Name: pressure-volume work in chemistry
Uploaded: 2020-09-03T13:38:27.000Z
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PS.Chm.7.2

Quantifying Work

Напомним, что внутренняя энергия системы определяется как сумма теплоты и работы. Теплота измеряется калориметрическим методом, но как измерить работу? Работа это результат воздействия силы на объект, например, когда его тянут, толкают или поднимают.Таким образом, работа равна силе, умноженной на расстояние. Если удар клюшки для гольфа перемещает мяч на 100 футов, значит, она выполнила работу. Энергия передается от клюшки к мячу.В этом случае мяч это система, над которой осуществляет работу окружение. В химических реакциях работа может быть связана с множеством физических или химических изменений в системе. Часто встречающийся тип работы это работа с давлением и объемом.Рассмотрим сгорание в цилиндре двигателя. При сгорании не только выделяется тепло, но и выделяется газ, который выполняет работу. Когда объем газа в цилиндре увеличивается, давление толкает поршень вниз, действуя против внешних сил окружающей среды.Давление определяется как сила, действующая на площадь. Здесь сила расширяющихся газов распределяется по площади основания поршня. Другими словами, сила равна давлению, умноженному на площадь.Таким образом, работу, которая является силой, умноженной на расстояние, можно представить как давление, умноженное на площадь и расстояние, на котором оно действует. Для цилиндра двигателя это разница высот, на которую перемещается поршень. Учитывая, что площадь, умноженная на высоту, это объем цилиндра, уравнение работы можно определить как умножение давления на изменение объема системы.Однако, поскольку объем увеличивается, и поршень выталкивается вниз, система фактически выполняет работу с окружающей средой, которая обычно имеет отрицательное значение. Поэтому работа определяется как отрицательное значение давления P, умноженное на дельта V, или изменение конечного и начального объема, которое произошло во время расширения. Единица измерения работы давления и объема обычно определяется как литр-атмосфера.Эту единицу можно преобразовать в обычную единицу энергии, Джоули, используя коэффициент преобразования, равный 101, 3 Джоуля.

<ol>
	<li>Шмидт-Рор, Клаус. "Работа по расширению без внешнего давления и термодинамики в условиях квазистатических необратимых процессов." Журнал химического образования 91, No3 (2014): 402-409.</li>
	<li>Гисласон, Эрик А., и Норман К. Крейг. "Общие определения работы и тепла в термодинамических процессах." Журнал химического образования 64, No8 (1987): 660.</li>
	<li>О'Лоан, Д. Кеннет. "Адиабатические изменения: обратимые и необратимые изменения, связанные только с давлением-объемом." Журнал химического образования 30, No4 (1953): 190. 
	 </li>
</ol>

Количественная оценка работы

По мере того как система претерпевает изменения, ее внутренняя энергия может меняться, и энергия может передаваться из системы в окружающую среду или из окружающей среды в систему.  
 
 Передача энергии происходит за счет тепла и работы. Взаимосвязь между внутренней энергией, теплом и работой представлена уравнением: 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-01.png"" style=""height: 40px;"" />
 
 В то время как тепло является функцией наблюдаемого изменения температуры, работа является функцией наблюдаемого изменения объема, называемого работой давления-объема. Рабочий (w) может быть определен как усилие (F), действующее через расстояние (D). 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/795/Equations-02.png"" style=""height: 40px;"" />
 
 Работа по давлению-объему (или работа по расширению) происходит, когда система отталкивает окружающую среду от сдерживающего давления или когда окружающая среда сжимает систему. Пример этого происходит во время работы двигателя внутреннего сгорания. Реакция горения бензина и кислорода экзотермична. Часть этой энергии выделяется в виде тепла, а часть выполняется в виде работы за счет расширения газов в цилиндре, тем самым выталкивая поршень наружу. Вещества, участвующие в реакции, - это система, а двигатель и остальная вселенная - это окружение. Система теряет энергию как при нагревании, так и при работе с окружающей средой, а ее внутренняя энергия уменьшается. 
 
 При увеличении объема цилиндра (т.е. газ расширяется) он толкает его против внешней силы, которая представляет собой давление, определяемое как сила на единицу площади. 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-03.png"" style=""height: 40px;"" />
 
 Из уравнений 2 и 3: 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-04.png"" style=""height: 40px;"" />
 
 Произведение площади и расстояния (A × D) равно изменению объема (ΔV) газа в баллоне. 
 
 Поэтому 
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-05.png"" style=""height: 40px;"" />
 
 Так как объем увеличивается во время расширения, VFinal > Vinitial, и ΔV является положительным. Однако для положительного расширения (т.е., когда система работает над окружающей средой), w должно быть отрицательным, и поэтому к уравнению добавляется отрицательный знак.  
 
 <img alt=""Eq1"" src=""https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-06.png"" style=""height: 40px;"" />
 
 В соответствии с этим уравнением работа давления-объема равна отрицательному значению внешнего давления (или противоположного давления), умноженным на изменение объема.  
 
 Единица измерения, основанная на этом уравнении, — L·atm. Некоторые другие полезные коэффициенты преобразования:  
 
 <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;"" />
 
 Этот текст адаптирован из <a href=""https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy"">Openstax, Химия 2е изд., раздел 5.3: Энтальпия</a>. "

По мере того, как система претерпевает изменения, ее внутренняя энергия может меняться, и энергия может передаваться от системы к окружающей среде или от ...

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KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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

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...

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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.

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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...

quantifying heat

Heat Transfer and Specific Heat

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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

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thermochemical equations

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hess's law

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standard enthalpy of formation

Using Standard Enthalpy Values in Chemistry

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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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PS.Chm.7.3

Understand the relationship among pressure, temperature, volume, and phase.

Pressure-Volume Work in Chemistry

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