BCHCT-137 Solved Assignment July 2025 | COORDINATION CHEMISTRY, STATES OF MATTER & CHEMICAL KINETICS | IGNOU BSCG

Q: According to valence bond theory, how do you account for the indicated molecular geometry for the following compounds:

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Q1. What is the general electronic configuration of elements of the second transition series? Which d orbitals are filled up here?

Key Points:

Second transition series includes elements from Y (Z=39) to Cd (Z=48).
General electronic configuration is [Kr] 4d¹⁻¹⁰ 5s⁰⁻².
The 4d orbitals are progressively filled in this series.
The five d orbitals filled are dxy, dyz, dxz, dx²-y², and dz².

Answer: The elements of the second transition series, also known as the 4d series, extend from Yttrium (Y, Z=39) to Cadmium (Cd, Z=48). Their general electronic configuration involves the progressive filling of the 4d orbitals, along with the 5s orbitals. The typical general electronic configuration can be represented as [Kr] 4d¹⁻¹⁰ 5s⁰⁻². The 5s orbital is generally filled before the 4d orbital according to the Aufbau principle, but electrons are often removed from 5s first during ionisation due to its...

Q2. In [Ni(H2O)6]2+ the observed magnetic moment is higher than the spin-only value. Explain the reason.

Key Points:

Ni²⁺ has 3d⁸ electronic configuration in [Ni(H2O)6]2+.
Octahedral d⁸ complexes with weak field ligands have t₂g⁶ eg² configuration with 2 unpaired electrons.
Spin-only magnetic moment (μ_s) for 2 unpaired electrons is ≈ 2.83 BM.
Observed magnetic moment for [Ni(H2O)6]2+ is higher (3.0-3.2 BM) than the spin-only value.

Answer: In [Ni(H2O)6]2+, nickel is in the +2 oxidation state. The electronic configuration of Ni²⁺ is [Ar]3d⁸. In an octahedral crystal field, like with water ligands, the 3d⁸ electrons occupy the t₂g and eg orbitals. Since water is a weak-field ligand, it's a high-spin complex, leading to the configuration t₂g⁶ eg². This configuration results in two unpaired electrons. Using the spin-only formula for magnetic moment, μ_s = √[n(n+2)] Bohr Magnetons (BM), where 'n' is the number of unpaired electrons. F...

Q3. Give the most common oxidation states of the lanthanoids. Give their configurations.

Key Points:

The most common and stable oxidation state for all lanthanoids is +3.
The +3 state results from the loss of two 6s electrons and one 5d or 4f electron.
Less common +2 and +4 oxidation states are observed for f⁰, f⁷, or f¹⁴ stability.
General ground state electronic configuration is [Xe] 4fⁿ 5d⁰⁻¹ 6s².

Answer: The most common and stable oxidation state exhibited by all lanthanoids is +3. This arises from the loss of the two 6s electrons and one 5d electron (if present) or one 4f electron, leading to stable ions. For instance, in the +3 state, the electron configuration typically becomes [Xe] 4fⁿ, where 'n' varies from 0 to 14, following the filling of the 4f subshell. Besides the ubiquitous +3 state, some lanthanoids also display +2 and +4 oxidation states, though these are less common and typically ...

Q4. What is meant by coordination entity and coordination sphere?

Key Points:

Coordination entity is the central metal ion/atom with its bonded ligands, acting as a single charged or neutral unit.
Ligands are ions or molecules directly attached to the central metal within the coordination entity.
Coordination sphere precisely defines the central metal and only its directly bonded ligands.
Square brackets ([ ]) are used to enclose and denote the coordination sphere in chemical formulas.

Answer: A coordination entity is the central, distinguishable unit of a coordination compound. It comprises a central metal atom or ion, typically a transition metal, bonded to a fixed number of surrounding ions or molecules known as ligands. This entire assembly behaves as a single unit and can carry a net positive, negative, or zero charge. For instance, in [Co(NH₃)₆]Cl₃, the [Co(NH₃)₆]³⁺ ion represents the coordination entity, where Co³⁺ is the central metal ion and NH₃ molecules are the ligands. Th...

