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Chemical Bonding and Molecular Structure - Hydrogen Bonding

Grade 11CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Hydrogen bonding is a special type of dipole-dipole attraction that occurs when a HH atom is covalently bonded to a highly electronegative atom (F,O,F, O, or NN) and is attracted to the lone pair of another electronegative atom.

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The conditions for hydrogen bonding include: (i) High electronegativity of the atom attached to HH, and (ii) Small atomic size of the electronegative atom to facilitate strong electrostatic attraction.

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Intermolecular Hydrogen Bonding: Occurs between two separate molecules of the same or different compounds, such as in H2OH_2O, HFHF, or NH3NH_3. This leads to association and higher boiling points.

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Intramolecular Hydrogen Bonding: Occurs within a single molecule when the two atoms (the one bonded to HH and the one with the lone pair) are in close proximity, such as in oo-nitrophenol.

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Influence on Properties: HH-bonding increases boiling points, increases solubility in water (for organic compounds like alcohols), and results in the unique 'open cage' structure of ice, making it less dense than liquid water.

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Strength Order: The strength of the hydrogen bond follows the order of electronegativity of the atoms involved: H−F⋯H>H−O⋯H>H−N⋯HH-F \cdots H > H-O \cdots H > H-N \cdots H.

📐Formulae

Xδ−−Hδ+⋯Yδ−X^{\delta-} - H^{\delta+} \cdots Y^{\delta-}

Strength of H-bond∝Electronegativity of X,Y\text{Strength of H-bond} \propto \text{Electronegativity of } X, Y

Density of Ice<Density of Water (at 273 K due to H-bonding)\text{Density of Ice} < \text{Density of Water (at } 273 \text{ K due to H-bonding)}

EH−bond≈10−40 kJ mol−1E_{H-bond} \approx 10 - 40 \text{ kJ mol}^{-1}

💡Examples

Problem 1:

Explain why H2OH_2O is a liquid at room temperature while H2SH_2S is a gas, even though SS is in the same group as OO.

Solution:

In H2OH_2O, the oxygen atom is highly electronegative (3.53.5), which allows for the formation of strong intermolecular hydrogen bonds. This causes the molecules to associate into a liquid state. In H2SH_2S, sulfur has a lower electronegativity (2.52.5) and a larger atomic size, making it incapable of forming hydrogen bonds. Thus, H2SH_2S molecules are held only by weak Van der Waals forces.

Explanation:

Intermolecular hydrogen bonding in water requires significant energy to break, leading to a higher boiling point compared to H2SH_2S.

Problem 2:

Compare the boiling points of oo-nitrophenol and pp-nitrophenol.

Solution:

pp-Nitrophenol has a higher boiling point than oo-nitrophenol.

Explanation:

oo-Nitrophenol undergoes intramolecular hydrogen bonding, forming a ring structure (chelation) within the molecule, which limits its ability to bond with other molecules. pp-Nitrophenol undergoes intermolecular hydrogen bonding, leading to the association of many molecules, which requires more thermal energy to overcome during boiling.

Problem 3:

Why does ice float on water?

Solution:

Ice floats because its density is lower than that of liquid water.

Explanation:

In ice, each H2OH_2O molecule is involved in four hydrogen bonds in a tetrahedral geometry, creating an open, cage-like structure with significant empty space. When ice melts, some of these HH-bonds break, and molecules pack closer together, increasing the density of the liquid state.