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Q1. Classify the following compounds on the basis of sharing or transfer of electrons:CH₄, NaCl, H₂O, CO₂, O₂, KBr.

Solution

Compounds formed by transfer of electrons are: NaCl, KBr.Compounds formed by sharing of electrons are: CH₄, H₂O, CO₂, O₂

Q2. Among H₂⁺ and H₂^- molecular ions which ion is more stable and why?

Solution

Among H₂⁺ and H₂^- molecular ions, H₂⁺ ion is more stable because the electron in H₂⁺ is present only in bonding orbital.

Q3. Name the noble gas which is an exception to the octet rule.

Solution

Helium is an exception to the octet rule as it has a duplet of electrons.

Q4. Why is the bond angle of water molecule less than the normal?

Solution

Water molecule is made of two hydrogen atoms and one oxygen atom. Oxygen atom has 2 lone pair of electrons. This causes a slight redistributing of the charges around the oxygen. The lone pair of electrons pushes the bonded electrons together, decreasing the size of the angle.

Q5. Explain why sucrose is quite soluble in water though it is a covalent compound.

Solution

A molecule of sucrose contains many –OH groups and hence is capable of forming H- bond with water. As a result of this it is soluble in water.

Q6. Explain why H₂O is liquid while H₂S is a gas.

Solution

The water molecules are associated with one another by strong intermolecular hydrogen bonds, whereas the intermolecular forces in liquid H₂S are weak dipole-dipole forces. As a result, the boiling point of H₂S is much lower than that of H₂O. So at ordinary temperature H₂O is liquid whereas H₂S is gas.

Q7. An atomic orbital is monocentric while a molecular orbital is polycentric. Explain

Solution

An atomic orbital is under the influence of one nucleus while a molecular orbital is influenced by two or more nuclei depending upon the number of atoms in the molecule.

Q8. Give examples of the compounds which show AB₅ and AB₆ geometry.

Solution

AB₆-SF₆, TeF₆ AB₅-PCl₅, SbCl₅

Q9. Two p-orbitals from one atom and two p-orbitals from other atom are combined to form molecular orbitals. How many molecular orbitals will result from this combination? Explain.

Solution

Four molecular orbitals are formed when two p-orbitals from one atom combines with the two p-orbitals of the other atom. Out of these four molecular orbitals, two are bonding molecular orbitals and the other two are antibonding molecular orbitals.

Q10. Name two species which have tetrahedral shape.

Solution

CH₄ and NH₄⁺ have tetrahedral shape.

Q11. Compare three types of bonding in terms of bond energy and directional characteristics.

Solution

  Type of Bond Bond Energy Other characteristics Ionic strong Non-directional   Covalent variable Directional   Metallic variable Non-directional  

Q12. What is the dipole moment and geometry of H₂ and CO₂ molecule?

Solution

The dipole moment of H₂ and CO₂ molecules is zero and their shapes are linear.

Q13. Name the d- orbitals that are involved in sp³d² hybridization

Solution

In sp³d² hybridization dx²-y² and dz² orbitals are involved.

Q14. What is lattice energy? Which compound NaCl or MgO has higher lattice energy and why?

Solution

The Lattice Enthalpy of an ionic solid is defined as the energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions.Lattice energy depends on:

• Size of the ions: Small is the size of ion, lesser is the inter-nuclear distance and greater will be the attraction, thus larger will be the magnitude of lattice energy.

 

• Charge on the ions: More the charge on the ion, higher is the lattice energy.

           Between NaCl and MgO, lattice energy of MgO is higher as the positive charge on Mg is higher than Na ion.

Q15. What is the hybrid state of B in BF₃, Al in AlCl₃, Be in BeCl₂, C in CO₂ and C₂H₄; S in SO₂ and SO₃.

Solution

Element Compound Hybrid state B BF₃ sp² Al AlCl₃ sp² Be BeCl₂ sp C CO₂ sp C C₂H₄ sp² S SO₂ and SO₃ sp² in both

Q16. What do you understand when we say bond energy for  O=O bond is 498 kJ/mole?