Q5. Give the rules of IUPAC nomenclature for naming complexes (give any five points).

Key Points:

Cation named first, then anion, for ionic coordination compounds.
Ligands named alphabetically before the central metal within the complex.
Use di-, tri- for simple ligands; bis-, tris- for complex ligands (parentheses needed).
Anionic ligands end in '-o'; neutral ligands retain common names (e.g., aqua, ammine).

Answer: IUPAC nomenclature provides a systematic way to name coordination complexes, ensuring clarity and unambiguous identification. Here are five key rules extracted from coordination chemistry principles, crucial for BCHCT-137. First, in an ionic compound, the cation is always named before the anion. This rule applies regardless of whether the cation or anion is the complex species. For instance, in [Co(NH₃)₆]Cl₃, 'hexamminecobalt(III)' (cation) is named first, followed by 'chloride' (anion). Secon...

Q6. According to valence bond theory, how do you account for the indicated molecular geometry for the following compounds:

(i)) [Fe(CN)6]3 - Octahedral (100 words)
(ii)) [GaCl4] - Tetrahedral (100 words)

Key Points:

Valence Bond Theory explains molecular geometry based on the hybridization of central metal orbitals.
[Fe(CN)₆]³⁻: Fe³⁺ (3d⁵) undergoes d²sp³ hybridization due to strong field CN⁻ ligands causing electron pairing.
d²sp³ hybridization in coordination complexes always leads to an octahedral geometry.
Strong field ligands force d-electron pairing, making inner d-orbitals available for hybridization (inner orbital complex).

Answer:

Q7. Explain ligand isomerism with suitable example.

Key Points:

Ligand isomerism occurs when the ligand itself exists in different isomeric forms.
Ligands have the same molecular formula but different internal atomic arrangements.
These isomeric ligands form distinct coordination compounds.
Example: 1,2-diaminopropane (pn) and 1,3-diaminopropane (tn) are ligand isomers.

Answer: Ligand isomerism is a specific type of structural isomerism observed in coordination compounds, where the isomerism arises from different isomeric forms of the ligand itself. This means that the ligand molecule can exist in two or more structurally distinct forms, even though they share the same molecular formula. When these isomeric ligands bind to a central metal ion, they give rise to different coordination complexes. This phenomenon occurs because the internal arrangement of atoms within th...

Q8. Compare the CFSE of an octahedral complex with that of a tetrahedral complex for four electrons in the d orbitals.

Key Points:

Octahedral d-orbitals split into t₂g (lower) and eg (higher).
Tetrahedral d-orbitals split into e (lower) and t₂ (higher).
Octahedral d⁴ high spin CFSE is -0.6Δo (t₂g³ eg¹).
Octahedral d⁴ low spin CFSE is -1.6Δo + P (t₂g⁴ eg⁰).

Answer: Crystal Field Stabilization Energy (CFSE) is the energy gained by placing d-electrons in a crystal field compared to a hypothetical spherical field. For four electrons in the d orbitals (d⁴ system), the CFSE differs significantly between octahedral and tetrahedral geometries due to distinct splitting patterns and ligand field strengths. In an octahedral field, d-orbitals split into three lower-energy t₂g orbitals and two higher-energy eg orbitals. For a d⁴ system, two scenarios exist. With a we...

Q9. What is meant by tetragonal distortion in octahedral geometry?

Key Points:

Tetragonal distortion is a deviation from ideal octahedral geometry.
It involves lengthening or shortening of two trans-ligand bonds along one axis (e.g., z-axis).
It leads to two types: tetragonal elongation (z-out) or tetragonal compression (z-in).
The primary cause is the Jahn-Teller effect, stabilizing degenerate electronic ground states.

