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

W procesie chemicznym różnica między energiami wewnętrznymi reagentów i produktów, reprezentowana przez ΔU, służy do określenia, czy układ zyskał, czy stracił energię podczas reakcji.

Jeśli ΔU jest większe od zera, końcowa energia wewnętrzna była wyższa niż początkowa energia wewnętrzna, a układ zyskał energię podczas reakcji.

Jeśli ΔU jest mniejsze od zera, końcowa energia wewnętrzna jest niższa niż początkowa energia wewnętrzna, co oznacza, że system stracił energię.

Zgodnie z pierwszą zasadą termodynamiki, każda zmiana energii układu musi być zrównoważona przez równą i przeciwną zmianę w jego otoczeniu.

Zatem zmiana energii wewnętrznej układu jest równa energii przekazywanej w postaci ciepła, symbolizowanej przez "q", plus energii przekazywanej jako praca symbolizowana przez "w" podczas procesu.

W chemii oznaki ciepła i pracy zależą od tego, czy system zyskuje, czy traci energię.

Rozważ przemianę dwutlenku węgla w węgiel elementarny i tlen. Reagent ma niższą energię wewnętrzną niż produkty, co oznacza, że ΔU jest dodatnie. Energia jest przekazywana z otoczenia do układu, zwiększając jego energię wewnętrzną.  

Alternatywnie, podczas reakcji między siarką a tlenem gazowym w celu wytworzenia dwutlenku siarki, energia jest przekazywana do otoczenia. Tutaj produkty mają niższą energię wewnętrzną niż reagenty, a ΔU jest ujemne. 

W związku z tym ΔU zależy tylko od początkowych i końcowych wewnętrznych stanów energetycznych układu oraz od wielkości ciepła i pracy wymienianej z otoczeniem.

energia wewnętrzna

Suma wszystkich możliwych rodzajów energii obecnej w substancji nazywana jest energią wewnętrzną (U), czasami symbolizowaną jako E. Załóżmy, że układ o początkowej energii wewnętrznej, Uinitial, ulega zmianie energii (przeniesieniu pracy lub ciepła) i końcowa energia wewnętrzna układu wynosi Ufinal. Zmiana energii wewnętrznej jest równa różnicy pomiędzy Ufinal i Uinitial.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/793/793_Equations-01.png" style="height: 40px;">

Chociaż dla układu nie można wyznaczyć wartości Ufinal i Uinitial, pierwsza zasada termodynamiki wymaga jedynie wartości ΔU, którą można wyznaczyć nawet nie znając wartości Ufinal i Uinitial. Dodatnia wartość ΔU pojawia się, gdy Ufinal > Uinitial i wskazuje, że system pozyskał energię z otoczenia. Ujemną wartość ΔU uzyskuje się, gdy Ukońcowa < Uinitial, i wskazuje, że system oddał energię do otoczenia.

Ciepło (energia cieplna) i praca (energia mechaniczna) to dwa różne sposoby wymiany energii przez system z otoczeniem. Energia jest przekazywana do układu, gdy pobiera ciepło (q) z otoczenia lub gdy otoczenie wykonuje pracę (w) nad układem.

Na przykład energia jest przekazywana do metalowego drutu o temperaturze pokojowej, jeśli zostanie on zanurzony w gorącej wodzie (drut pochłania ciepło z wody) lub gdy drut jest gwałtownie zginany w przód i w tył (drut nagrzewa się w wyniku pracy wykonanej nad To). Obydwa procesy zwiększają energię wewnętrzną drutu, co przekłada się na wzrost jego temperatury. I odwrotnie, energia jest przekazywana z systemu, gdy ciepło jest tracone z systemu lub gdy system oddziałuje na otoczenie. Na przykład spalanie paliwa rakietowego powoduje wyzwolenie ogromnej ilości ciepła, a także działanie na otoczenie poprzez przyłożenie siły na odległość (powodując oderwanie się promu kosmicznego od ziemi). Obydwa procesy zmniejszają energię wewnętrzną układu.

Zależność pomiędzy energią wewnętrzną, ciepłem i pracą można przedstawić za pomocą równania:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/793/793_Equations-02.png" style="height: 40px;">

Jest to jedna z wersji pierwszej zasady termodynamiki, która pokazuje, że energia wewnętrzna układu zmienia się w wyniku przepływu ciepła do lub z układu lub pracy wykonywanej w układzie lub przez układ. Znaki ciepła i pracy zależą od tego, czy układ zyskuje, czy traci energię. Dodatnie q to przepływ ciepła do układu z otoczenia, ujemne q to przepływ ciepła z układu. Praca w jest dodatnia, jeśli jest wykonywana na układzie, i ujemna, jeśli jest wykonywana przez układ.

Gdy oba q i w są dodatnie (> 0), ΔU jest zawsze dodatnie (> 0), a energia wewnętrzna układu wzrasta. Gdy oba q i w są ujemne (<0), ΔU jest zawsze ujemne (<0), a energia wewnętrzna układu maleje. Jeśli q i w mają różne konwencje znaków, wówczas znak ΔU zależy od względnych wielkości q i w.

Jednostką energii, ciepła i pracy w układzie SI jest dżul (J).

Energia wewnętrzna

Suma wszystkich możliwych rodzajów energii obecnych w substancji nazywana jest energią wewnętrzną (U), czasami symbolizowaną jako E. Załóżmy, że mamy do czynienia z układem z ...

Education

JoVE Core

Chemia

Jeden

Thermochemistry

Termochemia

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

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 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

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

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

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Intermolecular vs Intramolecular Forces

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Intermolecular Forces

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Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

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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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Vapor Pressure

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Clausius-Clapeyron Equation

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Phase Transitions: Sublimation and Deposition

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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

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Quantifying Heat

Thermal Energy&#160;

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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

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Constant Pressure Calorimetry

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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
Differential Scanning Calorimetry (DSC) is a method of thermodynamic analysis based on ...

Hess's Law

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Standard Enthalpy of Formation

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Enthalpies of Reaction

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Real Gases: Effects of Intermolecular Forces and Molecular Volume Deriving Van der Waals Equation

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Ideal Gas Law

Source: Laboratory of Dr. Andreas Z&#252;ttel - Swiss Federal Laboratories for Materials Science and Technology
The ideal gas law describes the behavior ...

Derivation of the Ideal Gas Law
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Source: Smaa Koraym at Johns Hopkins University, MD, USA

	Preparation of the LaboratoryExpand
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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.

Internal Energy, Heat, and Work

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