Name: enthalpy and heat at constant pressure
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 59 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Enthalpy

化学反応の中には ロケット燃料の燃焼で スペースシャトルが 地上から飛び立つような ものすごい熱を出して周囲に 働きかけるものがあります 熱量qと仕事量wの和は 熱力学の第一法則で与えられた 内部エネルギーの変化量ΔEです 大気圧で発生する 気体を含む化学反応の場合 行われる仕事は 体積の変化に伴う 機械的な仕事 つまり 膨張または収縮です つまり 仕事は圧力の負の値に 体積の変化を 乗算したものになります 熱力学の第一法則の wを代入し 式の項を並べ替えると qはΔEにPを加えた ΔVに等しくなり 一定圧力下での 熱流の式が得られます 食品を調理するために 木材を燃やすような 他の化学反応では 周囲の膨張作用の量を 測定するよりも 調理を行うために 放出される熱を定量化することが より重要です 内部エネルギーは 熱と仕事の両方に関係し 一定の圧力条件では ΔEは使用されません 熱の形でのエネルギーの 流れだけを議論するために 新しい熱力学関数として エンタルピーが定義されています エンタルピーHは 内部エネルギーEと 圧力-体積の仕事P-Vの 合計に等しくなります エネルギー 圧力 体積は 状態関数なので エンタルピーも状態関数です 特定の物質の 絶対エンタルピー値は 測定できません エンタルピーの変化のみを 測定することができます エンタルピーの変化ΔHは 内部エネルギーの変化ΔEと PXΔVの合計に 等しくなります エネルギーの変化は 熱と圧力-体積の仕事の 和であることを考えると この式を組み合わせることで 一定の圧力条件下では ΔHはシステムによって 得られた または 失われた熱 qに 等しいことがわかります システムが熱の形で 周囲にエネルギーを失う場合 木材の燃焼のように）周囲の温度は上昇します これはqの負の符号の規則 によって記述されます その結果 ΔHは負になり この仮定は 発熱性と呼ばれます 逆に 化学的な冷却材で 起こる反応のように システムが熱の形で 周囲からエネルギーを得ると 周囲の温度は下がります この場合の熱は 正の符号の規則によって 記述されます これによりΔHが正となり その過程は 吸熱性と呼ばれます

<ol>
	<li>Canagaratna、Sebastian G.「エンタルピー計算の視覚的補助」 Journal of Chemical Education 77、no. 9(2000):1178。</li>
	<li>ハワード、イルムガルドK.「Hはエンタルピー用です。Heike Kamerlingh OnnesとAlfred W. Porterのおかげです。」 Journal of chemical education 79, no. 6 (2002): 697.</li>
	<li>Van Ness、Hendrick C.非電解質溶液の古典的熱力学。エルゼビア, 2015. 
	 </li>
</ol>

エンタルピー

化学者は通常、エンタルピー（ H ）と呼ばれる特性を用いて、化学的・物理的プロセスの熱力学を記述します。 エンタルピーとは、システムの内部エネルギー（ E ）に、その圧力（ P ）と体積（ V）の数学的な積の合計を指します。 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/796/796_Equations-01.png" style="height: 40px;" />
 
 エンタルピーは状態関数です。 特定の物質のエンタルピー値は直接測定できません。化学的または物理的プロセスのエンタルピー変化のみを測定できます。 一定の圧力で行われるプロセス（多くの化学的および物理的変化の共通条件）では、エンタルピー変化（ Δ H ）は次のようになります。 
 
 <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/796/796_Equations-02.png" style="height: 40px;" />
 
 数学的な積PΔV は、仕事（ w ）、すなわち膨張または圧力 - 体積の仕事を表します。 その定義により、 Δ V と w の符号は常に逆になります。 
 
 <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/796/796_Equations-03.png" style="height: 40px;" />
 