Solution

If the bond energy for O=O bond is 498 kJ/mole, 498 kJ is required to break one mole of O=O bond. Bond energy is the amount of energy required to break one mole of bond of same kind, so as to separate the bonded atoms in the gaseous state.

Q17. The elements of the third period of the periodic table have expanded octet. Explain.

Solution

The elements present in the third period and beyond it have 3d orbitals apart from 3s and 3p orbitals for bonding. So in number of compounds of these elements there are more than eight valence electrons around the central atom.This is known as expanded octet.

Q18. What is the relationship between bond order and the dissociation energy of a molecule?

Solution

The dissociation energy of the molecule is directly proportional to the bond order.

Q19. Name one cation which is isostructural with NH₃.

Solution

H₃O⁺ is sp³ hybridized and has pyramidal shape so it is isostructural with NH₃.

Q20. How is ionic bond formed?

Solution

Ionic bond is formed by the transfer of one or more electrons from one atom to another. The atom which gives electron is the metal while the receiving atom is generally the non-metal.

Q21. Explain why reactions involving covalent compounds are generally slow?

Solution

Covalent bond compounds are formed by the mutual sharing of electrons between the atoms in the compound. These compounds do not produce ions in the aqueous solution.   Reactions of covalent bonded compounds involve breaking of bonds in reacting molecules and forming new covalent bonds in the products. Thus, these reactions are relatively slow.

Q22. Differentiate between bonding and antibonding molecular orbitals.

Solution

Bonding molecular orbital Antibonding molecular orbital i) It is formed by the addition overlap of atomic orbitals. i) It is formed by subtraction overlap of atomic orbital. ii) It may or may not have a node. ii) It always has a node in between the nuclei of bonded atoms. iii) In this orbital electron density is more in between the nuclei. Electrons in this orbital lead to attraction between atoms. iii) In this orbital electron density is less in between the nuclei. Electrons in this orbital lead to repulsion between atoms. iv) Its energy is less than the energy of the atomic orbitals. iv) Its energy is more than the energy of the atomic orbitals.

Q23. "BeH₂ molecule has zero dipole moment although the Be-H bonds are polar." Explain this sentence on the basis of the concept of dipole moment.

Solution

The BeH₂ molecule is linear with (H-Be-H) bond angle equal to 180^o. Although the Be-H bonds are polar due to the difference in the electronegativities of Be and H atom, the bond polarities cancel each other. As a result the resultant dipole moment is zero.

Q24. SO₂ and CO₂ are triatomic molecules. Compare their dipole moment. Justify your answer.

Solution

The dipole moment of CO₂ is zero due to its linear structure. But the dipole moment of SO₂ is more than 0 because of its bent shape.

Q25. How many lone pairs, bond pairs are present around S in SF₄ molecule? What is their arrangement?

Solution

In SF₄ molecule, 4 bond pair and 1 lone pair is present. The arrangement is trigonal bipyramidal.

Q26. What type of bond is present  in NH₃? Explain in brief.

Solution

Ammonia (NH₃) has polar covalent bond. A non-polar covalent bond is formed when the shared pair of electrons is not equally shared between the atoms of the molecule. The shared pair of electrons shifts towards the more electronegative atom. This results in formation of partial charges on the atoms in the molecule. Nitrogen has partial negative charge while hydrogen has partial positive charge. Thus, ammonia has polar covalent bond.

Q27. Define octet rule. What is its significance?

Solution

Atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet in their valence shells. This is known as octet rule. It is quite useful in explaining the normal valency of large number of elements.

Q28. What is the maximum number of covalent bonds an element with atomic number 16 can make with hydrogen? Give its formula and the type of bond formed.

Solution

Sulphur has atomic number = 16 Electronic configuration = 2, 8, 6 It needs 2 more electrons to complete its octet. Thus, it shares its 2 electrons with two different hydrogen atoms and forms two single-covalent bonds. The formula of the compound is H₂S and it has polar covalent bond.

Q29. What is bond pair and lone pair of electrons?

Solution

Bond pair of electrons is the electron pair that is directly involved in bond formation. The lone pair of electrons is the electron pair which is not involved directly in bonding.

Q30. Ionic compounds do not conduct electricity when solid. Why? When do they conduct electricity?