Answer: Tetragonal distortion in octahedral geometry refers to a deviation from the ideal O_h symmetry, where the six ligands are no longer equidistant from the central metal ion. Instead, the complex undergoes a change in its bond lengths along one specific axis, typically the z-axis, while maintaining a square planar arrangement in the perpendicular xy-plane. This distortion results in either two trans-ligands along the z-axis moving further away from the metal center (tetragonal elongation, or z-out...

Q10. Calculate the spin-only magnetic moment for low-spin octahedral complex of d⁶ and d⁹ ions.

Key Points:

Spin-only magnetic moment (μs) is calculated using μs = √[n(n+2)] BM.
'n' represents the number of unpaired electrons in the complex.
Low-spin octahedral d⁶ complexes have electron configuration t₂g⁶ eg⁰.
A d⁶ low-spin octahedral complex has 0 unpaired electrons, so μs = 0 BM.

Answer: The spin-only magnetic moment (μs) for a complex indicates the number of unpaired electrons present and is calculated using the formula derived from the course material: μs = √[n(n+2)] Bohr Magnetons (BM), where 'n' represents the number of unpaired electrons. For a low-spin octahedral complex of a d⁶ ion, the 6 d-electrons are filled according to Crystal Field Theory. In an octahedral field, the d-orbitals split into a lower energy t₂g set and a higher energy eg set. For a low-spin complex, st...

Q11. State the assumptions of kinetic theory of gases and derive the following expression for the pressure of a gas. p = mNu^2 / 3V

Key Points:

KTG assumes gas molecules have negligible volume and no intermolecular forces.
Gas molecules are in continuous, random motion, undergoing perfectly elastic collisions.
Average kinetic energy of gas molecules is directly proportional to absolute temperature.
Derivation starts from momentum change (2muₓ) imparted to the wall per collision.

Answer: The kinetic theory of gases (KTG) is built upon several fundamental assumptions that describe the behavior of an ideal gas. Firstly, gases are composed of a very large number of identical, infinitesimally small particles (molecules) whose individual volume is considered negligible compared to the total volume of the container. Secondly, these molecules are in continuous, rapid, and random motion, colliding with each other and the container walls. Furthermore, the KTG assumes that all collisions...

Q12. Answer the following:

a)) State the Dalton's law of partial pressure and give its significance. (80 words)
b)) Define critical constants and give their expressions in terms of van der Waals constants. (120 words)

Key Points:

Dalton's Law: Total pressure of non-reacting gas mixture equals sum of partial pressures.
Partial pressure: Pressure a single gas exerts if occupying the mixture's total volume.
Significance of Dalton's Law: Used to calculate dry gas pressure when collected over water.
Critical constants (T_c, P_c, V_c): Define the critical state of a substance.

Answer: This question delves into fundamental concepts of gas laws and real gas behavior, crucial for understanding states of matter. Sub-question (a) addresses Dalton's law of partial pressures, a cornerstone for ideal gas mixtures, while sub-question (b) explores critical constants derived from the van der Waals equation, essential for characterizing real gases and their liquefaction.

Q13. Explain the London or dispersion forces. Compare it with dipole-dipole forces.

Key Points:

London dispersion forces (LDFs) are the weakest intermolecular forces, present in all substances.
LDFs arise from instantaneous, temporary dipoles due to random electron motion.
LDF strength increases with molecular size and polarizability (more electrons).
Dipole-dipole forces occur only between polar molecules.

Answer: London dispersion forces (LDFs), also known as dispersion forces, are the weakest type of intermolecular force, present in all atoms and molecules. They originate from temporary, instantaneous dipoles caused by the continuous, random movement of electrons, creating momentary charge imbalances. This instantaneous dipole then induces a temporary dipole in an adjacent molecule, leading to a weak, transient attraction. LDFs are the sole intermolecular forces between nonpolar molecules, such as meth...

Q14. With suitable diagrams, explain the axes of symmetry in a cubic system.

Key Points:

Axes of symmetry are imaginary lines where rotation by a specific angle yields an identical object.
Cubic systems have three types of rotational axes: C₄, C₃, and C₂.
Three C₄ axes pass through centers of opposite faces, requiring a 90° rotation.
Four C₃ axes pass through opposite vertices (corners), requiring a 120° rotation.

