Anand Classes explains the Intermolecular and Intramolecular Hydrogen Bonding in HF, H2O, NH3, o-Nitrophenol, and o-Nitrobenzoic Acid, highlighting the types, formation, and effects of hydrogen bonds on molecular properties. Students will learn how hydrogen atoms bonded to highly electronegative elements (F, O, N) participate in hydrogen bonding, influencing the boiling point, solubility, and molecular association of compounds. The post clearly distinguishes intermolecular hydrogen bonding (between molecules) and intramolecular hydrogen bonding (within the same molecule) using structural diagrams and examples like o-nitrophenol and o-nitrobenzoic acid. This topic is crucial for Class 11, Class 12, JEE, and NEET Chemistry preparation and is supported with MCQs, Q&A, Assertion Reason, and Case Study questions for conceptual clarity. Click the print button to download study material and notes.
How Does Hydrogen Bonding Occur in Hydrogen Fluoride (HF)?
In the solid state, hydrogen fluoride consists of long zig-zag chains of molecules linked through hydrogen bonds.
- Each HF molecule forms hydrogen bonds with neighbors, creating polymeric chains represented as (HF)n.
- In the liquid or gaseous state, the chain becomes linear rather than zig-zag.
Key Takeaways:
- HF molecules associate via hydrogen bonding.
- Solid HF = zig-zag chains; liquid/gas HF = linear chains.
How Does Hydrogen Bonding Occur in Water (H2O)?
Water molecules exhibit extensive hydrogen bonding.
- Each oxygen atom forms two covalent O–H bonds.
- In addition, oxygen forms two hydrogen bonds with neighboring H2O molecules.
- Thus, every oxygen is tetrahedrally surrounded by four hydrogen atoms:
- Two from covalent bonds.
- Two from hydrogen bonds.
- This leads to associated clusters represented as (H2O)n.
Yes, each water molecule can form four hydrogen bonds with four other water molecules, resulting in a tetrahedral arrangement around each central molecule. This is because a water molecule has two hydrogen atoms that can each form a hydrogen bond with another molecule’s oxygen atom, and its oxygen atom has two lone pairs of electrons that can each form a hydrogen bond with another molecule’s hydrogen atom.
- Two hydrogen bonds: The two hydrogen atoms in a water molecule are attracted to the lone pairs of electrons on the oxygen atoms of two neighboring water molecules.
- Two lone pair bonds: The two lone pairs of electrons on the oxygen atom of a water molecule are attracted to the hydrogen atoms of two other neighboring water molecules.
H2O becomes solid due to four hydrogen bonds among water molecules, formed in a tetrahedral manner.
Key Takeaways:
- Water forms a tetrahedral network due to H-bonding.
- Explains water’s high boiling point, high surface tension, and anomalous density of ice.
How Does Hydrogen Bonding Occur in Ammonia (NH3)?
Ammonia has an electronegative nitrogen atom bonded to three hydrogen atoms.
- Due to electronegativity difference, each H atom gets a partial positive charge.
- Nitrogen carries a partial negative site.
- Thus, NH3 molecules associate via hydrogen bonds, forming long chains.
Key Takeaways:
- NH3 forms hydrogen bonds but weaker than H2O (because each N can form only one H-bond vs O forming two).
- Explains higher boiling point of NH3 compared to PH3.
What Are the Types of Hydrogen Bonds?
Hydrogen bonds are classified into two main types:
(a) Intermolecular Hydrogen Bond
- Formed between two different molecules of the same or different substances.
- Examples:
- H-bonding in HF molecules.
- H-bonding in H2O and alcohols.
- Effects:
- Causes association of molecules.
- Increases boiling point, melting point, viscosity, surface tension, solubility.
Key Takeaways:
- Intermolecular hydrogen bonding leads to stronger physical properties and molecular association.
(b) Intramolecular Hydrogen Bond
- Formed within the same molecule, between a hydrogen atom and an electronegative atom (F, O, N).
- Leads to cyclisation within the molecule and prevents intermolecular association.
- Examples:
- o-nitrophenol
- o-nitrobenzoic acid
- Effects:
- Minimal impact on boiling point or solubility.
- Often results in lower than expected physical properties compared to intermolecular H-bonded analogs.
Key Takeaways:
- Intramolecular hydrogen bonding stabilizes single molecules internally.
- Unlike intermolecular bonding, it does not significantly raise physical constants.
Summary
Hydrogen bonding plays a crucial role in structure and properties of molecules.
- Hydrogen fluoride (HF): Forms zig-zag chains in solid state, linear in liquid/gas.
- Water (H₂O): Exhibits a tetrahedral network due to two covalent and two hydrogen bonds, explaining its unique physical properties.
- Ammonia (NH₃): Forms chains through weaker hydrogen bonds, explaining its higher boiling point compared to PH₃.