 この方程式と一定圧力における内部エネルギーの定義（ ΔE = qp + w ）をエンタルピー変化方程式に置き換えると、次のようになります。 
 
 <img alt="Eq4" src="https://www.jovecore.com/CDNSource/lessons/796/796_Equations-04.png" style="height: 40px;" />
 
 ここで、 qp は一定圧力の条件下での反応熱です。  
 
 そのため、化学的または物理的なプロセスが一定の圧力で実行され、膨張または収縮による唯一の仕事（ P-V 仕事）が行われた場合、プロセスの熱の流れ（ qp ）とエンタルピー変化（ Δ H）は等しくなります。 
 
 バーナーの運転中に発生する熱は、メタン燃焼反応が本質的に大気中の一定圧力で発生することから、そのエンタルピー変化と等しくなります。 化学者は通常、通常の大気条件で実験を行い、外圧を一定にしてqp = ΔHとします。よって、化学反応の熱変化を求めるにはエンタルピーが最も便利です。 
 
 エンタルピー変化が負の値 Δ H < 0 は発熱反応（周囲に放出される熱）を示し、正の値 Δ H > 0 は吸熱反応（周囲から吸収される熱）を示します。 化学方程式の方向が逆になると、その Δ H の符号が変更される（一方向に吸熱するプロセスは、反対方向に発熱する）。 
 
 概念的には、 Δ E （熱と仕事の測定値）と Δ H （一定圧力での熱の測定値）は、どちらもシステムの状態機能の変化を表します。 体積変化が小さいプロセスでは Δ V が小さく（氷の融解）、 Δ E と Δ H が同一です。 ただし、体積の変化が大きい場合（水の蒸発）、仕事として伝達されるエネルギー量は大きくなります。したがって、 Δ E と Δ H の値は大きく異なります。 
 
 このテキストは <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">、 Openstax, Chemistry 2e, Section 5.3: Endalpy から引用</a>しています。

化学者は通常、エンタルピー(H)として知られる特性を使用して、化学的および物理的プロセスの熱力学を記述します。エンタルピーは合計として定義されます...

Education

JoVE Core

化学

Thermochemistry

熱化学

KEY TERMS AND CONCEPTS

主要用語と概念

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 represents expansion work, also called pressure-volume work. The signs of ΔV and w are always opposite, because expansion and work use opposite sign conventions.

At constant pressure, the internal energy change is ΔE = qp + w, where qp is the heat of reaction. When the only work done is due to expansion or contraction, qp and ΔH are equal. This is why enthalpy is a useful choice for measuring heat changes in many lab reactions and physical changes. A Bunsen burner is one example, since methane combustion occurs at about atmospheric pressure.

A negative ΔH means the process is exothermic, which means heat is given off to the surroundings. A positive ΔH means the process is endothermic, which means heat is absorbed from the surroundings. If a chemical equation is reversed, the sign of ΔH also changes. A process that is endothermic in one direction becomes exothermic in the other.

Enthalpy change and internal energy change both describe changes in a state function for the system. When the volume change, ΔV, is small, such as during melting of ice, ΔE and ΔH are essentially the same. When the volume change is larger, such as during evaporation of water, more energy is transferred as work. In that case, ΔE and ΔH can differ by a significant amount.

Enthalpy and Heat at Constant Pressure

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first law of thermodynamics

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internal energy

Internal Energy, Heat, and Work

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enthalpy

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

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standard enthalpy of formation

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key terms and concepts

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Enthalpy and Heat at Constant Pressure | JoVE K12

chemistry

JoVE Core 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

Matter and its Interactions

PS.Chm.4.1

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.a

CH.7.c

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New Jersey Student Learning Standards for Science

High School

Physical Science

Matter and Its Interactions

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Arizona Science Standards

HS+Phy.P4

HS+Phy.P4U1.6

Texas Essential Knowledge and Skills

CHEM.13

CHEM.13.A

CHEM.13.C

Next Generation Science Standards

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HS-PS3-1.

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SC5.

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Enthalpy and Heat at Constant Pressure

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