Name: pressure-volume work in chemistry
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 30 s
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

Quantifying Work

系统的内能是被定义为 热及功的总和。而热是以热量计测量，但是是如何将其量化的呢？功是力所造成的结果，其当一个物体被 拉扯、推挤、或者举起一段距离。因此功等于力乘以距离。假设一个高尔夫球棍打中高尔夫球，并让其飞了 100英尺，高尔夫球棍便有做功。能量从球杆转换至球裡。在这个情况下，球是一个系统，而其功是表现在其周围环境中。在一个系统中的化学反应下，功可以与多种物理及化学改 变有所关联。一个普遍的功为压（力容）积功。一起来想想在引擎汽缸内的燃烧反应。燃烧反应不仅生成热，他更生成气体，其表现功。当气体体积在汽缸中膨胀，压力推将活塞向下推挤 并推挤外围的外部压力。压力被定义为作用在一个区域上的力。在此，来自膨胀气体的压力 分佈在活塞底部面积。换句而言，力是压力乘以面积。因此，当功其是力量乘以距离，也可以写作压力乘以面 积和所作用的距离。对于發动机汽缸，这是活塞移动的高度差。设面积乘以高度是汽缸的体积，那功的公式可被定义为压 力乘以系统内体 积的改变。然而，因为体积增加而活 塞被往下推挤，系统实际上是在对周围环境做功。按照惯例，这会是一个负值。因此，功被定义为 负值的压力P乘以 delta V 也就是膨胀前 后改变的体积，压积功的单位 通常定义为公升一标准大气压 这个单位可以转换成能量的常规单位，焦耳，以一公升一标准大气压等于 101.3焦耳做转换。

<ol>
	<li>Schmidt-Rohr， Klaus. "就准静态不可逆过程而言，无外部压力和热力学的膨胀功。"化学教育杂志 91，第 3 期（2014）:402-409。</li>
	<li>Gislason、Eric A. 和 Norman C. Craig。"热力学过程中功和热的一般定义。"化学教育杂志 64，第 8 期（1987）:660。</li>
	<li>O'Loane， J. Kenneth. "绝热变化:仅涉及压力体积功的可逆和不可逆变化。"化学教育杂志 30，第 4 期（1953）:190. 
	 </li>
</ol>

量化做功

随着系统的变化，其内部能量也会发生变化，并且能量可以从系统传递到周围环境，或从周围环境传递到系统。
 
 通过热和功进行能量传递。内部能量，热量和功之间的关系由以下方程式表示：
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-01.png" style="height: 40px;" />
 
 虽然热量是观察到的温度变化的函数，但是功是观察到的体积变化的函数，称为压力-体积功。功（ w ）可以定义为作用距离（ D ）的力（ F ）。
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/Equations-02.png" style="height: 40px;" />
 
 当系统克服约束压力将周围环境向后推时，或者当周围环境压缩系统时，就会发生压力-体积功（或膨胀功）。这样的一个例子在内燃机的运行中发生。汽油和氧气的燃烧反应是放热的。这些能量中的一部分以热量的形式散发出，有些通过做功使气缸中的气体膨胀，从而向外推动活塞。反应涉及的物质是系统，引擎和宇宙的其余部分是周围的环境。该系统通过加热和在周围环境中工作而损失能量，其内部能量减少。
 
 当气缸的容积增加（即气体膨胀）时，它会抵抗外力，这是定义为每单位面积的力。
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-03.png" style="height: 40px;" />
 
 根据等式2和3：
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-04.png" style="height: 40px;" />
 
 面积与距离的乘积（ A ＆times; D ）等于体积的变化量（Δ V ）气瓶中的气体。
 
 因此
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-05.png" style="height: 40px;" />
 
 由于在扩展过程中体积会增加，因此 V final ＆gt; V initial ，而＆lt; V V 为正。但是，对于正扩展（即，当系统确实在环境中工作时）， w 应该为负，因此，方程式中会添加负号。
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/795/795_Equations-06.png" style="height: 40px;" />
 
 根据此等式，压力-体积功是外部压力（或反向压力）乘以体积变化的负值。
 
 基于该等式的工作单位为L·atm。其他一些有用的转换因素是：
 
 <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;" />
 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy"> Openstax，化学2e，第5.3节：焓</ a>。

当一个系统经历变化时，其内能可能发生改变，能量可以从系统传递到环境，或从环境传递到系统。

Education

教育

JoVE Core

化学

Thermochemistry

热化学

KEY TERMS AND CONCEPTS

关键术语与概念

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...

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, 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

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 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

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

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hess's law

Calculating Reaction Heat with Hess's Law

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standard enthalpy of formation

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enthalpies of reaction

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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

chemistry

JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

Matter and its Interactions

PS.Chm.4.1

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Physical Science

Energy And Waves

transfer-and-transformation-of-energy-including-work

Transfer and transformation of energy (including work)

Interactions Of Matter

Chemical reactions

Tennessee Academic Standards

High School

Physics

PHYS.PS3.7

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.a

Texas Essential Knowledge and Skills

CHEM.13

CHEM.13.A

CHEM.13.C

new-york-state-science-learning-standards

New York State Science Learning Standards

HS. Energy

HS-PS3-3.

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PS.Chm.7.1

Energy Basics

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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 ...

Internal Energy

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Quantifying Heat

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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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