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Dispersion Forces vs Hydrogen Bonds
Differentiation between van der Waals force and hydrogen bond
Between heaven and earth, everything is composed of particles, and the forces between particles are very different. Among them, van der Waals force and hydrogen bond have far-reaching influence on the properties of matter, so it is necessary to distinguish.
Van der Waals force exists between various molecules and is a weak interaction. Its cause is complex and related to the polarity and relative molecular mass of molecules. Between non-polar molecules, due to the instantaneous asymmetric distribution of electrons, an instantaneous dipole is generated, which triggers a weak attraction between molecules. This is the dispersion force, which is a kind of van der Waals force. Between polar molecules, in addition to the dispersion force, there are also orientation and induction forces. The van der Waals force is weak, usually only a few thousand joules per mole.
The hydrogen bond is actually a special intermolecular force, which is stronger than the van der Waals force. Its formation requires specific conditions, that is, after the hydrogen atom is combined with atoms with large electronegativity and small radius (such as fluorine, oxygen, nitrogen, etc.) in a covalent bond, the positive charge of the hydrogen atom is exposed due to electron cloud migration, and it can attract each other with such electronegativity atoms in another molecule. For example, in water, hydrogen is connected to oxygen in a water molecule, and one end of hydrogen is positive, forming a hydrogen bond with the oxygen atom of a neighboring water molecule. The force of hydrogen bonding is about tens of kilojoules per mole.
Compared with the two, the properties of substances are affected differently. Van der Waals force mainly affects the melting point and solubility of substances. Generally speaking, the greater the relative molecular weight, the stronger the van der Waals force, and the higher the melting boiling point of the substance. For example, alkane homologues, with the increase of carbon atoms, the relative molecular weight increases, and the melting boiling point gradually increases. The effect of hydrogen bonds on the properties of substances is more significant. Due to the existence of hydrogen bonds, the melting boiling point of water is higher than that of homologous hydrides, and the density of ice is less than that of water, because hydrogen bonds make water molecules form a regular tetrahedral structure, and the distance between water molecules in ice increases. In terms of solubility, substances that can form hydrogen bonds have better solubility in hydrogen-bonding solvents such as water. For example, ethanol and water can be miscible in any ratio, because hydrogen bonds can be formed between ethanol molecules and water molecules.
Identifying van der Waals forces and hydrogen bonds is of great benefit in understanding the relationship between the microstructure and macroscopic properties of matter, and can pave the way for exploring the mysteries of the material world.
Between heaven and earth, everything is composed of particles, and the forces between particles are very different. Among them, van der Waals force and hydrogen bond have far-reaching influence on the properties of matter, so it is necessary to distinguish.
Van der Waals force exists between various molecules and is a weak interaction. Its cause is complex and related to the polarity and relative molecular mass of molecules. Between non-polar molecules, due to the instantaneous asymmetric distribution of electrons, an instantaneous dipole is generated, which triggers a weak attraction between molecules. This is the dispersion force, which is a kind of van der Waals force. Between polar molecules, in addition to the dispersion force, there are also orientation and induction forces. The van der Waals force is weak, usually only a few thousand joules per mole.
The hydrogen bond is actually a special intermolecular force, which is stronger than the van der Waals force. Its formation requires specific conditions, that is, after the hydrogen atom is combined with atoms with large electronegativity and small radius (such as fluorine, oxygen, nitrogen, etc.) in a covalent bond, the positive charge of the hydrogen atom is exposed due to electron cloud migration, and it can attract each other with such electronegativity atoms in another molecule. For example, in water, hydrogen is connected to oxygen in a water molecule, and one end of hydrogen is positive, forming a hydrogen bond with the oxygen atom of a neighboring water molecule. The force of hydrogen bonding is about tens of kilojoules per mole.
Compared with the two, the properties of substances are affected differently. Van der Waals force mainly affects the melting point and solubility of substances. Generally speaking, the greater the relative molecular weight, the stronger the van der Waals force, and the higher the melting boiling point of the substance. For example, alkane homologues, with the increase of carbon atoms, the relative molecular weight increases, and the melting boiling point gradually increases. The effect of hydrogen bonds on the properties of substances is more significant. Due to the existence of hydrogen bonds, the melting boiling point of water is higher than that of homologous hydrides, and the density of ice is less than that of water, because hydrogen bonds make water molecules form a regular tetrahedral structure, and the distance between water molecules in ice increases. In terms of solubility, substances that can form hydrogen bonds have better solubility in hydrogen-bonding solvents such as water. For example, ethanol and water can be miscible in any ratio, because hydrogen bonds can be formed between ethanol molecules and water molecules.
Identifying van der Waals forces and hydrogen bonds is of great benefit in understanding the relationship between the microstructure and macroscopic properties of matter, and can pave the way for exploring the mysteries of the material world.
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