Name: pressure-volume work in chemistry
Uploaded: 2020-09-03T13:38:27.000Z
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Quantifying Work

Bedenk dat de inwendige energie van een systeem wordt gedefinieerd als de som van warmte en arbeid. Warmte wordt gemeten met calorimetrie, maar hoe wordt werk gekwantificeerd? Arbeid is het resultaat van een kracht die een object ervaart bijvoorbeeld door eraan te trekken, te duwen of te tillen over een afstand.Arbeid is dus gelijk aan kracht maal afstand. Als een golfclub een bal 30 meter raakt, heeft hij arbeid verricht. Energie wordt overgedragen van de club naar de bal.De bal is in dit geval het systeem waarop door de omgeving arbeid wordt verricht. Bij chemische reacties kan arbeid worden geassocieerd met meerdere fysische of chemische veranderingen in een systeem. Een veel voorkomend soort arbeid is druk-volume arbeid.Denk aan de verbranding in de cilinder van een motor. De verbranding genereert niet alleen warmte, maar ook gas, dat arbeid verricht. Wanneer het gasvolume in de cilinder uitzet, duwt de druk de zuiger naar beneden tegen krachten van buitenaf in de omgeving.Druk wordt gedefinieerd als een kracht die over een gebied inwerkt. Hier wordt de kracht van de uitzettende gassen verdeeld over het gebied van de basis van de zuiger. Met andere woorden, de kracht is druk maal gebied.Arbeid, dat kracht maal afstand is, kan dus worden herschreven als druk vermenigvuldigd met het gebied en de afstand waarover het werkt. Voor de motorcilinder is dit het hoogteverschil dat de zuiger beweegt. Aangezien oppervlakte maal hoogte het volume van een cilinder is, kan de vergelijking voor arbeid worden gedefinieerd als druk maal de volumeverandering van het systeem.Omdat het volume echter toeneemt en de zuiger naar beneden wordt gedrukt, verricht het systeem feitelijk arbeid aan de omgeving, wat volgens afspraak een negatieve waarde is. Daarom wordt werk gedefinieerd als het negatief van de druk P maal delta V of de verandering tussen het uiteindelijke en initiële volume, die plaatsvond tijdens de expansie. De eenheid voor druk-volume-arbeid wordt meestal gedefinieerd als liter-atmosfeer.Deze eenheid kan worden omgezet naar de conventionele energie-eenheid, Joules, door de conversiefactor te gebruiken dat één liter-atmosfeer gelijk is aan 101, 3 Joule.

kwantificeren van werk

Als een systeem een verandering ondergaat, kan de interne energie veranderen en kan energie worden overgedragen van het systeem naar de omgeving, of van de omgeving naar het systeem.  Energieoverdracht vindt plaats door warmte en arbeid. De relatie tussen interne energie, warmte en arbeid wordt weergegeven door de vergelijking: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-01.png" style="height: 40px;"> Terwijl warmte een functie is van een waargenomen temperatuurverandering, is arbeid een functie van een waargenomen volumeverandering, de zogenaamde druk-volumearbeid. Arbeid (w) kan worden gedefinieerd als een kracht (F) die over een afstand (D) werkt. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/Equations-02.png" style="height: 40px;"> Druk-volumewerk (of expansiewerk) vindt plaats wanneer een systeem de omgeving tegen een beperkende druk terugduwt, of wanneer de omgeving het systeem comprimeert. Een voorbeeld hiervan doet zich voor tijdens de werking van een verbrandingsmotor. De verbrandingsreactie van benzine en zuurstof is exotherm. Een deel van deze energie wordt afgegeven als warmte, en een deel wordt uitgevoerd als arbeid door de gassen in de cilinder uit te zetten, waardoor de zuiger naar buiten wordt geduwd. De stoffen die bij de reactie betrokken zijn, zijn het systeem, en de motor en de rest van het universum zijn de omgeving. Het systeem verliest energie door zowel verwarming als werkzaamheden aan de omgeving, en de interne energie neemt af. Wanneer het volume van een cilinder toeneemt (dat wil zeggen, het gas zet uit), duwt het tegen een externe kracht, namelijk de druk gedefinieerd als kracht per oppervlakte-eenheid. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-03.png" style="height: 40px;"> Uit vergelijkingen 2 en 3: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-04.png" style="height: 40px;"> Het product van oppervlakte en afstand (A × D) is gelijk aan de verandering in het volume (ΔV) van het gas in de cilinder. Daarom, <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-05.png" style="height: 40px;"> Omdat het volume tijdens de expansie toeneemt, Vfinal > Vinitial, en ΔV is positief. Voor een positieve expansie (dat wil zeggen wanneer het systeem op de omgeving werkt) moet w echter negatief zijn, en daarom wordt een negatief teken aan de vergelijking toegevoegd.  <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-06.png" style="height: 40px;"> Volgens deze vergelijking is druk-volumearbeid het negatief van de externe druk (of tegendruk) vermenigvuldigd met de verandering in volume.  De op deze vergelijking gebaseerde werkeenheid is L·atm. Enkele andere nuttige conversiefactoren zijn:  <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-07.png" style="height: 40px;"> <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-08.png" style="height: 40px;"> Deze tekst is aangepast van <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">Openstax, Chemistry 2e, Sectie 5.3: Enthalpie</a>.

Kwantificeren van werk

As a system undergoes a change, its internal energy can change, and energy can be transferred from the system to the surroundings, or from the ...

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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 called expansion work, happens when a system pushes against a restraining pressure. It can also happen when the surroundings compress the system.

A common example is an internal combustion engine. The combustion of gasoline and oxygen is exothermic, so it gives off energy. Some of that energy is released as heat, and some is used as work that expands the gases in the cylinder and pushes the piston outward. In this situation, the reacting substances are the system, and the engine and the rest of the universe are the surroundings. The system loses energy by heating and by doing work on the surroundings, so its internal energy decreases.

When a gas expands in a cylinder, it pushes against an external force. That force is pressure, which is force per unit area. The area of the piston times the distance it moves equals the change in gas volume, or ΔV. Since the volume increases during expansion, ΔV is positive.

For expansion, the work is written with a negative sign because the system is doing work on the surroundings. Pressure-volume work is the negative of the external pressure multiplied by the change in volume. The unit of this work is L·atm, and it can be converted using other common unit factors.

Pressure-Volume Work in Chemistry

energy basics

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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...

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Internal Energy, Heat, and Work

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quantifying work

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...

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standard enthalpy of formation

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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

Learn how pressure-volume work links gas expansion, internal energy, and sign conventions in chemistry, with an engine example and the L·atm unit.

Pressure-Volume Work in Chemistry | JoVE K12

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Pressure-Volume Work in Chemistry

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