Answer: A cubic system possesses several axes of symmetry, which are imaginary lines passing through the center of the cube, about which the cube can be rotated by a specific angle to achieve an identical orientation. These rotational axes are fundamental symmetry elements described as C_n axes, where 'n' denotes the order of rotation (360°/n). There are three distinct types of rotational axes in a cubic system. Firstly, three four-fold rotational axes (C₄ axes) pass through the centers of opposite fac...

Q15. Explain the Bravais Lattices in orthorhombic crystals. Give suitable diagrams.

Key Points:

Orthorhombic system: a ≠ b ≠ c, α = β = γ = 90°.
Four Bravais lattices exist within the orthorhombic crystal system.
Primitive (P) orthorhombic: Lattice points only at unit cell corners.
Body-Centered (I) orthorhombic: Points at corners and the cell center.

Answer: The Bravais lattices describe the fourteen distinct types of unit cell arrangements of lattice points in three-dimensional space, falling under the seven crystal systems. The orthorhombic crystal system is characterized by unit cell edge lengths that are unequal (a ≠ b ≠ c) but mutually perpendicular interaxial angles (α = β = γ = 90°). This system allows for four unique Bravais lattices. First is the **Primitive (P)** orthorhombic lattice. In this arrangement, lattice points are located only a...

Q16. The density and cell-edge length of sodium chloride are 2.163 X 10^3 kg m-3 and 5.63 X 10^-10 m, respectively. Using these data, arrive at the number of formula units per unit cell of sodium chloride crystal.

Key Points:

The density of a unit cell (ρ) relates to its formula units (Z), molar mass (M), edge length (a), and Avogadro's number (N_A).
The formula used is ρ = (Z × M) / (a³ × N_A), rearranged to solve for Z = (ρ × a³ × N_A) / M.
Ensure unit consistency: density in kg m⁻³, cell edge in m, molar mass in kg mol⁻¹, and Avogadro's number in mol⁻¹.
Molar mass of NaCl (Na=22.99, Cl=35.45) is 58.44 g/mol, converted to 0.05844 kg mol⁻¹ for calculation.

Answer: To determine the number of formula units per unit cell (Z) for sodium chloride, we utilize the relationship between the crystal's density, molar mass, unit cell volume, and Avogadro's number, a fundamental concept covered in BCHCT-137 Block 1, Unit 2, 'Unit Cells and Crystal Systems'. The key formula connecting these parameters is given by: ρ = (Z × M) / (a³ × N_A), where ρ is the density, M is the molar mass, a is the cell-edge length, and N_A is Avogadro's number. We need to rearrange this fo...

Q17. Answer the following:

a)) List different factors affecting the rates of chemical reactions. (80 words)
b)) Derive the integrated rate equation for a second order reaction in which a single reactant forms the product and the reaction rate depends on the second power of the reactant concentration. (120 words)

Key Points:

Reaction rates are affected by concentration, temperature, nature of reactants, catalysts, and surface area.
Higher reactant concentration increases collision frequency, speeding up reactions.
Catalysts lower activation energy, accelerating reactions without being consumed.
The differential rate law for a second-order reaction (A → Product) is -d[A]/dt = k[A]².

Answer:

Q18. What is meant by rate of a reaction? List and explain different types of rates used in chemical kinetics.

Key Points:

Rate of reaction: Change in reactant/product concentration per unit time.
Indicates reaction speed; negative for reactants, positive for products.
Average rate: Change in concentration over a measurable time interval (Δ[C]/Δt).
Instantaneous rate: Rate at a specific moment in time (d[C]/dt).

Answer: The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time. It quantifies how quickly reactants are consumed or products are formed over a given period. A higher rate indicates a faster reaction, while a lower rate signifies a slower one. Mathematically, for a general reaction where a reactant R converts to a product P, the rate can be expressed as: Rate = -Δ[R]/Δt or Rate = +Δ[P]/Δt. The negative sign for reactants denotes a decrease in con...

