25 important short questions with answers in inorganic chemistry for graduation students
1. What are the postulates of VSEPR theory?
Answer: VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the geometry of molecules based on the repulsion between electron pairs in the valence shell of the central atom. The main postulates are:
Electron pairs around the central atom arrange themselves to minimize repulsion.
Electron pairs can be bonding pairs (shared) or lone pairs (unshared).
Lone pair-lone pair repulsion > lone pair-bonding pair repulsion > bonding pair-bonding pair repulsion.
Multiple bonds are treated as a single electron pair for geometry prediction.
2. What is hybridization? Explain sp3 hybridization with an example.
Answer: Hybridization is the concept of intermixing atomic orbitals of slightly different energies to form new degenerate orbitals of equivalent energy and shape, suitable for bond formation.
sp3 Hybridization: In sp3 hybridization, one s orbital and three p orbitals of an atom mix to form four equivalent sp3 hybrid orbitals. These orbitals are directed towards the corners of a tetrahedron.
Example: Methane (CH4). The carbon atom undergoes sp3 hybridization to form four sp3 hybrid orbitals, each overlapping with the 1s orbital of a hydrogen atom to form four C-H sigma (σ) bonds.
3. What is the Born-Haber cycle? What is its use?
Answer: The Born-Haber cycle is a thermodynamic cycle that relates the lattice energy of an ionic compound to other energetic terms, such as ionization energy, electron affinity, sublimation energy, and bond dissociation energy.
Use: It allows the determination of the lattice energy of an ionic compound, which cannot be measured directly. It is based on Hess's Law.
4. Define lattice energy. What factors affect it?
Answer: Lattice energy is the energy released when one mole of an ionic compound is formed from its gaseous ions. It is a measure of the strength of the forces holding the ions together in the crystal lattice.
Factors affecting lattice energy:
Charge of the ions: Higher charges lead to stronger electrostatic attractions and higher lattice energy. (E∝q1q2)
Size of the ions: Smaller ions lead to shorter interionic distances and higher lattice energy. (E∝1/r)
5. What are the types of defects found in ionic solids?
Answer: Common types of defects in ionic solids include:
Schottky defects: Vacancy defects where an equal number of cations and anions are missing from their lattice sites, maintaining electrical neutrality.
Frenkel defects: A cation leaves its lattice site and occupies an interstitial site, creating a cation vacancy and a cation interstitial.
Metal excess defects: Can occur due to anionic vacancies or the presence of extra cations in interstitial sites.
Metal deficiency defects: Can occur due to cationic vacancies or the presence of extra anions in interstitial sites.
6. What is the inert pair effect? Give an example.
Answer: The inert pair effect refers to the reluctance of the two s electrons in the outermost valence shell of heavier post-transition metals (especially in groups 13-16) to participate in bonding. This results in the stability of oxidation states that are two units lower than the group oxidation state.
Example: In Group 15, the stable oxidation state of nitrogen and phosphorus is +5, but for bismuth, the +3 oxidation state (Bi3+) is more stable than the +5 state (Bi5+). The 6s2 electrons in bismuth are relatively inert.
7. What are pseudohalides? Give two examples.
Answer: Pseudohalides are polyatomic ions that exhibit chemical behavior similar to that of halide ions (e.g., Cl−,Br−,I−). They typically form insoluble silver salts, colored mercury(II) salts, and form acids with hydrogen.
Examples:
Cyanide ion (CN−)
Thiocyanate ion (SCN−)
8. What are interhalogen compounds? Give one example and its structure.
Answer: Interhalogen compounds are covalent compounds formed between two or more different halogen atoms.
Example: Chlorine trifluoride (ClF3). It has a T-shaped structure due to the presence of three bonding pairs and two lone pairs of electrons around the central chlorine atom (sp3d hybridization).
9. What are the allotropes of carbon? Briefly describe their structures.
Answer: Allotropes of carbon are different structural modifications of carbon atoms in the same physical state. Some important allotropes include:
Diamond: Each carbon atom is sp3 hybridized and bonded tetrahedrally to four other carbon atoms, forming a giant covalent network structure. It is very hard and a poor conductor of electricity.
Graphite: Carbon atoms are sp2 hybridized and arranged in layers of hexagonal rings. Each carbon is bonded to three others within the layer. The layers are held together by weak van der Waals forces, making it soft and a good conductor of electricity.
Fullerenes (e.g., C60 - Buckminsterfullerene): Carbon atoms are arranged in closed cages or tubes. C60 has a soccer ball-like structure with 60 carbon atoms arranged in pentagons and hexagons.
Graphene: A single layer of graphite, a two-dimensional sheet of carbon atoms arranged in a hexagonal lattice. It has exceptional strength and electrical conductivity.
