Search Previous Year Questions

CONCEPT:

Periodic Trends: Atomic Radius

• In a Period: Atomic radius generally decreases from left to right across a period. This occurs because the number of protons in the nucleus increases, leading to a higher effective nuclear charge that pulls the electron cloud closer to the nucleus.
• In a Group: Atomic radius increases moving down a group. This is because each successive element down the group has an additional electron shell (energy level), which increases the distance between the nucleus and the outermost electrons, despite the increase in nuclear charge.

EXPLANATION:

• Analysis of Statement I:
• Statement I says atomic radius increases as we go from left to right in a period.
• As we move from left to right, electrons are added to the same valence shell while the nuclear charge increases. This results in a greater inward pull on the electrons, making the atom smaller.
• Therefore, Statement I is incorrect.
• Analysis of Statement II:
• Statement II says atomic size increases as we go down a group.
• As we move down a group, the principal quantum number increases, meaning new electron shells are added. These additional shells outweigh the increase in nuclear charge, causing the atom to expand.
• Therefore, Statement II is correct.
• Conclusion:
• Statement I is false.
• Statement II is true.

Therefore, only statement II is correct.

CONCEPT:

Highest Oxidation State of Transition Metals

• For the 3d transition series, the oxidation states are determined by the loss of electrons from both the 4s and 3d subshells.
• The highest oxidation state of a transition element is generally equal to the total number of electrons present in both the (n-1)d and ns orbitals.
• This trend increases from Scandium (+3) to Manganese (+7) and then decreases as electrons begin to pair up in the d-orbitals.

EXPLANATION:

• Chromium (Cr):
• The atomic number of Chromium is 24.
• Its electronic configuration is [Ar] 3d⁵ 4s¹.
• The total number of valence electrons available for bonding is 5 (from 3d) + 1 (from 4s) = 6.
• Thus, the highest oxidation state of Cr is +6 (as seen in compounds like K₂Cr₂O₇).
• Manganese (Mn):
• The atomic number of Manganese is 25.
• Its electronic configuration is [Ar] 3d⁵ 4s².
• The total number of valence electrons available for bonding is 5 (from 3d) + 2 (from 4s) = 7.
• Thus, the highest oxidation state of Mn is +7 (as seen in compounds like KMnO₄).

Therefore, the highest oxidation states of Cr and Mn are +6 and +7, respectively.

CONCEPT:

Peptide Linkage and Amino Acids

• Proteins are polymers of amino acids linked together by a specific type of covalent bond known as a peptide bond.
• A peptide bond is formed through a condensation reaction between two amino acids, involving the loss of a water molecule.
• Chemically, this linkage is classified as an amide functional group.

EXPLANATION:

• Amino acids contain both an amino group (-NH₂) and a carboxyl group (-COOH).
• During protein synthesis, the carboxyl group of one amino acid reacts with the amino group of another amino acid.
• This process releases a molecule of water (H₂O) and forms a bond between the carbon of the carbonyl group and the nitrogen of the amine group:

-COOH + H₂N- → -CO-NH- + H₂O

• The resulting -CO-NH- linkage is called a peptide bond in biochemistry, but in organic chemistry, it is fundamentally an amide bond.
• Other options like Hydrogen bonds, Ionic bonds, and Dipole-Dipole interactions are intermolecular forces that help in the folding (secondary and tertiary structure) of proteins, but they do not join the amino acids into a primary chain.

Therefore, amino acids are joined together by amide bonds.

CONCEPT:

• Homohaptic (Homoleptic) Complexes: These are coordination complexes where the central metal atom or ion is bound to only one kind of donor group or ligand.
• Heterohaptic (Heteroleptic) Complexes: These are coordination complexes where the central metal atom or ion is bound to more than one kind of donor group or ligand.
• Polydentate Ligands: These are ligands that can bond to a central metal ion through multiple donor atoms. Examples include EDTA, which is a hexadentate ligand.
• Bidentate Ligands: These are ligands that have two donor atoms and can coordinate to the metal ion at two positions simultaneously. Examples include the oxalate ion (C₂O₄^2-).

EXPLANATION:

• (I) Homohaptic: This refers to a complex with only one type of ligand. In the given options, [Fe(CNH₃)]³⁺ (B) represents a complex with a single ligand type surrounding the metal.

Match: (I) → B

• (ii) Heterohaptic: This refers to a complex with different types of ligands. The complex [Re(H₂O)₂(NH₃)₂] (C) contains both water (H₂O) and ammonia (NH₃) ligands.

Match: (ii) → C

• (iii) Polydentate: EDTA (A), or ethylenediaminetetraacetate, is a well-known polydentate ligand that can form six bonds with a metal ion (hexadentate).

Match: (iii) → A

• (iv) Bidentate: The oxalate ion, C₂O₄^2- (D), is a bidentate ligand as it coordinates to the central metal atom through two oxygen atoms.