Types of hydrogen bonding:
- Intermolecular hydrogen bond – between molecules (e.g., HF, H₂O, alcohols). Causes association and increases boiling point, viscosity, solubility.
- Intramolecular hydrogen bond – within a single molecule (e.g., o-nitrophenol, o-nitrobenzoic acid). Leads to cyclisation, has negligible effect on bulk physical properties.
👉 Understanding hydrogen bonding is essential for explaining the anomalous behavior of water, molecular association, and stability of organic and biological molecules like proteins and DNA.
Short Answer Type (SAT) Questions on Hydrogen Bonding
Q1. Why does HF form zig-zag chains in the solid state?
Answer: In solid HF, molecules are linked through intermolecular hydrogen bonds, forming long zig-zag chains because in solid state it occurs in lattice shape. In liquid or gaseous state, the chain becomes linear.
Q2. Explain the tetrahedral hydrogen bonding arrangement in water.
Answer: Each oxygen atom in water forms two covalent O–H bonds and two hydrogen bonds, resulting in a tetrahedral network of molecules. This explains water’s high boiling point and unusual density of ice.
Q3. Why does NH3 show weaker hydrogen bonding compared to H2O?
Answer: Nitrogen has only one lone pair, so it forms fewer hydrogen bonds than oxygen, which has two lone pairs. Thus, hydrogen bonding in NH₃ is weaker than in H₂O.
Q4. Differentiate between intermolecular and intramolecular hydrogen bonding.
Answer:
- Intermolecular H-bond: Between molecules, causes association, increases boiling point, viscosity (e.g., HF, H₂O).
- Intramolecular H-bond: Within the same molecule, causes cyclisation, negligible effect on physical constants (e.g., o-nitrophenol).
Multiple Choice Questions (MCQs) on Hydrogen Bonding
Q5. Which of the following molecules shows intramolecular hydrogen bonding?
(a) HF
(b) H2O
(c) o-nitrophenol
(d) NH3
Answer: (c) o-nitrophenol
Explanation: The –OH group in ortho-position forms hydrogen bond with –NO2 group within the same molecule.
Q6. In water, each oxygen atom is surrounded tetrahedrally by:
(a) Four covalent bonds
(b) Two covalent and two hydrogen bonds
(c) One covalent and three hydrogen bonds
(d) Four hydrogen bonds
Answer: (b) Two covalent and two hydrogen bonds
Explanation: Each H₂O has two covalent O–H bonds and forms two hydrogen bonds with neighboring molecules.
Q7. Which type of hydrogen bonding decreases solubility of a compound in water?
(a) Intermolecular
(b) Intramolecular
(c) Both (a) and (b)
(d) None
Answer: (b) Intramolecular
Explanation: Intramolecular H-bond prevents molecule from associating with water, thus lowering solubility.
Q8. Which of the following shows the strongest hydrogen bonding?
(a) NH3
(b) H2O
(c) HF
(d) PH3
Answer: (b) H2O
Explanation: Oxygen forms two covalent bonds and two hydrogen bonds, giving maximum H-bonding per molecule.
Assertion–Reason Type Questions (ARQs) on Hydrogen Bonding
Q9.
Assertion (A): Water has an abnormally high boiling point.
Reason (R): Water molecules are associated through strong intermolecular hydrogen bonding.
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): o-nitrophenol is less soluble in water than p-nitrophenol.
Reason (R): o-nitrophenol shows intramolecular hydrogen bonding while p-nitrophenol shows intermolecular hydrogen bonding.
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): HF exists as associated molecules in the solid state.
Reason (R): Each HF molecule forms zig-zag chains via intermolecular hydrogen bonding.
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 on Hydrogen Bonding
Read the passage and answer the questions:
Hydrogen bonding is responsible for the unique properties of HF, H2O, and NH3.
- In HF, molecules form zig-zag chains in solid state.
- In H2O, each oxygen atom is tetrahedrally surrounded, leading to high boiling point and ice floating on water.
- In NH3, weaker hydrogen bonding explains why its boiling point is lower than water but higher than PH3.
Hydrogen bonding can be intermolecular (association of molecules, e.g., H2O) or intramolecular (cyclisation, e.g., o-nitrophenol).
Q12.1. Why does ice float on water?
Answer: Ice has an open cage-like structure due to intermolecular hydrogen bonding, making it less dense.
Q12.2. Why is NH3 more soluble in water than PH3?
Answer: NH3 can form hydrogen bonds with water, while PH3 cannot due to low electronegativity of phosphorus.
Q12.3. Which type of hydrogen bonding is present in o-nitrophenol?
Answer: Intramolecular hydrogen bonding.
Q12.4. How does intermolecular hydrogen bonding affect boiling point?
Answer: It increases boiling point by causing association of molecules, requiring more energy to separate them.
