Anand Classes explains the Formation, Conditions, and Causes of Hydrogen Bonding in molecules, along with the Difference between Hydrogen Bond and Covalent Bond. Students will understand how hydrogen atoms form special weak bonds with highly electronegative atoms like F, O, and N, and how this affects the physical and chemical properties of substances. The topic also covers types of hydrogen bonding (intermolecular and intramolecular), its conditions for formation, and how it differs from covalent bonding in strength, nature, and directionality. This concept is crucial for Class 11, Class 12, JEE, and NEET Chemistry and includes MCQs, Q&A, Assertion Reason, and Case Study questions for complete understanding. Click the print button to download study material and notes.
What Is Hydrogen Bonding?
When a hydrogen atom is bonded to highly electronegative elements such as fluorine, oxygen, or nitrogen, the hydrogen atom forms a weak bond with the electronegative atom of another molecule. This weak attraction is known as a hydrogen bond.
Example: In hydrogen fluoride (HF), hydrogen forms a covalent bond with one fluorine atom while also forming a hydrogen bond with the fluorine atom of a neighboring molecule:
$$H — F \; \cdots H — F \; \cdots H — F$$
- Solid line (—): Covalent bond
- Dotted line (···): Hydrogen bond
As a result, HF exists as a cluster of molecules represented as $(HF)_n$.
Thus, the attractive force which binds hydrogen atom of one molecule with electronegative atom (F, O or N) of another molecule is known as hydrogen bond or hydrogen bonding.
Key Takeaways:
- Hydrogen bonding is a weak intermolecular attraction.
- It requires hydrogen attached to F, O, or N.
- Molecules like HF, H2O, NH3 exist as associated clusters due to H-bonding.
How Strong Is a Hydrogen Bond Compared to a Covalent Bond?
Strength of hydrogen bond : Hydrogen bond is much weaker than a covalent bond. The strength of hydrogen bond ranges from 10–40 kJ/mol while that of a normal covalent bond is of the order of 400 kJ/mol. Thus, a hydrogen bond is about one-tenth of the strength of a covalent bond. It may also be noted that the bond length of a hydrogen bond is larger than that of a covalent bond.
For example, in case of HF molecule, the covalent bond between H and F is 109 pm while the bond length of hydrogen bond between F and H is 155 pm.
$$H — F \; \cdots H — F \; \cdots H — F$$
- Solid line (—): Covalent bond
- Dotted line (···): Hydrogen bond
Key Takeaways:
- Hydrogen bonds are weaker and longer than covalent bonds.
- Even though weaker, they strongly affect boiling points and physical properties.
Why Does Hydrogen Bonding Form?
When hydrogen (H) is bonded to strongly electronegative element, A (such as F, O or N) the electron pair shared between the two atoms lies far away from the hydrogen atom. As a result, hydrogen atom becomes highly electropositive with respect to the other atom, A. Since the electrons are displaced towards A, it acquires partial negative charge (δ–) while hydrogen atom gets partial positive charge (δ+).
When hydrogen is bonded to a strongly electronegative atom (F, O, or N), the electron pair shifts away from hydrogen, leaving it highly electropositive.
- Hydrogen gets a partial positive charge (δ+).
- The electronegative atom gets a partial negative charge (δ-).
The H—A bond becomes polar:
$$H^{\delta+} — A^{\delta-}$$
The electrostatic force of attraction between $H^{\delta+}$ of one molecule and $A^{\delta-}$ of neighbouring molecule results in the formation of hydrogen bond :
Key Takeaways:
- Hydrogen bonding arises from bond polarity.
- Hydrogen acts as a bridge between two electronegative atoms.
What Are the Conditions Necessary for Hydrogen Bonding?
- High electronegativity of atom bonded to H : The molecule should contain an atom of high electronegativity such as F, O or N bonded to hydrogen atom.
- Examples: HF, H2O, NH3.
- Small size of electronegative atom
- Smaller atoms attract bonding electrons strongly.
- This increases polarity in the bond between H and electronegative atom and strengthens hydrogen bonding.
- Example: NH3 shows hydrogen bonding, but HCl does not, even though N and Cl have the same electronegativity (3.0).
- Reason: Cl is larger than N, so polarity is weaker.
Key Takeaways:
- Requires both high electronegativity and small atomic size.
- Explains why HF, H2O, NH3 form H-bonds but HCl does not.
How Is a Hydrogen Bond Different From a Covalent Bond?
The main points of differences between hydrogen bond and covalent bond are :
| Hydrogen Bond | Covalent Bond |
|---|---|
| Hydrogen bond Involves dipole–dipole attractions. | Covalent bond Involves electron sharing. |
| Hydrogen bond formed only between hydrogen atom and a highly electronegative atom (F, O or N). | Covalent bond can form between any two electronegative atoms which may be of the same element or of different elements.. |
| Strength of hydrogen bond is very small e.g. bond strength of H ……….. F bond is 41.83 kJ/mol | Bond strength of covalent bond is sufficiently high. For example, bond strength of H–H bond is 433 kJ/mol. |
| Example: H···F = 41.83 kJ mol⁻¹. | Example: H–H = 433 kJ mol⁻¹. |
Key Takeaways:
- Hydrogen bonds are weak and intermolecular.