Solution

Ionic compounds are formed by attraction between the positive and negative ions. These ions cannot move out of their fixed positions. Thus, solid ionic compounds do not conduct electricity. When ionic compounds are in molten state or dissolved in water forming a solution, the ions can move. Then ionic compounds conduct electricity.

Q31. How are the shapes of molecular orbitals determined?

Solution

The shapes of the molecular orbitals depend upon the shapes of the combining atomic orbitals.

Q32. Differentiate between valence bond theory and Lewis concept with regard to the formation of covalent bond.

Solution

Lewis concept Valence bond theory i) Lewis concept considers the formation of covalent bond by mutual sharing of electrons. i) Valence bond theory considers the formation of covalent bond by overlap of half filled atomic orbitals. ii) It does not provide explanation for different shape of molecules. ii) Valence bond theory explains the shape of molecules. iii) Lewis structure does not explain the bond strength. iii) Valence bond theory explains the bond strength.

Q33. Which has higher boiling point o-nitro phenol or p-nitro phenol? Give reason for your answer.

Solution

p-nitrophenol has higher boling point than o-nitrophenol. This is due to intermolecular hydrogen bonding i.e. p-nitrophenol molecules can associate with themselves. o-nitrophenol does not undergo intermolecular hydrogen bonding because of chelation.  

Q34. "In the molecule H-Br the H would have a partial negative charge and the Br would have a partial positive charge." Is it true? Explain.

Solution

No, the statement is wrong. In HBr, the H has partial positive charge while Br has partial negative charge. Br being more electronegative attracts the shared pair of electrons towards itself. The compound HBr is a polar covalent compound.

Q35. (i) Describe the conditions necessary for hydrogen bonding. (ii) Differentiate between intermolecular and intramolecular hydrogen bonding. (iii) How does energy of the conical structures contribute to the stability of resonance hybrid?

Solution

(i) The two conditions that are necessary for hydrogen bonding are:    (a) Hydrogen atom should be bonded to a highly electronegative atom.   (b) The size of the electronegative atom should be small.(ii) Intermolecular hydrogen bonding Intramolecular hydrogen bonding (i) It is formed between two different molecules of the same or different substances. (i) It is formed between the hydrogen atom and highly electronegative atom present within the same molecule. (ii) Example: HF, alcohol, water molecule. (ii) Example: o-nitro phenol, o-nitro benzoic acid. (iii) The canonical structures of similar energy contribute equally to the resonance hybrid whereas the structure with higher energy is less stable and has lesser contribution to the resonance hybrid.

Q36. Explain why the bond order of N₂ is greater than N₂⁺ but the bond order of O₂ is less than that of O₂⁺.

Solution

When N₂ changes to N₂⁺, the electron is removed from the bonding molecular orbital while when O₂ changes to O₂⁺, the electron is removed from antibonding molecular orbital. This is the reason why the bond order of N₂ is greater than N₂⁺ but the bond order of O₂ is less than that of O₂⁺.

Q37. What is meant by the term chemical bond? How does Kossel-Lewis approach of bonding differ from the modern views?

Solution

The attractive force which holds various constituents (atoms, ions etc) together in different chemical species is called chemical bond. According to Kossel-Lewis approach, a chemical bond is formed between two atoms either by the transfer of electrons or by mutual sharing of electrons. But according to modern view a chemical bond is formed only when there is a net decrease of energy between the two approaching atoms due to attractive and repulsive forces.

Q38. Why does the pair of electrons in the valence shell repel each other?

Solution

The pair of electrons in the valence shell repel each other because their electron clouds are negatively charged.

Q39. What do you understand by the term valence electrons?

Solution

The electrons present in the outershell of an atom that take part in a chemical reaction are known as valence electrons.

Q40. MgCl₂ is a linear molecule while SnCl₂ is angular.Explain.

Solution

In MgCl₂, the central atom i.e. the magnesium ion is surrounded by only two bond pairs so there is no repulsion and the compound posses a regular geometry whereas in  case of SnCl₂ molecule the central atom is surrounded by two bond pairs and two lone pairs of electrons which leads to repulsion among them as a result the molecule posses distorted geometry.