Name: boiling point and vapor pressure
Uploaded: 2020-03-26T07:22:27.000Z
Duration: 4 min 25 s
Channel: JoVE

K12 Standards

Georgia Standards of Excellence

9-12

Physical Science

SPS2.

Physical Properties

Boiling Points

液体は、分子間の弱い相互作用によって一緒に保持されています。気相に移行するには、分子はこれらの相互作用を克服するために、熱または他のエネルギー源から十分なエネルギーを得る必要があります。
室温で純粋な液体の密閉容器を考えてみましょう。表面の液相分子の一部は蒸発するのに十分なエネルギーを獲得し、その蒸気の一部はエネルギーを失い、凝縮して液体に戻ります。蒸発速度と凝縮速度が同じ場合、液体と蒸気は平衡状態に達します。この時点で、液体の上の空間は蒸気で飽和しており、容器内の液体と蒸気の量に正味の変化はありません。蒸気によって加えられる圧力は蒸気圧と呼ばれます。
さて、液体の開いた容器を考えてみましょう。ここでは、蒸気が逃げることができ、液体の上の領域が飽和するのを防ぎます。したがって、液体がなくなるまで蒸発が続く可能性があります。液体の温度が上昇すると、蒸発速度も上昇し、したがって蒸気圧も上昇します。十分な熱が加えられると、液体の奥深くにある分子が気化し始め、蒸気の泡が形成されます。
これは沸騰と呼ばれ、化合物の蒸気圧が大気圧と等しくなったときに始まります。これが発生する温度が沸点です。純粋な物質はそれぞれ分子間相互作用の種類と強度が異なるため、物質が異なれば沸点も異なります。
では、分子間相互作用が沸点にどのように影響するかを見てみましょう。多くの分子間相互作用には、双極子と呼ばれる電子密度が不均一な領域が関与しています。すべての分子は電子分布に短い揺らぎがあるため、非極性分子でも一時的に双極子を持つことがあります。これに反応して、近くの分子に対向する双極子が形成されます。
一時的な双極子と誘導された双極子の間の引力はロンドン分散力と呼ばれ、炭化水素のような非極性分子間の支配的な相互作用です。ロンドン分散力は、電子を多く持つ大きな分子間では、それらの分子が電子を弱く保持し、隣接する分子が電子分布に影響を与えやすくなるため、より強くなります。
例えば、高分子量の非分岐アルカンは、低分子量の非分岐アルカンよりも沸点が高くなります。さらに、非分岐アルカンは、表面積が大きいため、同等の分子量を持つ分岐アルカンよりも沸点が高くなります。
極性結合を持つ分子は永久双極子を持ち、それらの間の規則的な双極子-双極子相互作用を可能にします。双極子-双極子相互作用はロンドン分散力よりも強いため、通常、極性液体を沸騰させるには非極性液体よりも多くの熱エネルギーが必要です。例えば、極性分子であるクロロペンタン1つの沸点は108°Cで、表面積は似ているが双極子を持たないヘキサンは沸点が低くなっています。
水素がより電気陰性の原子に共有結合した分子と、孤立電子対の電子を持つ電子吸引原子の両方を持つ分子は、水素結合が可能です。水素結合は双極子-双極子相互作用やロンドン分散力よりも強いため、それらを克服するにはさらに多くの熱エネルギーが必要です。例えば、プロパンとエタノールは分子量と表面積が似ています。しかし、プロパンの沸点はエタノールよりもはるかに低いのは、エタノール分子が互いに水素結合を形成できるのに対し、プロパンは水素結合を形成できないためです。
このラボでは、キャピラリー法を使用して、2つの有機溶媒の沸点を決定します。

沸点

沸点測定
融点と同様に、沸点は物理的特性です。サンプルが純粋な化合物の場合、沸点を使用して化合物の同一性を判断できます。結局のところ、正確な沸点を実験的に決定することは困難です。融点と同様に、実験の沸点は範囲として与えられ、実際の文献値から数度異なります。
蒸気圧
溶媒が沸騰する理由を理解するには、溶液のよく知られた泡立ちを特徴とするため、液相と気相の間のダイナミクスを理解することが重要です。密閉容器に入った純粋な液体化合物を考えてみましょう。液体の表面にある分子の一部は、分子間力に打ち勝ち、気相に入るのに十分なエネルギーを持っています。ただし、気相の分子もエネルギーを失い、凝縮して液体に戻ることがあります。したがって、このシステムには、蒸発と凝縮の2つの競合するプロセスがあります。
蒸発速度が凝縮速度と等しい場合、システムは平衡状態に達しています。これは、気相に入る分子ごとに、別の分子が液相に凝縮し、容器内の液体または気体の量の純増減がないことを意味します。平衡が確立されると、液体の上の蒸気によって加えられる圧力は蒸気圧と呼ばれます。液体が気化する傾向は、その揮発性と呼ばれます。 揮発性の高い液体は蒸気圧が高く、揮発性の低い液体は蒸気圧が低くなります。
蒸気圧は温度によって異なります。溶液の温度を上げると、より多くの分子が液相から逃げるのに十分なエネルギーを持っているため、蒸気圧が上昇します。最終的に、十分な熱が加えられると、液体と気体の間の界面にない分子が気相に遷移し、沸騰に関連するおなじみの泡を形成します。
液体の全蒸気圧が大気圧と同等になると、液体の沸点に達します。これが発生する温度は沸点と呼ばれます。標高が高いほど、つまり大気圧が低いと、蒸気圧を大気圧まで上げるために必要な熱が少なくて済むため、液体はより低い温度で沸騰します。さらに、揮発性、または溶媒が気化する能力も蒸気圧に影響を与えます。揮発性の高い溶剤は、揮発性の低い溶剤よりも蒸気圧が高くなります。
沸点に影響を与える要因
融点と沸点の類似性は、化合物の融点に影響を与えるのと同じ要因が沸点にも影響を与えることを意味します。したがって、液体化合物内に見られる分子間力の強度と種類は、沸点に影響を与えます。分子力には、水素結合、双極子-双極子相互作用、ロンドン分散力の3つのタイプがあることを思い出してください。これらはそれぞれ異なる引力強度を持ち、克服するために異なる量のエネルギーを必要とします。水素結合が可能な化合物は、ロンドン分散力によってのみ相互作用できる化合物よりも沸点が高くなります。沸点に関する追加の考慮事項には、化合物の蒸気圧と揮発性が含まれます。通常、化合物の揮発性が高いほど、その沸点は低くなります。
沸点を決定するためのキャピラリー法
有機化合物の沸点を測定する簡単な方法は、キャピラリー法を利用することです。このセットアップでは、空のガラスキャピラリーチューブを液相の純粋な化合物の容器に反転させます。液体が加熱されると、サンプルの蒸気圧が上昇し、ガス状の蒸気がガラスキャピラリーチューブに入り始めます。これにより、内部に閉じ込められた空気が押し出され、毛細管の底から気泡が発生します。この時点で、液体は冷まされます。サンプルの蒸気圧がガラスキャピラリーチューブ内の大気圧と同じになると、液体がチューブに入り始めます。この現象が発生したときの溶液の温度が液体化合物の沸点です。

