Name: how intermolecular forces affect matter
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 56 s
Channel: JoVE

K12 Standards

Louisiana Student Standards for Science

High School

Chemistry

Matter And Its Interactions

HS-PS1-3

intermolecular-forces-and-physical-properties

Intermolecular Forces and Physical Properties

Elektrostatische Wechselwirkungen, die zwischen Molekülen bestehen, werden als intermolekulare Kräfte bezeichnet. Diese Anziehungskräfte beeinflussen verschiedene physikalische Eigenschaften wie Schmelzpunkt, Siedepunkt und Löslichkeit.
Zu den drei Haupttypen intermolekularer Kräfte gehören starke Ionen-Dipol-Wechselwirkungen zwischen Ionen und polaren Molekülen, Dipol-Dipol-Wechselwirkungen zwischen polaren Molekülen und schließlich die schwächste von allen – Dispersionskräfte – die in allen Molekülen vorhanden sind, polare und unpolare.
Ionen-Dipol-Wechselwirkungen sind bei Lösungen üblich. Wenn eine ionische Verbindung wie Natriumchlorid in einem polaren Lösungsmittel wie Wasser gelöst wird, richten sich die Ionen an den entgegengesetzten geladenen Enden der Wassermoleküle aus, was eine maximale elektrostatische Anziehungskraft ermöglicht.
Alternativ werden Dipol-Dipol-Wechselwirkungen von polaren Molekülen mit permanenten Dipolen gezeigt, bei denen das positive Ende eines Moleküls elektrostatisch mit dem negativen Ende des benachbarten Moleküls wechselwirkt.
Wenn ein Wasserstoffatom, das entweder an Sauerstoff, Stickstoff oder Fluor gebunden ist, von dem elektronegativen Atom eines benachbarten Moleküls angezogen wird, entsteht eine Wasserstoffbrückenbindung – eine spezielle Art der Dipol-Dipol-Wechselwirkung. Insbesondere Verbindungen, die Wasserstoffbrückenbindungen aufnehmen können, weisen hohe Schmelz- und Siedepunkte auf.
Dispersionskräfte sind das Ergebnis temporärer Dipole und neigen dazu, mit der molaren Masse zuzunehmen. Atome mit höheren Massen haben mehr Elektronen und größere Elektronenwolken, was zu erhöhten Dispersionskräften führt. Infolgedessen wird mehr Energie benötigt, um die Anziehungskräfte zwischen benachbarten Atomen zu überwinden, was zu höheren Siede- und Schmelzpunkten führt.
Dies erklärt, warum die Siedepunkte von Alkanen mit ihrer molaren Masse steigen.
Die Molmasse ist jedoch nicht das einzige Kriterium. Bei gleichen Massen hat n-Pentan aufgrund der größeren Oberfläche, die für die Dispergierkräfte zur Verfügung steht, einen höheren Siedepunkt als Neopentan.
Intermolekulare Kräfte sind auch entscheidend für die Bestimmung der Löslichkeit. Flüssigkeiten ähnlicher Art und Größenordnung intermolekularer Kräfte, wie Wasser und Ethanol, sind in allen Anteilen vollständig löslich oder mischbar.
Im Gegensatz dazu sind Flüssigkeiten wie Hexan und Wasser mit unterschiedlichen Arten und Größen intermolekularer Kräfte in den meisten Anteilen unlöslich oder nicht mischbar.
Insgesamt sind die intermolekularen Kräfte relativ schwach, da kleine oder partielle Ladungen über große Entfernungen wechselwirken.

intermolekulare kräfte und physikalische eigenschaften

Intermolekulare Kräfte sind Anziehungskräfte, die zwischen Molekülen wirken. Sie bestimmen mehrere Masseneigenschaften, wie z.B. Schmelzpunkte, Siedepunkte und Löslichkeiten (Mischbarkeiten) von Substanzen. Beispielsweise weist eine Flüssigkeit mit hohem Siedepunkt wie Wasser (H2O, Siedepunkt 100 °C) stärkere intermolekulare Kräfte auf als eine Flüssigkeit mit niedrigem Siedepunkt wie Hexan (C6H14, Siedepunkt 68,73 °C). Zu den drei Arten intermolekularer Wechselwirkungen gehören i) Ionen-Dipol-Kräfte, ii) Dipol-Dipol-Wechselwirkungen und iii) Van-der-Waals-Kräfte, zu denen auch Londonsche Dispersionskräfte gehören.

1. Ionen-Dipol-Kräfte

Ionen-Dipol-Kräfte sind die elektrostatischen Anziehungskräfte zwischen einem Ion und einem Dipol. Sie kommen häufig in Lösungen vor und spielen eine wichtige Rolle bei der Auflösung ionischer Verbindungen wie KCl in Wasser. Die Stärke der Ion-Dipol-Wechselwirkungen ist direkt proportional zu i) der Ladung des Ions und ii) der Größe des Dipols polarer Moleküle.

2. Dipol-Dipol-Wechselwirkungen

Polare Moleküle haben eine teilweise positive Ladung an einem Ende und eine teilweise negative Ladung am anderen Ende des Moleküls – eine Ladungstrennung, die als Dipol bezeichnet wird. Die Anziehungskraft zwischen zwei permanenten Dipolen wird als Dipol-Dipol-Anziehung bezeichnet – die elektrostatische Kraft zwischen dem teilweise positiven Ende eines polaren Moleküls und dem teilweise negativen Ende eines anderen. Wasserstoffbrückenbindungen sind eine Art von Dipol-Dipol-Wechselwirkung zwischen Molekülen mit Wasserstoff, der an ein stark elektronegatives Atom wie O, N oder F gebunden ist. Das daraus resultierende teilweise positiv geladene H-Atom auf einem Molekül (der Wasserstoffbrückenbindungs-Donor) kann starke Wechselwirkungen mit einem einsamen Elektronenpaar eines teilweise negativ geladenen O-, N- oder F-Atoms auf einem benachbarten Molekül (dem Wasserstoffbrückenbindungsakzeptor) aufweisen. Wasserstoffbrücken erhöhen den Siedepunkt erheblich.