10. What are silicones? How are they prepared? Give one use.
Answer: Silicones are polymers containing silicon-oxygen chains with organic groups attached to the silicon atoms. They have the general formula (R2SiO)n.
Preparation: Silicones are typically prepared by the hydrolysis of dialkyldichlorosilanes (R2SiCl2) followed by polymerization. nR2SiCl2+nH2O⟶[R2Si(OH)2]n+2nHCl [R2Si(OH)2]n⟶(R2SiO)n+nH2O
Use: Silicones have various applications due to their properties like thermal stability, water repellency, and electrical insulation. They are used in lubricants, sealants, adhesives, and medical implants.
11. What are metal carbonyls? Give one example and its structure.
Answer: Metal carbonyls are coordination complexes in which carbon monoxide (CO) ligands are bonded to a central metal atom.
Example: Nickel tetracarbonyl (Ni(CO)4). It has a tetrahedral structure with the nickel atom at the center and four CO ligands bonded to it.
12. What is the 18-electron rule? Does V(CO)6 obey it?
Answer: The 18-electron rule is a guideline used to predict the stability of transition metal complexes. It states that thermodynamically stable low-valent transition metal complexes tend to have 18 valence electrons (the same electron configuration as the next noble gas). This count includes the metal's d, s, and p valence electrons and the electrons donated by the ligands.
V(CO)6: Vanadium (V) is in Group 5, so it has 5 valence electrons. Each CO ligand donates 2 electrons. Therefore, the total electron count for V(CO)6 is 5+(6×2)=17 electrons. Thus, V(CO)6 does not obey the 18-electron rule. It is a paramagnetic species and tends to dimerize or undergo reduction to achieve an 18-electron configuration.
13. What are organometallic compounds? Give one example and its application.
Answer: Organometallic compounds are chemical compounds containing at least one chemical bond between a carbon atom of an organic group and a metal, including transition metals, alkali metals, alkaline earth metals, and metalloids like boron, silicon, and tin.
Example: Grignard reagent (RMgX, where R is an alkyl or aryl group and X is a halogen, e.g., CH3MgBr).
Application: Grignard reagents are widely used in organic synthesis for forming carbon-carbon bonds.
14. What is crystal field theory (CFT)? What are its main assumptions?
Answer: Crystal Field Theory (CFT) is a model that describes the electronic structure of transition metal complexes by considering the electrostatic interaction between the metal ion and the ligands.
Main assumptions:
Ligands are treated as point charges (or dipoles).
There is no covalent interaction between the metal ion and the ligands; the bonding is purely electrostatic.
The d orbitals of the central metal ion are degenerate in a spherical field but split into different energy levels in the presence of the ligand field.
15. Explain the splitting of d orbitals in an octahedral field.
Answer: In an octahedral complex, the six ligands approach the central metal ion along the x, y, and z axes. The d orbitals lying along these axes (dx2−y2 and dz2) experience greater repulsion from the ligand electron pairs compared to the d orbitals lying between the axes (dxy, dyz, and dxz). This repulsion causes the d orbitals to split into two sets of different energies:
eg orbitals: The higher energy set consisting of dx2−y2 and dz2 orbitals.
t2g orbitals: The lower energy set consisting of dxy, dyz, and dxz orbitals. The energy difference between these two sets is denoted by Δo (crystal field splitting energy for octahedral complexes).
16. What factors affect the magnitude of crystal field splitting (Δo)?
Answer: The magnitude of Δo is affected by several factors:
Nature of the metal ion: The charge on the metal ion and its position in the periodic table (e.g., Δo increases down a group and with increasing charge).
Nature of the ligands: Strong field ligands cause a larger splitting than weak field ligands (spectrochemical series).
Geometry of the complex: Different geometries (e.g., tetrahedral, square planar) result in different splitting patterns and magnitudes.
Oxidation state of the metal ion: Higher oxidation states generally lead to larger Δo.
17. What is the spectrochemical series? Give an example.
Answer: The spectrochemical series is an empirical ordering of ligands based on their ability to split the d orbital energies in a metal complex (i.e., the magnitude of Δ). Ligands that cause a large splitting are called strong field ligands, and those that cause a small splitting are weak field ligands.
Example (partial series): I−<Br−<S2−<SCN−<Cl−<NO3−<F−<OH−<C2O42−≈H2O<NCS−<CH3CN<NH3<en<bpy<phen<NO2−<PPh3<CN−≈CO
18. What is meant by "high spin" and "low spin" complexes? When do they arise?
Answer: High spin and low spin complexes arise in octahedral complexes when the crystal field splitting energy (Δo) is comparable to the pairing energy (P), which is the energy required to pair two electrons in the same d orbital.
