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1. Concept: A special type of intermolecular force. 2. Conditions for formation: ① H bonded to atoms with high electronegativity and small radius (F, O, N)； ②Nearby are atoms with high electronegativity and small radius (F, O, N). 3. Notation: X—H... Y—. A hydrogen bond is an electrostatic force, representing another type of intermolecular force besides van der Waals forces ； The strength of a hydrogen bond lies between that of chemical bonds and van der Waals forces; it is not considered a chemical bond, but it does have a bond length and bond energy. Hydrogen bonds are characterized by saturation and directionality. The existence of hydrogen bonds: 1. Intermolecular hydrogen bonds: Examples include C2H5OH, CH3COOH, H2O, HF, and NH3, where these molecules form bonds with each other. 2. Intramolecular hydrogen bonds: For example, in phenol, when there are -CHO, -COOH, -OH, or -NO2 groups in the ortho position, special ring structures are formed due to hydrogen bonds. The strength of hydrogen bonds: (1) X—H … Y—: The greater the electronegativity of X and Y, the stronger their ability to attract electrons, and thus the stronger the hydrogen bond. For example, F has the highest electronegativity and the strongest ability to attract electrons; therefore, the F-H…F bond is the strongest hydrogen bond. (2) Order of hydrogen bond strength: F-H…F > O-H…O > O-H…N > N-H…N (Note: Carbon atoms have a weak ability to attract electrons, so hydrogen bonds generally do not form between them.) Effect on melting and boiling points: Substances that can form hydrogen bonds between their molecules usually have higher melting and boiling points. This is because, in order for a solid to melt or a liquid to vaporize, it is necessary not only to break the van der Waals forces but also the hydrogen bonds between molecules, which requires more energy. Among compounds of the same type, those that can form intermolecular hydrogen bonds have higher melting and boiling points than those that cannot form such bonds. For example, in the hydrides of Group VIA elements, from H2Te, H2Se to H2S, as the relative molecular weight decreases, the intermolecular forces diminish, and the melting and boiling points decrease accordingly ； However, O–H…O hydrogen bonds are formed between H2O molecules, strengthening the intermolecular forces and causing the melting and boiling points of H2O to rise sharply. The formation of intramolecular hydrogen bonds lowers the melting and boiling points of substances: for example, the melting points of o-, m-, and p-cresols are 45°C, 96°C, and 114°C respectively. This is because m- and p-cresols have intermolecular hydrogen bonds, and some of these bonds must be broken during melting, resulting in higher melting points ； In contrast, o-***phenol forms intramolecular hydrogen bonds but no intermolecular hydrogen bonds, hence it has a lower melting point. Effect on solubility of substances: If hydrogen bonds form between solute molecules and solvent molecules, the solubility of the solute increases significantly. For example, ammonia has a higher solubility in water than other gases; at 20°C, 1 volume of water can absorb 700 volumes of ammonia. The high solubility of ammonia in water is due to the formation of ammonia hydrates, which result from the interaction between water molecules and ammonia molecules through hydrogen bonds ； Ethanol, ethylene glycol, glycerol, and the like can mix with water in any ratio, and this is why. If solute molecules form intramolecular hydrogen bonds, their solubility decreases in polar solvents and increases in non-polar solvents. Effect on the acidity of organic compounds: Taking carboxylic acids as an example, there are many factors that influence their acidity. Any factor that makes the carboxylate anion more stable than the carboxylic acid itself increases the acidity of the carboxylic acid ； Conversely, it weakens the acidity of carboxylic acids. Hydrogen bonds stabilize the carboxylate anion, thereby increasing the acidity of the carboxylic acid. In protonic solvents, if the carboxylate anion can be stabilized by the solvent through hydrogen bonding, an increase in its acidity can also be observed. The formation of intramolecular hydrogen bonds also affects the acidity of carboxylic acids. The most typical example is the acidity of ortho-hydroxybenzoic acid; since its carboxylate anion can form hydrogen bonds with the ortho hydroxyl group, this increases the stability of the anion, and as a result its acidity (pKa=2.98) is much stronger than that of para-hydroxybenzoic acid (pKa=4.57). Effect on the viscosity and surface tension of substances: When hydrogen bonds form between