio para produzir dióxido de enxofre, a energia é transferida para o meio envolvente. Aqui, os produtos têm uma energia interna mais baixa do que os reagentes, e ΔU é negativo. Assim, ΔU depende apenas dos estados energéticos internos iniciais e finais do sistema e da magnitude de calor e trabalho trocadas com o meio envolvente.

energia interna

O total de todos os tipos possíveis de energia presentes em uma substância é chamado de energia interna (U), às vezes simbolizada como E. Suponha que um sistema com energia interna inicial, Uinicial, sofre uma mudança de energia (transferência de trabalho ou calor), e a energia interna final do sistema é Ufinal. A alteração na energia interna é igual à diferença entre Ufinal e Uinicial. 

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/793/793_Equations-01.png" style="height: 40px;" />

Embora os valores de Ufinal e Uinicial não possam ser determinados para um sistema, a primeira lei da termodinâmica requer apenas o valor de ΔU, que pode ser determinado mesmo sem conhecer os valores de Ufinal e Uinicial. Um valor positivo de ΔU resulta quando Ufinal > Uinicial, e indica que o sistema ganhou energia das imediações. Um valor negativo de ΔU é obtido quando Ufinal < Uinicial e indica que o sistema perdeu energia para as imediações.

O calor (energia térmica) e o trabalho (energia mecânica) são as duas maneiras diferentes que um sistema pode trocar energia com as suas imediações. A energia é transferida para um sistema quando absorve calor (q) das imediações ou quando as imediações realizam trabalho (W) sobre o sistema. 

Por exemplo, a energia é transferida para um fio metálico à temperatura ambiente se estiver imerso em água quente (o fio absorve calor da água) ou quando o fio é rapidamente dobrado para a frente e para trás (o fio fica mais quente devido ao trabalho realizado). Ambos os processos aumentam a energia interna do fio, o que se reflecte em um aumento da temperatura do fio. Por outro lado, a energia é transferida para fora de um sistema quando o calor é perdido do sistema ou quando o sistema realiza trabalho sobre as imediações. Por exemplo, a queima de combustível de um foguete liberta uma quantidade tremenda de calor e também realiza trabalho sobre as imediações ao aplicar uma força ao longo de uma distância (fazendo com que um vaivém espacial se levante do solo). Ambos os processos diminuem a energia interna do sistema.

A relação entre energia interna, calor, e trabalho pode ser representada pela equação:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/793/793_Equations-02.png" style="height: 40px;" />

Esta é uma versão da primeira lei da termodinâmica, e mostra que a energia interna de um sistema muda através do fluxo de calor para dentro ou para fora do sistema ou quando é realizado trabalho sobre ou pelo sistema. Os sinais para calor e trabalho dependem de se o sistema ganha ou perde energia. q positivo é o fluxo de calor para o sistema a partir das imediações, enquanto que q negativo é o fluxo de calor para fora do sistema. O trabalho, w, é positivo se for feito sobre o sistema e negativo se for feito pelo sistema.

Quando q e w são ambos positivos (>0), ΔU é sempre positivo (>0), e a energia interna do sistema aumenta. Quando q e w são ambos negativos (<0), ΔU é sempre negativo (<0), e a energia interna do sistema diminui. Se q e w tiverem convenções diferentes de sinais, o sinal de ΔU depende das magnitudes relativas de q e w.    

A unidade SI de energia, calor, e trabalho é o joule (J).

Este texto é adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">OpenStax Chemistry 2e, Section 5.3: Enthalpy</a>.

Energia Interna

O total de todos os tipos possíveis de energia presentes em uma substância é chamado de energia interna (U), às vezes simbolizada como E. Suponha um sistema com ...

Education

JoVE Core

Química

Thermochemistry

Termoquímica

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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 - Química dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o mundo.

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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Physical Properties of Matter

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Intermolecular Forces

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Surface Tension, Capillary Action, and Viscosity

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States of Matter and Phase Changes

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Phase Transitions

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Phase Transitions: Vaporization and Condensation

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Clausius-Clapeyron Equation

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Heating and Cooling Curves

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Phase Diagrams

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Use models to explain how changes in energy affect the arrangement and movement of the particles in solids, liquids, and gases, as well as the relative strengths of their intermolecular forces.

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

Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some form ...

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Quantifying Heat

Thermal Energy&#160;

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Quantifying Work

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Enthalpy

Chemists ordinarily use a property known as enthalpy (H) to describe the thermodynamics of chemical and physical processes. Enthalpy is defined as the sum ...

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

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

Enthalpy changes are typically tabulated for reactions in which both the reactants and products are at the same conditions. A standard state is a commonly ...

Enthalpies of Reaction

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Definition and Measurement of Pressure: Atmospheric Pressure, Barometer, and Manometer

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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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