High spin complexes: Occur when Δo<P. Electrons tend to occupy the higher energy eg orbitals before pairing in the lower energy t2g orbitals to minimize electron-electron repulsion. These complexes have the maximum number of unpaired electrons. This is favored by weak field ligands and smaller Δo.
Low spin complexes: Occur when Δo>P. It is energetically more favorable for electrons to pair up in the lower energy t2g orbitals before occupying the higher energy eg orbitals. These complexes have fewer unpaired electrons. This is favored by strong field ligands and larger Δo.
19. What is the difference between lanthanides and actinides?
Answer: Both lanthanides and actinides are f-block elements.
Lanthanides: The 14 elements following lanthanum (La, Z=57) where the 4f subshell is being filled. They generally exhibit a +3 oxidation state.
Actinides: The 14 elements following actinium (Ac, Z=89) where the 5f subshell is being filled. They exhibit a greater variety of oxidation states due to the smaller energy difference between the 5f, 6d, and 7s orbitals. All actinides are radioactive.
20. What are the common oxidation states exhibited by lanthanides? Why?
Answer: The most common and stable oxidation state exhibited by lanthanides is +3. This is because the removal of the three valence electrons (two 6s and one 5d or sometimes three 6s depending on the element) leads to a relatively stable electronic configuration, often resembling that of the preceding noble gas or having a stable f0, f7 (half-filled), or f14 (fully filled) configuration. However, some lanthanides also exhibit +2 and +4 oxidation states under certain conditions due to the stability associated with these fn configurations.
21. What are the main differences between double salts and coordination complexes?
Answer:
Double salts: These are formed by the combination of two simple salts in a definite stoichiometric ratio and crystallize as a single substance. When dissolved in water, they completely dissociate into their constituent ions.
Example: Potash alum (K2SO4⋅Al2(SO4)3⋅24H2O) dissociates into K+, SO42−, and Al3+ ions in solution.
Coordination complexes: These consist of a central metal atom or ion bonded to a group of ligands (ions or neutral molecules) by coordinate bonds. In solution, the complex ion usually persists, and it does not completely dissociate into the metal ion and free ligands.
Example: Potassium ferrocyanide (K4[Fe(CN)6]) in solution gives K+ ions and the complex ion [Fe(CN)6]4−, which does not dissociate significantly into Fe2+ and CN− ions.
22. What is chelation? Give an example of a chelating ligand.
Answer: Chelation is the formation of a complex in which a polydentate ligand (a ligand that can bind to the central metal ion through more than one donor atom) binds to a single metal ion, forming a ring structure. The resulting complexes are called chelates and are generally more stable than complexes with monodentate ligands.
Example of a chelating ligand: Ethylenediamine (en), H2NCH2CH2NH2, which has two nitrogen donor atoms and can form a five-membered ring with a metal ion.
23. What are the different types of isomerism found in coordination complexes?
Answer: Coordination complexes exhibit various types of isomerism, including:
Structural isomerism:
Ionization isomerism (e.g., [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br)
Hydrate isomerism (a type of solvate isomerism, e.g., [Cr(H2O)6]Cl3, [Cr(H2O)5Cl]Cl2⋅H2O, and [Cr(H2O)4Cl2]Cl⋅2H2O)
Linkage isomerism (e.g., [M(SCN)]n and [M(NCS)]n)
Coordination isomerism (occurs in complexes with both cationic and anionic complex ions, e.g., [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6])
Stereoisomerism:
Geometrical isomerism (cis- and trans- isomers in square planar and octahedral complexes)
Optical isomerism (non-superimposable mirror images, enantiomers, often found in chiral octahedral complexes with polydentate ligands)
24. What is the role of hemoglobin and myoglobin in biological systems?
Answer: Both hemoglobin and myoglobin are iron-containing proteins involved in oxygen transport and storage.
Hemoglobin: Found in red blood cells, its primary role is to transport oxygen from the lungs to the tissues and carbon dioxide from the tissues back to the lungs. It is a tetrameric protein with four heme units, each capable of binding one oxygen molecule.
Myoglobin: Found in muscle tissue, its main function is to store oxygen and release it when needed, particularly during intense muscular activity. It is a monomeric protein with one heme unit that can bind one oxygen molecule.
25. What are the essential elements in biological systems? Classify them.
Answer: Essential elements are those elements that are required for the normal growth, development, and reproduction of an organism. They are typically classified based on their abundance in biological systems:
Macronutrients (Major elements): Elements required in relatively large amounts (e.g., Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Sulfur (S), Potassium (K), Calcium (Ca), Magnesium (Mg), Sodium (Na), Chlorine (Cl)).