Q19. If the half-life of a first order reaction in A is 15 min, how long will it take for [A] to reach 10 percent of the initial concentration?

Key Points:

First-order half-life (t₁/₂) is calculated using t₁/₂ = ln(2)/k.
The integrated rate law for first-order reactions is ln([A]₀/[A]) = kt.
The rate constant 'k' must be determined from the given half-life first.
Use the calculated 'k' and ln([A]₀/[A]) to find the required time 't'.

Answer: The problem asks for the time required for the concentration of reactant A to decrease to 10% of its initial value for a first-order reaction. This is a common application of chemical kinetics principles, specifically related to the integrated rate law and half-life for first-order reactions, as covered in Unit 14 of BCHCT-137. The half-life (t₁/₂) of a first-order reaction is independent of the initial concentration and is related to the rate constant (k) by the formula: t₁/₂ = ln(2) / k. We a...

Q20. Compare the collision theory with Arrhenius theory.

Key Points:

Arrhenius theory relates rate constant (k) to temperature (T) using k = A * e^(-Ea/RT).
Activation energy (Ea) is the minimum energy required for reaction according to Arrhenius.
Collision theory postulates molecules must collide with sufficient energy (≥Ea) and correct orientation.
Collision frequency and steric factor are key components in collision theory.

Answer: The Arrhenius theory, as covered in BCHCT-137, primarily describes the empirical relationship between the rate constant (k) of a reaction and temperature (T). It is mathematically represented by the Arrhenius equation: k = A * e^(-Ea/RT), where 'A' is the pre-exponential factor and 'Ea' is the activation energy. This theory quantifies how reaction rates change with temperature, suggesting that molecules must overcome an energy barrier (Ea) to react. Collision theory, on the other hand, provide...

BCHCT-137 Solved Assignment — July 2025

COORDINATION CHEMISTRY, STATES OF MATTER & CHEMICAL KINETICS | IGNOU Bachelor of Science (General) (CBCS) (BSCG)

BCHCT-137—July 2025

COORDINATION CHEMISTRY, STATES OF MATTER & CHEMICAL KINETICS

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BCHCT-137 Assignment Questions — July 2025

Q1. What is the general electronic configuration of elements of the second transition series? Which d orbitals are filled up here?

Q2. In [Ni(H2O)6]2+ the observed magnetic moment is higher than the spin-only value. Explain the reason.

Q3. Give the most common oxidation states of the lanthanoids. Give their configurations.

Q4. What is meant by coordination entity and coordination sphere?

Q5. Give the rules of IUPAC nomenclature for naming complexes (give any five points).

Q6. According to valence bond theory, how do you account for the indicated molecular geometry for the following compounds:

Q7. Explain ligand isomerism with suitable example.

Q8. Compare the CFSE of an octahedral complex with that of a tetrahedral complex for four electrons in the d orbitals.

Q9. What is meant by tetragonal distortion in octahedral geometry?

Q10. Calculate the spin-only magnetic moment for low-spin octahedral complex of d⁶ and d⁹ ions.

Q11. State the assumptions of kinetic theory of gases and derive the following expression for the pressure of a gas. p = mNu^2 / 3V

Q12. Answer the following:

Q13. Explain the London or dispersion forces. Compare it with dipole-dipole forces.

Q14. With suitable diagrams, explain the axes of symmetry in a cubic system.

Q15. Explain the Bravais Lattices in orthorhombic crystals. Give suitable diagrams.

Q16. The density and cell-edge length of sodium chloride are 2.163 X 10^3 kg m-3 and 5.63 X 10^-10 m, respectively. Using these data, arrive at the number of formula units per unit cell of sodium chloride crystal.

Q17. Answer the following:

Q18. What is meant by rate of a reaction? List and explain different types of rates used in chemical kinetics.

Q19. If the half-life of a first order reaction in A is 15 min, how long will it take for [A] to reach 10 percent of the initial concentration?

Q20. Compare the collision theory with Arrhenius theory.

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