Question

For pairs of molecules in the gas phase, average \(\mathrm{H}\) -bond dissociation energies are \(17 \mathrm{~kJ} / \mathrm{mol}\) for \(\mathrm{NH}_{3}, 22 \mathrm{~kJ} / \mathrm{mol}\) for \(\mathrm{H}_{2} \mathrm{O}\) and \(29 \mathrm{~kJ} / \mathrm{mol}\) for \(\mathrm{HF}\). Explain this increase in \(\mathrm{H}\) -bond strength.

Short answer

Hydrogen bond strength increases from NH3 to H2O to HF due to increasing electronegativity of the central atoms (N < O < F).

Step-by-step solution

- Understand Hydrogen Bonding

Hydrogen bonding is an interaction involving a hydrogen atom located between a pair of other atoms having a high affinity for electrons; such bonds form among molecules (or within the same molecule) where hydrogen is covalently bound to a more electronegative atom, such as nitrogen (N), oxygen (O), or fluorine (F).

- Analyze Electronegativity

Compare the electronegativity of N, O, and F. Fluorine (F) is the most electronegative with a value of 3.98, followed by oxygen (O) with 3.44, and nitrogen (N) with 3.04. Electronegativity affects the strength of the hydrogen bond because a more electronegative atom can attract the bonding electrons more strongly, increasing the bond strength.

- Relate Electronegativity to Bond Strength

The hydrogen bond strength increases as the electronegativity of the atom bonded to hydrogen increases. Therefore, the stronger hydrogen bond in HF (29 kJ/mol) compared to H2O (22 kJ/mol) and NH3 (17 kJ/mol) can be attributed to the higher electronegativity of fluorine (F) compared to oxygen (O) and nitrogen (N).

- Understand Bond Dissociation Energy

Bond dissociation energy is the energy required to break a bond in a molecule in the gas phase. Higher bond dissociation energy indicates a stronger bond. Thus, higher numbers for HF indicate that it takes more energy to break its hydrogen bonds compared to H2O and NH3.

- Conclusion

The increase in hydrogen bond strength from NH3 to H2O to HF is due to the increasing electronegativity of the central atom in each molecule, which leads to a stronger attraction between the hydrogen atom and the electronegative atom, thereby increasing the bond dissociation energy.

Key concepts

Electronegativity

Electronegativity is a crucial concept in understanding hydrogen bonding. It refers to an atom's ability to attract and keep electrons. When a hydrogen atom bonds with a highly electronegative atom like fluorine (F), oxygen (O), or nitrogen (N), the bonding electrons are drawn closer to this more electronegative atom. This shift makes the hydrogen atom slightly positive and the other atom slightly negative.
Fluorine, oxygen, and nitrogen are among the most electronegative elements, with electronegativity values of 3.98, 3.44, and 3.04, respectively.
This gradient in electronegativity explains why HF has stronger hydrogen bonds than H2O and NH3. Fluorine's high electronegativity means it pulls electrons more effectively, resulting in a stronger bond.

Bond Dissociation Energy

Bond dissociation energy (BDE) is a measure of how much energy is required to break a bond in a molecule when it's in the gas phase. A higher BDE means a stronger bond.
In the case of hydrogen bonding, the BDE values for NH3, H2O, and HF show a trend: Hydrogen bonds in HF (29 kJ/mol) are stronger than those in H2O (22 kJ/mol) and NH3 (17 kJ/mol).
The higher BDE for HF indicates that it takes more energy to break its hydrogen bonds, reaffirming that HF has the strongest hydrogen bonds among the three.

Gas Phase Molecules

In the gas phase, molecules do not interact with each other as they would in a liquid or solid state. This isolation allows a clearer understanding of intrinsic bond strengths without external interference.
When comparing NH3, H2O, and HF in the gas phase, the isolated molecules show their true hydrogen bond strengths. This isolation is vital for accurate measurement of their bond dissociation energies.
It emphasizes that the observed trends in bond strength are due solely to the molecules' intrinsic properties, particularly the electronegativity of the atoms involved.

Hydrogen Bond Strength

Hydrogen bond strength is determined mainly by the electronegativity of the atom bonded to hydrogen and the bond dissociation energy.
Fluorine's high electronegativity makes HF have the strongest hydrogen bonds (29 kJ/mol), followed by H2O (22 kJ/mol), and then NH3 (17 kJ/mol).
The stronger the electronegativity, the more the electrons are pulled towards the electronegative atom, making the bond with hydrogen stronger. Understanding this helps explain why certain molecules have stronger hydrogen bonding and hence higher dissociation energies.
This concept is key to various fields, including biochemistry, where hydrogen bonding plays a vital role in the structure and function of proteins and nucleic acids.