Hydrogen Bonding: MDCAT Chemistry notes
Hydrogen Bonding MDCAT notes: conditions for H-bonds, strength order in HF, H2O and NH3, effect on boiling points, examples and non-examples.
What a hydrogen bond is
A hydrogen bond is the electrostatic attraction between a hydrogen atom that is covalently bonded to a small, highly electronegative atom (F, O or N) and a lone pair on another such atom nearby. The very polar bond leaves hydrogen with a large partial positive charge, and because hydrogen is so small this charge is concentrated.
Conditions for hydrogen bonding:
- Hydrogen must be bonded directly to F, O or N.
- The other atom must be small, highly electronegative and have a lone pair.
A hydrogen bond is much weaker than a covalent bond but it is the strongest intermolecular force.
Which substances hydrogen bond?
| Hydrogen bonding present | No hydrogen bonding |
|---|---|
| $\mathrm{H_2O}$, $\mathrm{HF}$, $\mathrm{NH_3}$ | $\mathrm{H_2S}$, $\mathrm{H_2Se}$, $\mathrm{H_2Te}$ |
| Alcohols, e.g. ethyl alcohol | $\mathrm{HCl}$, $\mathrm{HI}$ (Cl and I too large) |
| Carboxylic acids (form dimers) | $\mathrm{SiH_4}$, $\mathrm{CH_4}$ |
| Proteins, DNA | Hydrocarbons |
Silicon, sulphur and carbon are not electronegative enough; chlorine is electronegative but too large.
Strength of hydrogen bonds
The strength of a single hydrogen bond follows the electronegativity of the atom bonded to hydrogen:
$$\mathrm{F{-}H\cdots F} \gt \mathrm{O{-}H\cdots O} \gt \mathrm{N{-}H\cdots N}$$
So the strongest hydrogen bond is in HF. However, water has a higher boiling point than HF because each water molecule has two hydrogens and two lone pairs and forms more hydrogen bonds per molecule, making a three-dimensional network.
Effect on physical properties
- Boiling points of hydrides: for group 16, $\mathrm{H_2S} \lt \mathrm{H_2Se} \lt \mathrm{H_2Te}$ rise with size, but $\mathrm{H_2O}$ boils far higher than all of them because it alone hydrogen bonds. $\mathrm{H_2O}$ has the largest intermolecular forces in this group. $\mathrm{HF}$ and $\mathrm{NH_3}$ are similarly anomalous in their groups.
- Solubility: alcohols, carboxylic acids and sugars dissolve in water because they hydrogen bond with it. Hydrocarbons do not.
- Carboxylic acids such as acetic acid exist as hydrogen-bonded dimers, which raises their boiling points.
- Biological molecules: hydrogen bonds hold the two strands of DNA together (between base pairs) and maintain the secondary structure of proteins such as the alpha helix.
- Ice: the open, hydrogen-bonded structure makes ice less dense than water.
Joining of ice cubes
When two ice cubes are pressed together, the pressure melts a thin film of ice at the contact surface. When the pressure is released, this water refreezes and hydrogen bonds form across the join, uniting the two cubes into one.
Common MDCAT traps
- Strongest single hydrogen bond: HF. Highest boiling point among $\mathrm{HF}$, $\mathrm{H_2O}$, $\mathrm{NH_3}$: $\mathrm{H_2O}$.
- $\mathrm{H_2S}$, $\mathrm{HCl}$, $\mathrm{HI}$ and $\mathrm{SiH_4}$ have polar or non-polar bonds but no hydrogen bonding.
- Hydrocarbons never hydrogen bond with each other.
- Pressed ice cubes join because of hydrogen bonding, not van der Waals or covalent forces.
Quick revision
- Hydrogen bonding needs H attached to F, O or N.
- It is the strongest intermolecular force but weaker than a covalent bond.
- Ammonia hydrogen bonds; hydrogen sulphide does not.
- Water forms more hydrogen bonds per molecule than HF.
- Hydrogen bonds stabilize DNA and protein structure.