Explore Chapter 3: Chemical Bonding from the new PTB syllabus. This SLO-based chapter covers ionic bonds, covalent bonds, coordinate covalent bonds, metallic bonds, and more. Ideal for students following the latest curriculum updates.
Question i:
When molten copper and molten zinc are mixed together, they give rise to a new substance called brass. Predict what type of bond is formed between copper and zinc.
- (a) Coordinate covalent bond
- (b) Ionic bond
- (c) Metallic bond
- (d) Covalent bond
Answer: (c) Metallic bond
Explanation: Brass is an alloy of copper and zinc, and the atoms are held together by metallic bonds, which involve a sea of delocalized electrons.
Question ii:
Which element is capable of forming all three types of bonds: covalent, coordinate covalent, or ionic?
- (a) Carbon
- (b) Oxygen
- (c) Magnesium
- (d) Silicon
Answer: (a) Carbon
Explanation: Carbon can form covalent bonds (e.g., methane), coordinate covalent bonds (e.g., carbon monoxide), and ionic bonds (e.g., in carbides).
Question iii:
Why is H₂O a liquid while H₂S is a gas?
- (a) Because in water, the atomic size of oxygen is smaller than that of sulfur
- (b) Because water is a polar compound and there exists strong forces of attraction between its molecules
- (c) Because H₂O molecules are lighter than H₂S
- (d) Because water can easily freeze into ice
Answer: (b) Because water is a polar compound and there exists strong forces of attraction between its molecules
Explanation: Water molecules form hydrogen bonds, leading to strong intermolecular forces, making H₂O a liquid. H₂S does not form hydrogen bonds.
Question iv:
Which of the following bonds is expected to be the weakest?
- (a) C–C
- (b) Cl–Cl
- (c) O–O
- (d) F–F
Answer: (d) F–F
Explanation: The F–F bond is weak due to the high electron-electron repulsion between the lone pairs on the fluorine atoms, despite its high bond energy.
Question v:
Which form of carbon is used as a lubricant?
- (a) Coal
- (b) Diamond
- (c) Graphite
- (d) Charcoal
Answer: (c) Graphite
Explanation: Graphite is used as a lubricant because its layers can slide over one another due to weak Van der Waals forces between layers.
Question vi:
Keeping in view the intermolecular forces of attraction, indicate which compound has the highest boiling point.
- (a) H₂O
- (b) HF
- (c) H₂S
- (d) NH₃
Answer: (a) H₂O
Explanation: Water has the highest boiling point due to its extensive hydrogen bonding, which requires more energy to break.
Question vii:
Which metal has the lowest melting point?
- (a) Li
- (b) Na
- (c) K
- (d) Rb
Answer: (d) Rb
Explanation: Rubidium (Rb) has the lowest melting point because its metallic bonds are weaker due to the large atomic size.
Question viii:
Which ionic compound has the highest melting point?
- (a) NaCl
- (b) KCl
- (c) LiCl
- (d) RbCl
Answer: (a) NaCl
Explanation: Sodium chloride (NaCl) has the highest melting point due to strong ionic bonds between smaller-sized ions.
Question ix:
Which compound contains both covalent and ionic bonds?
- (a) NH₄Cl
- (b) PCl₃
- (c) NaCl
- (d) CH₄
Answer: (a) NH₄Cl
Explanation: Ammonium chloride (NH₄Cl) contains ionic bonds between NH₄⁺ and Cl⁻ and covalent bonds within the NH₄⁺ ion.
Question x:
Which among the following has a double covalent bond?
- (a) Ethane
- (b) Ethene
- (c) Ethyne
- (d) Methane
Answer: (b) Ethene
Explanation: Ethene (C₂H₄) contains a double covalent bond between the two carbon atoms.
2. Questions for Short Answers
i. What type of elements lose their outer electron easily, and what type of elements gain electrons easily?
Answer:
Metals lose their outer electrons easily because they have low electronegativity and a small number of electrons (usually 1, 2, or 3) in their outermost shell. For example, sodium (Na) easily loses one electron to form Na⁺.
Non-metals gain electrons easily because they have high electronegativity and a nearly complete outer shell (usually 5, 6, or 7 electrons). For example, chlorine (Cl) gains one electron to form Cl⁻.
ii. Why does lower molecular mass covalent compounds exist as gases or low boiling liquids?
Answer:
Lower molecular mass covalent compounds, such as methane (CH₄) or hydrogen (H₂), exist as gases or low boiling liquids because the forces between their molecules (intermolecular forces) are very weak. These forces, such as Van der Waals forces or dipole-dipole interactions, require very little energy to break. As a result, these substances have low boiling and melting points and often exist in the gaseous state at room temperature.
iii. Give one example of an element which exists as a crystalline solid and has covalent bonds in its atoms.
Answer:
Diamond is an example of a crystalline solid where each carbon atom is bonded covalently to four other carbon atoms, forming a tetrahedral structure. This strong network of covalent bonds makes diamond extremely hard and gives it a high melting point.
iv. Which property of metals makes them malleable and ductile?