molecules, the intermolecular forces increase, fluidity decreases, and viscosity rises. Generally, substances that can form intermolecular hydrogen bonds have a higher viscosity than those that cannot. Alcohols and carboxylic acids can form intermolecular hydrogen bonds, whereas alkanes, ethers, and esters cannot; as a result, alcohols and carboxylic acids have higher viscosities than alkanes, ethers, and esters with the same molecular weight. Polyhydroxyl compounds such as glycerol, phosphoric acid, and concentrated sulfuric acid are typically viscous liquids because numerous hydrogen bonds can form between their molecules. The effect of intramolecular hydrogen bonds on the viscosity of compounds is different from that of intermolecular hydrogen bonds. Compared to compounds with intermolecular hydrogen bonds, compounds with intramolecular hydrogen bonds have weaker intermolecular forces, greater molecular mobility, and lower viscosity. For example, the viscosity ratio of o-hydroxybenzaldehyde is lower than that of its para isomer ； Among the isomers of ***phenol, the ortho isomer has a lower viscosity. Water has high surface tension, which is also due to the hydrogen bonds between water molecules. The magnitude of the surface energy of a substance is related to the strength of the intermolecular forces; since the surface molecules are attracted by the molecules inside the liquid, they are pushed toward the interior of the liquid, resulting in higher energy and a tendency for the surface to contract on its own. Additionally, water has the highest surface energy because of the strong hydrogen bonding between water molecules. Adding a surfactant to disrupt the hydrogen bond system in the surface layer can reduce the surface energy, which is of great significance in industrial production. Factors affecting the density of a substance: The greater the intermolecular forces between the molecules, the tighter their arrangement, and thus the higher the density. As the number of carbon atoms in straight-chain alkane molecules increases, the intermolecular forces increase and the density rises. Intermolecular hydrogen bonds also affect the density of compounds: for example, alcohols can form intermolecular hydrogen bonds, and the density of lower-carbon alcohols is higher than that of alkanes with similar molecular weights ； As the molecular weight increases, the proportion of the hydrocarbon portion rises, which hinders the formation of intermolecular hydrogen bonds; as a result, the difference in density between higher-carbon alcohols and alkanes with similar molecular weights gradually decreases. Bioligol molecules contain two hydroxyl groups, which give them a stronger ability to form hydrogen bonds. The density of ethylene glycol is 1.113 g·cm⁻³, which is higher than that of ethanol with the same number of carbon atoms (0.789 g·cm⁻³), as well as higher than that of propanol, which has a similar molecular weight (0.804 g·cm⁻³). Carboxylic acids can form strong hydrogen bonds; their density is higher than that of the corresponding alkanes and ethers, as well as higher than that of the corresponding alcohols. If hydrogen bonds form between liquid molecules, association can occur, and the result of this molecular association affects the density of the substance. For example, in liquid water at room temperature, in addition to simple H2O molecules, there are also associated molecules such as (H2O)2, (H2O)3, …, (H2O)n. Lowering the temperature facilitates the association of water molecules. The role of hydrogen bonds in biological molecules: Biological molecules are composed of organic substances such as proteins, nucleic acids, carbohydrates, and lipids, as well as water and inorganic salts. When these substances combine, they acquire biological properties, and hydrogen bonds play a key role in this process. Proteins are polypeptide chain molecules formed by the condensation of amino acids in a specific sequence, and they possess a strong ability to form hydrogen bonds. In the peptide backbone, the N–H group acts as a proton donor, while the C=O group acts as a proton acceptor; they form C=O…H–N hydrogen bonds, which determine the secondary structure of the protein. In a DNA molecule, the two polynucleotide strands are connected to each other through hydrogen bonds formed between bases (C=O…H–N and C=N…H–N). Specifically, adenine (A) pairs with thymine (T), forming 2 hydrogen bonds, while guanine (G) pairs with cytosine (C), forming 3 hydrogen bonds. The turns of the double helix structure are also held together by hydrogen bonds, which enhances its stability. Once the hydrogen bonds are broken, the spatial structure of the molecule changes, and its biological functions are lost.