Hydrogen Bonding

Identifying Hydrogen Bonding Potential

• Step 1: Check the molecular formula for the presence of H atoms directly bonded to N, O, or F. If H is only bonded to C (as in $CH_4$), hydrogen bonding cannot occur.
• Step 2: Identify the presence of lone pairs on N, O, or F atoms within the system. These act as the 'acceptors' for the hydrogen bond.
• Step 3: Determine the maximum number of bonds. For example, water can form up to four hydrogen bonds (two via its H atoms and two via the lone pairs on Oxygen).

Drawing Conventions

• Always represent the hydrogen bond using a dashed or dotted straight line to distinguish it from a solid covalent bond.
• Ensure the dashed line originates exactly at a lone pair and terminates at the $\delta+$ hydrogen atom.

It is vital to distinguish hydrogen bonding from other intermolecular forces to predict physical properties accurately.

• Unlike London forces, which increase with molecular mass, hydrogen bonding provides a 'boost' to boiling points that defies simple mass-based trends.

• Boiling and Melting Points: Water has significantly higher boiling points than other Group 16 hydrides (like $H_2S$) because a large amount of thermal energy is required to overcome the extensive hydrogen bonding network.

• Surface Tension: Molecules at the surface of liquid water are pulled inward and downward by hydrogen bonds with molecules below them, creating a 'skin' effect that resists external force.

• Density of Ice: In the solid state, water molecules arrange into a rigid open lattice to maximize hydrogen bonding. This increases the distance between molecules, making ice less dense than liquid water, which is why ice floats.

• Common Exam Pattern: You will often be asked to explain why a small molecule like $NH_3$ has a higher boiling point than a larger molecule like $PH_3$. The answer is always the presence of hydrogen bonding in $NH_3$ vs. only dipole-dipole/London forces in $PH_3$.

• Verification: When drawing, always label your partial charges ($\delta+$ and $\delta-$). Examiners often award marks specifically for the correct placement of these charges and the lone pairs.

• Sanity Check: If a molecule contains Fluorine but the H is not bonded to the Fluorine (e.g., in some organic isomers), it cannot form hydrogen bonds with itself.