Answer:
Metals are malleable (can be hammered into thin sheets) and ductile (can be drawn into wires) because of the unique nature of metallic bonds. In metals, atoms are arranged in layers surrounded by a “sea of electrons.” When force is applied, the layers of atoms can slide over one another without breaking the bonds, allowing metals to change shape without cracking.
v. Is coordinate covalent bond a strong bond?
Answer:
Yes, a coordinate covalent bond is a strong bond. It forms when one atom donates a pair of electrons to another atom that has an empty orbital. For example, in the ammonium ion (NH₄⁺), the nitrogen atom donates a pair of electrons to a hydrogen ion (H⁺) to form a coordinate covalent bond. The strength of this bond is similar to that of a regular covalent bond.
vi. Write down and cross the formula of HNO₃.
Answer:
The structural formula of HNO₃ (nitric acid) is:
In this molecule, one oxygen atom is double-bonded to nitrogen, one is single-bonded, and the other is bonded to hydrogen, forming a hydroxyl group (-OH). The nitrogen atom is the central atom, and it follows the octet rule.
3. Constructed Response Questions
i. Why HF is a liquid while HCl is a gas?
Answer:
Hydrofluoric acid (HF) is a liquid at room temperature because it forms strong hydrogen bonds between its molecules. Hydrogen bonds occur when a hydrogen atom is bonded to a highly electronegative atom like fluorine, creating strong attractions between neighboring HF molecules.
In contrast, hydrogen chloride (HCl) does not form hydrogen bonds because chlorine is less electronegative than fluorine. The molecules of HCl are held together by weak Van der Waals forces, making it a gas at room temperature.
ii. Why covalent compounds are generally not soluble in water?
Answer:
Covalent compounds are usually not soluble in water because they are non-polar, while water is a polar solvent. Water molecules have partial positive and negative charges, allowing them to dissolve ionic or polar substances. Non-polar covalent compounds, like oil or methane, do not interact with water molecules and, therefore, do not dissolve in water.
iii. How do metals conduct heat?
Answer:
Metals conduct heat efficiently due to the presence of free electrons in their structure. These electrons are delocalized and can move freely throughout the metallic lattice. When heat is applied to a metal, the free electrons gain energy and move faster, transferring the energy to other parts of the metal by colliding with atoms and other electrons.
iv. How many oxides does nitrogen form? Write down the formulae of oxides.
Answer:
Nitrogen forms five oxides:
- Nitrous oxide (N₂O): A gas used as a mild anesthetic (laughing gas).
- Nitric oxide (NO): A colorless gas involved in biological signaling.
- Dinitrogen trioxide (N₂O₃): A blue solid that exists at low temperatures.
- Nitrogen dioxide (NO₂): A reddish-brown toxic gas.
- Dinitrogen pentoxide (N₂O₅): A solid used in organic synthesis.
v. What will happen if NaBr is treated with AgNO₃ in water?
Answer:
When sodium bromide (NaBr) reacts with silver nitrate (AgNO₃) in water, a white precipitate of silver bromide (AgBr) forms due to a double displacement reaction:
NaBr + AgNO₃ → AgBr (precipitate) + NaNO₃
This reaction is used to test for the presence of bromide ions in a solution.
vi. Why does iodine exist as a solid while Cl₂ exists as a gas?
Answer:
Iodine exists as a solid because it has a larger atomic size and molecular mass, leading to stronger Van der Waals forces between its molecules. These forces hold the iodine molecules together in a solid structure.
Chlorine, on the other hand, has a smaller molecular size and weaker Van der Waals forces, so it exists as a gas at room temperature.
4. Descriptive Questions
i. Explain the formation of an ionic bond and a covalent bond.
Answer:
- Ionic Bond: This bond forms when one atom transfers its electrons to another atom. For example, in NaCl, sodium (a metal) loses one electron to form Na⁺, and chlorine (a non-metal) gains one electron to form Cl⁻. These oppositely charged ions are held together by strong electrostatic forces.
- Covalent Bond: This bond forms when two atoms share a pair of electrons. For example, in H₂, each hydrogen atom contributes one electron, forming a shared pair of electrons between them.
ii. How do ions arrange themselves to form NaCl crystal?
Answer:
In a sodium chloride (NaCl) crystal, ions arrange themselves in a 3D lattice structure. Each sodium ion (Na⁺) is surrounded by six chloride ions (Cl⁻), and each chloride ion is surrounded by six sodium ions. This arrangement ensures maximum electrostatic attraction and stability.
iii. Explain the properties of metals, keeping in view the nature of metallic bonds.
Answer:
Metals have the following properties due to metallic bonding:
- Malleability and ductility: The layers of metal atoms can slide over each other without breaking the bond.
- Electrical conductivity: Free electrons in the metal move freely, allowing current to flow.
- Thermal conductivity: Free electrons transfer heat efficiently.
- Shininess: Free electrons absorb and reflect light, giving metals a shiny appearance.
iv. Compare the properties of ionic and covalent compounds.
Answer:
| Property | Ionic Compounds | Covalent Compounds |
|---|---|---|
| Bond type | Ionic bonds (transfer of electrons) | Covalent bonds (sharing electrons) |
| State | Solids at room temperature | Gases or liquids at room temperature |
| Solubility | Soluble in water | Insoluble in water |
| Conductivity | Conduct in molten/aqueous state | Non-conductors |