- Covalent bonds are strong and intramolecular.
Short Answer Type Questions on Hydrogen Bonding
Q1. Why does HF exist as associated molecules while HCl exists as discrete molecules?
Answer: HF shows hydrogen bonding due to high electronegativity and small size of fluorine. HCl does not show hydrogen bonding because chlorine is larger in size, even though its electronegativity is similar to nitrogen.
Q2. Compare the strength of hydrogen bond and covalent bond.
Answer: Hydrogen bond strength = 10–40 kJ mol⁻¹; covalent bond strength ≈ 400 kJ mol⁻¹. Thus, hydrogen bond is about 1/10th as strong as a covalent bond.
Q3. Why is the boiling point of water abnormally high compared to ?
Answer: Water molecules form strong intermolecular hydrogen bonds, leading to high association and high boiling point. H2S lacks hydrogen bonding.
Q4. Explain why NH3 forms hydrogen bonds but PH3 does not.
Answer: Nitrogen is small and highly electronegative, hence NH3 forms hydrogen bonds. Phosphorus is larger and less electronegative, so PH3 does not form hydrogen bonds.
Multiple Choice Questions (MCQs) on Hydrogen Bonding
Q5. Which of the following molecules does not show hydrogen bonding?
(a) HF
(b) H2O
(c) HCl
(d) NH3
Answer: (c) HCl
Explanation: Chlorine is large in size, so hydrogen bonding is not possible.
Q6. In hydrogen fluoride, the H–F bond length is 109 pm while the F···H hydrogen bond length is 155 pm. What does this show?
(a) Hydrogen bond is shorter than covalent bond
(b) Hydrogen bond is longer than covalent bond
(c) Both have equal length
(d) None of these
Answer: (b) Hydrogen bond is longer than covalent bond.
Q7. Which type of attraction is present in a hydrogen bond?
(a) Van der Waals forces
(b) Dipole–dipole attraction
(c) Covalent sharing
(d) Metallic bond
Answer: (b) Dipole–dipole attraction.
Q8. Which of the following has the highest boiling point due to hydrogen bonding?
(a) H₂O
(b) H₂S
(c) H₂Se
(d) H₂Te
Answer: (a) H₂O
Explanation: Strongest hydrogen bonding due to O–H bonds.
Assertion–Reason Questions (ARQs) on Hydrogen Bonding
Q9.
Assertion (A): Hydrogen bond is weaker than a covalent bond.
Reason (R): Hydrogen bond arises from dipole–dipole interaction, while covalent bond arises from electron sharing.
Options:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, R is false.
(d) A is false, R is true.
Answer: (a) Both A and R are true, and R explains A.
Q10.
Assertion (A): NH₃ shows hydrogen bonding but PH₃ does not.
Reason (R): Nitrogen has higher electronegativity and smaller atomic size compared to phosphorus.
Options:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, R is false.
(d) A is false, R is true.
Answer: (a) Both A and R are true, and R explains A.
Q11.
Assertion (A): Ice floats on water.
Reason (R): Ice has an open cage-like structure due to hydrogen bonding, making it less dense than liquid water.
Options:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is not the correct explanation of A.
(c) A is true, R is false.
(d) A is false, R is true.
Answer: (a) Both A and R are true, and R explains A.
Case Study Question on Hydrogen Bonding
Read the passage and answer the following questions:
Hydrogen bonding plays a crucial role in determining the physical and chemical properties of compounds. In water, strong hydrogen bonding leads to an abnormally high boiling point and gives ice its open cage-like crystalline structure, making ice less dense than water. In biological systems, hydrogen bonding stabilizes DNA double helix and the secondary structure of proteins (α-helix and β-sheets).
Q12.1. Why is the boiling point of water higher than expected from its molecular mass?
Answer: Due to extensive intermolecular hydrogen bonding between water molecules.
Q12.2. Why does ice float on water?
Answer: Ice has an open cage-like hydrogen bonded structure, which makes it less dense than liquid water.
Q12.3. Name two biomolecules where hydrogen bonding plays an essential role.
Answer: DNA (base pairing) and proteins (secondary structure).
Summary
Hydrogen bonding is a special type of weak intermolecular force that occurs when hydrogen is covalently bonded to electronegative atoms like F, O, or N. It explains many unique properties:
- Strength: Hydrogen bond (10–40 kJ mol⁻¹) is much weaker than covalent bond (≈400 kJ mol⁻¹).
- Cause: Occurs due to bond polarity, where H becomes partially positive and interacts with electronegative atoms of neighboring molecules.
- Conditions: Requires high electronegativity and small atomic size (F, O, N).
- Applications: Explains high boiling point of water, floating of ice, solubility, and biomolecular stability.
👉 In exams, hydrogen bonding is tested through conceptual reasoning, bond strength comparisons, physical property explanations, and biological applications. Mastery of this topic helps in both inorganic and biochemistry.
👉 In conclusion, hydrogen bonding is a cornerstone concept in chemistry, crucial for understanding the properties of water, biomolecules like DNA and proteins, and many inorganic and organic compounds.
