Hydrogen Bonding (College Board AP® Chemistry) : Study Guide

Oluwapelumi Kolawole

Written by: Oluwapelumi Kolawole

Reviewed by: Stewart Hird

Updated on

Hydrogen Bonding

  • This is a special type of dipole-dipole interaction found in covalent molecules in which hydrogen is directly bonded to either a nitrogen, oxygen or fluorine atom

    • These elements are very electronegative

    • Therefore, they form polar bonds with hydrogen

Polar covalent bond H—F bond

polar-h-f-bond

A polar covalent bond in hydrogen fluoride is described by the presence of δ+ on the hydrogen and δ- charges on the electronegative fluorine atom

  • The hydrogen atom is significantly less electronegative than a nitrogen, oxygen or fluorine atom

    • Therefore, the hydrogen atom has a partial positive (δ+) charge

    • The very electronegative element has a partial negative charge (δ-)

  • Hydrogen bonding describes the attraction between the δ+ hydrogen atom (Hδ+) in this polar bond and the nonbonding electron pairs on a nearby small electronegative atom or ion

    • Compared to dipole-dipole or London dispersion forces, hydrogen bonds are a stronger intermolecular force

Hydrogen Bonding

hydrogen-bond

Hydrogen bonding between molecules of ammonia, water and hydrogen fluoride

  • The strength of the hydrogen bond is highest in molecules where the hydrogen atom is bonded to a fluorine atom

    • This is because fluorine has the highest electronegativity

  • Generally, the strength of hydrogen bonds increases in the order:
                       H—N < H—O < H—F

  • However, the strength of hydrogen bond in a molecule is also dependent on how many hydrogen bonds can be formed per pair of molecules

  • For example, acetic acid, CH3COOH, and propanol, CH3CH2CH2OH:

    • They both have the same molecular mass, 60 amu

    • They are both capable of forming hydrogen bonds

  • However, a pair of acetic acid molecules can form two hydrogen bonds, whereas a pair of propanol molecules can form only one

    • Hence, the boiling point of acetic acid is greater than propanol

Multiple Hydrogen Bonds

multiple-hydrogen-bonds

Hydrogen bonding in acetic acid and propanol. Each acetic acid can form two hydrogen bonds while propanol can form only one hydrogen bond.

  • Similarly, the hydrogen bonds in water are greater than that in hydrogen fluoride.

    • Water can form two hydrogen bonds

      • This is because there are 2 lone pairs of electrons on the oxygen atom AND two available δ+ hydrogen atoms

    • Hydrogen fluoride forms only one hydrogen bond

      • This is because, although there are 3 lone pairs of electrons on the fluorine atom, there is only one available δ+ hydrogen atom

    • Hence water has a higher boiling point than hydrogen fluoride 

Hydrogen Bonding in H2O and HF

hydrogen-bonding-in-h2o-and-hf

The diagram shows two hydrogen bonds per pair of water molecules compared to only one per pair of hydrogen fluoride

Examiner Tips and Tricks

  • A covalent molecule may not be able to form hydrogen bonds with itself due to the lack of a polar hydrogen atom yet forms hydrogen bonds with another molecule with a polar hydrogen atom

  • For example, acetaldehyde, CH3CHO, can form hydrogen bonds with water molecules which makes it soluble in water

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Oluwapelumi Kolawole

Author: Oluwapelumi Kolawole

Expertise: Chemistry Content Creator

Oluwapelumi is a Pharmacist with over 15000+ hours of AP , IB, IGCSE, GCSE and A-Level chemistry tutoring experience. His love for chemistry education has seen him work with various Edtech platforms and schools across the world. He’s able to bring his communication skills as a healthcare professional in breaking down seemingly complex chemistry concepts into easily understood concepts for students.

Stewart Hird

Reviewer: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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