Name: how intermolecular forces affect matter
Uploaded: 2025-05-22T09:05:33.000Z
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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

As interações eletrostáticas que existem entre as moléculas são chamadas de forças intermoleculares. Essas forças atrativas influenciam várias propriedades físicas, como ponto de fusão, ponto de ebulição e solubilidade.
Os três principais tipos de forças intermoleculares incluem fortes interações íon-dipolo entre íons e moléculas polares, interações dipolo-dipolo entre moléculas polares e, finalmente, a mais fraca de todas - forças de dispersão - presentes em todas as moléculas, polares e apolares.
As interações íon-dipolo são comuns às soluções. Quando um composto iônico como o cloreto de sódio é dissolvido em um solvente polar como a água, os íons se alinham com as extremidades de carga oposta das moléculas de água, permitindo a máxima atração eletrostática.
Alternativamente, as interações dipolo-dipolo são exibidas por moléculas polares com dipolos permanentes, onde a extremidade positiva de uma molécula interage eletrostaticamente com a extremidade negativa da molécula vizinha.
Quando um átomo de hidrogênio ligado a oxigênio, nitrogênio ou flúor é atraído para o átomo eletronegativo de uma molécula vizinha, uma ligação de hidrogênio - um tipo especial de interação dipolo-dipolo - é formada. Notavelmente, compostos capazes de ligações de hidrogênio exibem altos pontos de fusão e ebulição.
As forças de dispersão são resultado de dipolos temporários e tendem a aumentar com a massa molar. Átomos com massas mais altas têm mais elétrons e nuvens de elétrons maiores, levando a maiores forças de dispersão. Como resultado, mais energia é necessária para superar as forças atrativas entre os átomos vizinhos, resultando em pontos de ebulição e fusão mais altos.
Isso explica por que os pontos de ebulição dos alcanos aumentam com sua massa molar.
No entanto, a massa molar não é o único critério. Apesar de ter as mesmas massas, o n-pentano tem um ponto de ebulição mais alto do que o neopentano devido ao aumento da área de superfície disponível para as forças de dispersão.
As forças intermoleculares também são cruciais na determinação da solubilidade. Líquidos de tipo e magnitude semelhantes de forças intermoleculares, como água e etanol, são totalmente solúveis em todas as proporções ou são miscíveis.
Em contraste, líquidos como hexano e água, com diferentes tipos e magnitudes de forças intermoleculares, são insolúveis na maioria das proporções ou imiscíveis.
No geral, as forças intermoleculares são relativamente fracas porque cargas pequenas ou parciais interagem em grandes distâncias.

forças intermoleculares e propriedades físicas

As forças intermoleculares são forças de atração que existem entre as moléculas. Elas ditam várias propriedades de volume, como pontos de fusão, pontos de ebulição e solubilidades (miscibilidades) de substâncias. Por exemplo, um líquido de alto ponto de ebulição, como a água ((H2, p.e. 100 °C), exibe forças intermoleculares mais fortes em comparação com um líquido de baixo ponto de ebulição, como o hexano (C6H14, p.e. 68,73 °C). Os três tipos de interações intermoleculares incluem I) forças íon-dipolo, II) interações dipolo-dipolo e III) forças de van der Waals, que incluem as forças de dispersão de London.

1. Forças íon-dipolo

As forças íon-dipolo são as atrações eletrostáticas entre um íon e um dipolo. São comuns em soluções e desempenham um papel importante na dissolução de compostos iônicos, como o KCl, em água. A força das interações íon-dipolo é diretamente proporcional I) à carga do íon e II) à magnitude do dipolo das moléculas polares.

2. Interações dipolo-dipolo

As moléculas polares têm uma carga parcial positiva em uma extremidade e uma carga parcial negativa na outra extremidade, ou seja, uma separação de carga chamada dipolo. A força atrativa entre dois dipolos permanentes é chamada de atração dipolo-dipolo, ou seja, a força eletrostática entre a extremidade parcialmente positiva de uma molécula polar e a extremidade parcialmente negativa de outra. A ligação de hidrogênio é um tipo de interação dipolo-dipolo entre moléculas com hidrogênio, ligadas a um átomo altamente eletronegativo, como O, N ou F. O átomo de H parcialmente carregado positivamente resultante em uma molécula (o doador de ligação de hidrogênio) poderia interagir fortemente com um par solitário de elétrons de um átomo de O, N ou F parcialmente carregado negativamente em moléculas adjacentes (o aceitador de ligação de hidrogênio). A ligação de hidrogênio aumenta consideravelmente o ponto de ebulição.

3. Força de van der Waals e de dispersão de London

A mais fraca de todas as forças são as forças de van der Waals, que dependem das distâncias intermoleculares entre átomos e moléculas. As forças de dispersão de London, um subconjunto das forças de van der Waals, são experimentadas como resultado de interações entre átomos/moléculas não carregados devido a mudanças espontâneas e temporárias na distribuição de elétrons. A intensidade dessas forças parece aumentar com o aumento do peso molecular devido ao aumento da área superficial. Como resultado, compostos de pesos moleculares mais elevados geralmente entrarão em ebulição a temperaturas mais elevadas. É digno de nota que um hidrocarboneto ramificado (neopentano) normalmente tem uma área superficial menor do que seu respectivo isômero de cadeia linear (n-pentano) e, portanto, um ponto de ebulição mais baixo.

4. Solubilidade de compostos orgânicos em água

Os líquidos que podem ser misturados homogeneamente em qualquer proporção são considerados miscíveis. Líquidos miscíveis têm polaridades semelhantes. Por exemplo, o metanol e a água são polares e capazes de formar ligações de hidrogênio. Na mistura, o metanol e a água interagem através de ligações de hidrogênio intermoleculares de força comparável às interações metanol-metanol e água-água; portanto, eles são miscíveis. Da mesma forma, líquidos apolares como o hexano e o bromo são miscíveis entre si através de forças de dispersão. O axioma químico “semelhante dissolve semelhante” é útil para prever a miscibilidade de compostos. Dois líquidos que não se misturam de forma apreciável são chamados de imiscíveis. Por exemplo, o hexano apolar é imiscível em água polar. Forças de atração relativamente fracas entre o hexano e a água não superam adequadamente as forças de ligação de hidrogênio mais fortes entre as moléculas de água.

Este texto foi adaptado do<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>.

Forças intermoleculares e propriedades físicas


As forças intermoleculares são forças atrativas que existem entre as moléculas. Eles ditam várias propriedades em massa, como pontos de fusão, pontos de ebulição, ...

Education

JoVE Core

Organic Chemistry

Química Orgânica

Unit 1

Unidade 1

Covalent Bonding and Structure

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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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