沸点測定
融点と同様に、沸点は物理的特性です。サンプルが純粋な化合物である場合、沸騰...

Education

JoVE Lab Manual

Chemistry

化学

boiling point and vapor pressure

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value.

Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas molecules lose energy and condense back into the liquid. When evaporation and condensation happen at the same rate, the system is at equilibrium. The pressure from the vapor above the liquid is the vapor pressure.

Volatility is the tendency of a liquid to vaporize. A more volatile liquid has a higher vapor pressure, while a less volatile liquid has a lower vapor pressure. Vapor pressure also changes with temperature. As temperature rises, more molecules can escape the liquid, so vapor pressure increases. With enough heat, molecules inside the liquid also change to gas and form the bubbles seen during boiling.

A liquid boils when its total vapor pressure matches atmospheric pressure. That temperature is the boiling point. Because atmospheric pressure changes with elevation, a liquid boils at a lower temperature at higher elevations. Less atmospheric pressure means less heat is needed for the vapor pressure to reach that level.

Intermolecular forces also affect boiling point. These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. They differ in strength, so different amounts of energy are needed to overcome them. Compounds that can hydrogen bond usually have higher boiling points than compounds that rely only on London dispersion forces. Volatility matters too. In general, more volatile compounds have lower boiling points.

One simple way to measure a boiling point is the capillary method. An empty glass capillary tube is placed upside down in the liquid sample. As the sample is heated, its vapor pressure rises and vapor enters the tube. This pushes out trapped air and causes bubbles to appear from the bottom of the capillary tube. The sample is then allowed to cool. When liquid begins to rise into the tube, the vapor pressure inside the tube matches the atmospheric pressure. The temperature at that moment is the boiling point of the compound.

Boiling Point and Vapor Pressure

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How Impurities Change Melting Points

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boiling points

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value. Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas...

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Purifying Crystals by Solubility

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Separating Mixtures with Solvent Extraction

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simple distillation

Separating Liquids by Boiling Point

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Steam Distillation and Low-Boiling Separation

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TLC Separation by Polarity

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Separating Mixtures by Polarity

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identification-of-unknown-aldehydes-and-ketones

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chemistry

入門化学ラボコースの指導者と学生のための、包括的でカリキュラムに焦点を当てたビデオ。

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Explore how vapor pressure, volatility, and intermolecular forces affect boiling point, and learn how the capillary method measures it in the lab.

Boiling Point and Vapor Pressure | JoVE K12

沸点 を教える準備を、この 1 分間の教員向けガイドで行いましょう。JoVE Lab Manual の Chemistry ラボの主要な準備手順を解説します。

Chemistry の 沸点 教員準備 - 4.9K+ 回の視聴 | JoVE Lab Manual

この 6 分間の学生プロトコル動画で、沸点 の手順をステップごとに学びましょう。JoVE Lab Manual の Chemistry コレクションの一部です。

Chemistry の 沸点 学生プロトコル - 90K+ 回の視聴 | JoVE Lab Manual

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Similar to the melting point, the boiling point is a physical property. If the sample is a pure compound, then the boiling ...

Melting Points

Melting Points in Organic Chemistry
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Melting Point

Source: Vy M. Dong and Jan Riedel, Department of Chemistry, University of California, Irvine, CA
One of the most important properties of a crystalline ...

Electrolyte and Nonelectrolyte Solutions

Substances that undergo either a physical or a chemical change in solution to yield ions that can conduct electricity are called electrolytes. If a ...

ionic-compounds-formulas-and-nomenclature

Ionic Compounds: Formulas and Nomenclature

An element composed of atoms that readily lose electrons (a metal) can react with an element composed of atoms that readily gain electrons (a nonmetal) to ...

molecular-compounds-formulas-and-nomenclature

Molecular Compounds: Formulas and Nomenclature

Molecular compounds or covalent compounds result when atoms share electrons to form covalent bonds. Since there is no electron transfer, molecular ...

Chemical Symbols

A chemical symbol is an abbreviation that is used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. We use the ...

Chemical Formulas

A chemical formula presents information about the proportions of atoms constituting a particular chemical compound or molecule, mainly using symbols of ...

Chemical Names and Chemical Formulas

Obtain, evaluate, and communicate information to explain how atoms bond to form stable compounds.

Boiling Point and Vapor Pressure

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