3. Van der Waals Kräfte und London-Dispersions-Wechselwirkungen

Die schwächste aller Kräfte ist die Van-der-Waals-Kraft, die von den intermolekularen Abständen zwischen Atomen und Molekülen abhängt. Londonsche Dispersionskräfte, eine Teilmenge der Van-der-Waals-Kräfte, treten als Folge von Wechselwirkungen zwischen ungeladenen Atomen/Molekülen aufgrund vorübergehender, spontaner Verschiebungen in der Elektronenverteilung auf. Die Stärke dieser Kräfte scheint mit zunehmendem Molekulargewicht aufgrund der Vergrößerung der Oberfläche zuzunehmen. Infolgedessen sieden Verbindungen mit höherem Molekulargewicht im Allgemeinen bei höheren Temperaturen. Bemerkenswert ist, dass ein verzweigter Kohlenwasserstoff (Neopentan) normalerweise eine kleinere Oberfläche als sein entsprechendes geradkettiges Isomer (n-Pentan) und daher einen niedrigeren Siedepunkt hat.

4. Löslichkeit organischer Verbindungen in Wasser

Flüssigkeiten, die in jedem Verhältnis homogen vermischt werden können, werden als mischbar bezeichnet. Mischbare Flüssigkeiten haben ähnliche Polaritäten. Beispielsweise sind Methanol und Wasser beide polar und zur Bildung von Wasserstoffbrücken fähig. Beim Mischen treten Methanol und Wasser durch intermolekulare Wasserstoffbrückenbindungen in Wechselwirkung, die ähnlich stark sind wie die Wechselwirkungen zwischen Methanol und Methanol und zwischen Wasser und Wasser; sie sind also mischbar. Ebenso sind unpolare Flüssigkeiten wie Hexan und Brom durch Dispersionskräfte miteinander mischbar. Der chemische Grundsatz „Gleiches löst Gleiches“ ist nützlich, um die Mischbarkeit von Verbindungen vorherzusagen. Zwei Flüssigkeiten, die sich nicht nennenswert vermischen, werden als nicht mischbar bezeichnet. Beispielsweise ist unpolares Hexan mit polarem Wasser nicht mischbar. Die relativ schwachen Anziehungskräfte zwischen Hexan und Wasser reichen nicht aus, um die stärkeren Wasserstoffbindungskräfte zwischen den Wassermolekülen zu überwinden.

Dieser Text wurde angepasst von'<a href="https://openstax.org/books/chemistry-2e/pages/10-1-intermolecular-forces"> Openstax, Chemistry 2e, Section 10.1: Intermolecular Forces</a>, <a href="https://openstax.org/books/chemistry-2e/pages/11-3-solubility">Section 11.3: Solubility</a>, and <a href="https://openstax.org/books/chemistry-2e/pages/10-introduction">Chapter 10: Liquids and Solids</a>.

Intermolekulare Kräfte und physikalische Eigenschaften


Intermolekulare Kräfte sind Anziehungskräfte, die zwischen Molekülen bestehen. Sie bestimmen verschiedene Bulk-Eigenschaften, wie z.B. Schmelzpunkte, Siedepunkte, ...

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Organic Chemistry

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Unit 1

Einheit 1

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how intermolecular forces affect matter

Intermolecular forces are the attractions between molecules, and they help explain how matter behaves. These forces influence melting points, boiling points, and whether substances mix with one another. Water has a much higher boiling point than hexane, which shows that water has stronger intermolecular forces.

Three main kinds of intermolecular interactions are covered here. They include ion-dipole forces, dipole-dipole interactions, and van der Waals forces, which include London dispersion forces. Each type depends on how charges are arranged in or between molecules.

Ion-dipole forces are electrostatic attractions between an ion and a dipole. They are common in solutions and are important when ionic compounds such as KCl dissolve in water. Their strength depends on the charge on the ion and the size of the dipole in the polar molecule.

Dipole-dipole interactions happen between polar molecules. A polar molecule has a partial positive end and a partial negative end, which is called a dipole. Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is bonded to O, N, or F, and it can raise boiling points a great deal.

Van der Waals forces are the weakest intermolecular forces discussed here. London dispersion forces are a type of van der Waals force and arise from temporary shifts in electron distribution in uncharged atoms or molecules. These forces usually get stronger as molecular weight increases because surface area increases, so compounds with higher molecular weights tend to boil at higher temperatures. A branched hydrocarbon such as neopentane has less surface area than straight-chain n-pentane, so neopentane has the lower boiling point.

Intermolecular forces also help predict solubility in water. Liquids that mix in any proportion are called miscible, and they usually have similar polarities. Methanol and water are miscible because both are polar and can form hydrogen bonds. Nonpolar hexane and bromine are also miscible with each other through dispersion forces. The idea of “like dissolves like” helps explain why this happens. Hexane and water are immiscible because the weak attraction between them does not overcome the stronger hydrogen bonding among water molecules.

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How Intermolecular Forces Affect Matter

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