Giving reasons, explain which of the following can form hydrogen bonds with water? (a) CH
OCH
(b) CH
(c) F
(d) HCOOH (e) Na
(f) C
H
OH 3
The Correct Answer and Explanation is:
Correct Answer
The substances that can form hydrogen bonds with water are (a) CH₃OCH₃, (c) F⁻, (d) HCOOH, and (f) C₂H₅OH.
Explanation
A hydrogen bond is a strong type of intermolecular dipole-dipole attraction. For a substance to form a hydrogen bond with water, it must meet at least one of two criteria:
- Be a hydrogen bond donor: The substance must contain a hydrogen atom covalently bonded to a highly electronegative atom, specifically nitrogen (N), oxygen (O), or fluorine (F). This creates a highly polarized bond and a significant partial positive charge on the hydrogen atom.
- Be a hydrogen bond acceptor: The substance must contain a highly electronegative atom (N, O, or F) that has at least one lone pair of electrons. This lone pair can attract the partially positive hydrogen atom from a water molecule.
Based on these principles, the given substances are analyzed as follows:
(a) CH₃OCH₃ (Dimethyl ether): This molecule cannot act as a hydrogen bond donor because all its hydrogen atoms are bonded to carbon, which is not sufficiently electronegative. However, the oxygen atom possesses two lone pairs of electrons. It can therefore act as a hydrogen bond acceptor, forming a hydrogen bond with a hydrogen atom from a water molecule.
(b) CH₄ (Methane): This molecule cannot form hydrogen bonds. The carbon-hydrogen bond has very low polarity, and carbon is not one of the highly electronegative atoms (N, O, F) required. It has no lone pairs and cannot act as a donor or an acceptor.
(c) F⁻ (Fluoride ion): The fluoride ion is a single atom with a full negative charge and available lone pairs. It cannot donate a hydrogen atom, but it is an excellent hydrogen bond acceptor. Its high electronegativity and negative charge strongly attract the partially positive hydrogen atoms of water molecules.
(d) HCOOH (Formic acid): This molecule can both donate and accept hydrogen bonds. It has a hydrogen atom bonded to an oxygen atom in its hydroxyl group (-OH), making it a hydrogen bond donor. Additionally, both of its oxygen atoms have lone pairs, allowing it to act as a hydrogen bond acceptor.
(e) Na⁺ (Sodium ion): This is a positive ion (cation) and has no hydrogen atoms or lone pairs of electrons. It cannot form hydrogen bonds. It interacts with water via a different attractive force called an ion-dipole interaction, where the positive charge of the ion is attracted to the partial negative charge on the oxygen atom of water.
(f) C₂H₅OH (Ethanol): Similar to formic acid, ethanol can both donate and accept hydrogen bonds. It contains a hydrogen atom bonded to oxygen in its hydroxyl group (-OH), making it a hydrogen bond donor. The oxygen atom also has lone pairs, making it a hydrogen bond acceptor. This ability is why ethanol is fully miscible with water.thumb_upthumb_down
