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

En un proceso químico, la diferencia entre las energías internas de los reactivos y los productos se representa como ΔU y se utiliza para determinar si el sistema ganó o perdió energía durante la reacción. Si ΔU es mayor que cero, la energía interna final es mayor que la energía interna inicial, es decir, que el sistema ganó energía durante la reacción. Si la variación de energía interna es menor que cero, la energía interna final es menor que la energía interna inicial, lo que se traduce en que el sistema perdió energía.De acuerdo con la primera ley de la termodinámica, cualquier cambio en la energía de un sistema debe equilibrarse mediante un cambio opuesto e igual en su entorno. Por lo tanto, el cambio en la energía interna de un sistema es igual a la energía que se transfiere en forma de calor, representada como q"más la energía que se transfiere en forma de trabajo, representada como w"durante el proceso. En química, los signos del calor y del trabajo dependen de si el sistema gana o pierde energía.Tomemos como ejemplo la transformación del dióxido de carbono en carbono y oxígeno elementales. El reactivo tiene una energía interna menor que los productos, lo que quiere decir que la variación de energía interna ΔU es positiva. La energía se transfiere del entorno al sistema y esto aumenta su energía interna.De manera alternativa, durante la reacción entre el azufre y el oxígeno para formar dióxido de azufre, la energía se transfiere del sistema al entorno. En este caso, los productos tienen una energía interna menor que los reactivos y la variación de energía interna es negativa. Por tanto, ΔU sólo depende de los estados iniciales y finales de la energía interna del sistema y la magnitud de calor y trabajo intercambiada con el entorno.

energía interna

El total de todos los tipos de energía posibles presentes en una sustancia se llama energía interna (U), a veces simbolizada como E. Suponga que un sistema con energía interna inicial, Uinicial, sufre un cambio en la energía (transferencia de trabajo o calor), y la energía interna final del sistema es Ufinal. El cambio en la energía interna equivale a la diferencia entre Ufinal y Uinicial. 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/793/793_Equations-01.png" style="height: 40px;" />
 
 Aunque los valores de Ufinal y Uinicial no pueden ser determinados para un sistema, la primera ley de la termodinámica sólo requiere el valor de ΔU, que puede determinarse incluso sin conocer los valores de Ufinal  y Uinicial. Un valor positivo de ΔU resulta cuando Ufinal > Uinicial, e indica que el sistema ha ganado energía del entorno. Se obtiene un valor negativo de ΔU cuando Ufinal < Uinicial, e indica que el sistema ha perdido energía en el entorno.
 
 El calor (energía térmica) y el trabajo (energía mecánica) son las dos formas diferentes en que un sistema puede intercambiar energía con su entorno. La energía se transfiere a un sistema cuando absorbe calor (q) del entorno o cuando el entorno realiza trabajo (w) en el sistema. 
 
 Por ejemplo, la energía se transfiere a un alambre metálico a temperatura ambiente si este es sumergido en agua caliente (el alambre absorbe calor del agua), o cuando el alambre es doblado rápidamente hacia adelante y hacia atrás (el alambre se calienta debido al trabajo hecho en él). Ambos procesos aumentan la energía interna del alambre, lo que se refleja en un aumento en la temperatura del mismo. Por el contrario, la energía se transfiere fuera de un sistema cuando se pierde calor del sistema o cuando el sistema hace trabajo sobre el entorno. Por ejemplo, la quema de combustible para cohetes libera una enorme cantidad de calor y también realiza trabajo en los alrededores aplicando una fuerza a lo largo de una distancia (haciendo que un transbordador espacial se levante del suelo). Ambos procesos disminuyen la energía interna del sistema.
 
 La relación entre la energía interna, el calor y el trabajo se puede representar mediante la ecuación:
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/793/793_Equations-02.png" style="height: 40px;" />
 
 Esta es una versión de la primera ley de la termodinámica y muestra que la energía interna de un sistema cambia a través del flujo de calor dentro o fuera del sistema o el trabajo hecho en o por el sistema. Las señales de calor y trabajo dependen de si el sistema gana o pierde energía. El valor q positivo es el flujo de calor que entra en el sistema desde el entorno, mientras que el valor q negativo es el flujo de calor que sale del sistema. El trabajo, w, es positivo si es realizado sobre el sistema y negativo si es realizado por el sistema.
 
 Cuando q y w son positivos (>0), ΔU siempre es positivo (>0), y la energía interna del sistema aumenta. Cuando q y w son negativos (<0), ΔU siempre es negativo (<0) y la energía interna del sistema disminuye. Si q y w tienen convenciones de signos diferentes, entonces el signo de &DeltaU depende de las magnitudes relativas de q y w.  
 
 La unidad SI de energía, calor y trabajo es el joule (J).
 
 Este texto ha sido adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">OpenStax, Química 2e, Sección 5.3: Entalpía</a>.

Energía Interna

El total de todos los tipos posibles de energía presentes en una sustancia se llama energía interna (U), a veces simbolizada como E. Supongamos un sistema con ...

Education

Educación

JoVE Core

Química

Thermochemistry

Termoquímica

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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 da vida a los conceptos de química general mediante lecciones de vídeo basadas en animaciones, concisas y fáciles de entender. Además, en los vídeos de científicos en acción, se muestran experimentos de investigación clásicos y originales realizados en laboratorios de todo el 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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Classifying Matter by Composition

Matter: Pure Substances and Mixtures
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Physical and Chemical Properties of Matter

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Physical Properties of Matter

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Surface Tension, Capillary Action, and Viscosity

Surface Tension
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States of Matter and Phase Changes

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Phase Transitions

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Phase Transitions: Melting and Freezing

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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

There are two ways to determine the amount of heat involved in a chemical change: measure it experimentally, or calculate it from other experimentally ...

Standard Enthalpy of Formation

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Enthalpies of Reaction

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Ideal Gas 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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