Chemistry · Chemical Bonding · NEET
To boil a liquid you must pull the molecules apart into a gas. Hydrogen bonds are extra attractions between molecules. Breaking them needs extra heat. So more hydrogen bonds means a higher boiling point. This is why H2O, HF and NH3 boil much higher than expected for their small size.
Each water molecule has 2 H atoms and 2 lone pairs on oxygen. So one water molecule can form up to 4 hydrogen bonds with neighbours. This makes a strong 3D network. That is why water boils at 100 C, far higher than H2S (about -60 C), even though H2S is heavier.
F is very small and very electronegative, so H-F forms strong hydrogen bonds. Cl is bigger and less electronegative, so HCl does NOT form real hydrogen bonds, only weak van der Waals forces. Fewer/weaker attractions means HCl boils lower than HF, even though HCl is heavier. This breaks the normal 'heavier boils higher' rule.
It LOWERS it compared to intermolecular. In intramolecular H-bonding (like o-nitrophenol), the H bonds to an atom inside the SAME molecule. So it is not available to link neighbouring molecules. With fewer molecule-to-molecule links, less heat is needed, so the boiling point is lower and the compound is more volatile. o-nitrophenol boils lower than p-nitrophenol for this reason.
A carboxylic acid (-COOH) has both a C=O and an -OH, so two acid molecules join by two hydrogen bonds to make a dimer. This double linking is stronger than the single H-bond chain in alcohols. Breaking the dimer needs more heat, so acids boil higher than alcohols, aldehydes and ketones of similar mass. This is a common NEET question.
In ice, hydrogen bonds hold water molecules in an open, cage-like structure with empty space inside. This makes ice less dense than liquid water. Less dense things float. So the same hydrogen bonds that raise boiling point also make ice lighter than water. This is an important property effect to remember.
Hydrogen bonding also raises melting point, viscosity (thickness of liquid, like glycerol), surface tension, and solubility in water. Substances that can H-bond with water (alcohols, sugars, NH3) dissolve well. It also shapes DNA and proteins. So H-bonding controls many physical properties, not just boiling point.
Carboxylic acids have higher boiling points than aldehydes, ketones and even alcohols of comparable molecular mass. This is due to their:
Which one of the following compounds shows the presence of intramolecular hydrogen bond?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Weaker. A hydrogen bond (about 10-40 kJ/mol) is much weaker than a covalent bond, but stronger than ordinary van der Waals forces. It is strong enough to change boiling point, melting point and viscosity a lot.
Only F, O and N. They are small and highly electronegative. When H is attached to one of these, it becomes partly positive and is attracted to a lone pair on another F, O or N atom. Remember: FON.
N is small and electronegative, so NH3 forms hydrogen bonds. P is larger and less electronegative, so PH3 forms only weak van der Waals forces. So NH3 boils higher even though PH3 is heavier.
It increases it. Molecules that can hydrogen bond with water (like alcohols, sugars, ammonia) mix well with water. Molecules that cannot H-bond with water (like oils) do not dissolve.
In ice, hydrogen bonds lock water into an open, hollow, cage-like arrangement with empty space. This spreads the molecules out, so ice is less dense and floats on liquid water.