Match: (iv) → D

Therefore, the correct matching is: I-B, II-C, III-A, IV-D.

CONCEPT:

Vitamins and Deficiency Diseases

• Vitamins are organic compounds required by the body in small amounts for various metabolic processes, growth, and health maintenance.
• A lack of specific vitamins in the diet over a long period leads to deficiency diseases. Each vitamin has a characteristic deficiency symptom or disease associated with it.

EXPLANATION:

• (a) Vitamin E: It acts as an antioxidant and is important for the immune system and cell signaling. Its deficiency can lead to muscular weakness and increased fragility of red blood cells.
• Match: (a) → (iii)
• (b) Vitamin C: Also known as ascorbic acid, it is essential for the synthesis of collagen and maintenance of healthy gums. Deficiency of Vitamin C leads to Scurvy, characterized by bleeding gums and loose teeth.
• Match: (b) → (iv)
• (c) Vitamin A: It is critical for maintaining healthy vision, especially in low light. The deficiency of Vitamin A results in Night blindness (inability to see well at night).
• Match: (c) → (i)
• (d) Vitamin B: Vitamin B1 (Thiamine) deficiency is well known to cause Beri-Beri, a disease that affects the nervous system and the heart.
• Match: (d) → (ii)

Therefore, the correct matching is a-iii, b-iv, c-i, d-ii.

CONCEPT:

Identification of Aldehydes and Ketones

• Tollens Test (Silver Mirror Test):
• Tollens reagent is an ammoniacal solution of silver nitrate, [Ag(NH₃)₂]⁺.
• It is a mild oxidizing agent that oxidizes aldehydes to carboxylate ions. During this reaction, Ag⁺ ions are reduced to metallic silver, forming a silver mirror on the inner wall of the reaction vessel.
• Ketones generally do not respond to Tollens test.
• Fehling Test:
• Fehling solution consists of a mixture of copper(II) sulfate and sodium potassium tartrate in an alkaline medium.
• It oxidizes aliphatic aldehydes to carboxylate ions, while the copper(II) ions are reduced to cuprous oxide (Cu₂O), which appears as a reddish-brown precipitate.
• Ketones and aromatic aldehydes do not typically give a positive Fehling test.

EXPLANATION:

• Statement I: All Aldehyde and Ketone are given Positive Tollen Test.
• This statement is incorrect. Tollens test is a characteristic test for aldehydes. While aldehydes (both aliphatic and aromatic) give a positive result, ketones do not. Therefore, the test is used to distinguish between these two functional groups.
• Statement II: only aldehyde are given positive Fehlling test.
• This statement is correct in the context of comparing aldehydes and ketones. Since ketones do not show a positive response to Fehling solution, the ability to give a positive Fehling test is limited to the aldehyde group (specifically aliphatic aldehydes).

Therefore, Statement I is incorrect and Statement II is correct.

CONCEPT:

Osmotic Pressure (Π)

• Osmotic pressure is a colligative property, meaning it depends on the number of solute particles in a solution rather than their identity.
• It is the minimum pressure that must be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane.
• The osmotic pressure (Π) is given by the equation: Π = iCRT, where C is the molar concentration, R is the gas constant, T is the temperature, and i is the Van't Hoff factor.

EXPLANATION:

• Solutions are categorized based on their relative osmotic pressures:
• Isotonic Solutions: Two solutions that have the same osmotic pressure at the same temperature are called isotonic solutions. When these solutions are separated by a semipermeable membrane, there is no net movement of solvent (osmosis) between them.
• Hypertonic Solutions: If one solution has a higher osmotic pressure than another, it is called hypertonic relative to the second solution.
• Hypotonic Solutions: If one solution has a lower osmotic pressure than another, it is called hypotonic relative to the second solution.
• Since the question asks for solutions having the same osmotic pressure, they are referred to as isotonic.

Therefore, solutions having the same osmotic pressure are called Isotonic.

CONCEPT:

Criteria for Spontaneity

• The spontaneity of a process at constant temperature and pressure is determined by the change in Gibbs Free Energy ($\Delta G$).
• According to the Second Law of Thermodynamics, a spontaneous process must result in an increase in the total entropy of the universe ($\Delta S_{total} > 0$).
• For a system at constant temperature and pressure, the relationship between these thermodynamic properties is defined by the equation:

$\Delta G = \Delta H - T\Delta S$

EXPLANATION:

• A process is considered spontaneous if it can occur without continuous external intervention.
• The mathematical conditions for spontaneity at constant pressure and temperature are:
• Gibbs Free Energy Change: The change must be negative, meaning $\Delta G < 0$. This indicates that the free energy of the system decreases.
• Entropy Change: For a process to be naturally favorable, the entropy change ($\Delta S$) is typically positive ($\Delta S > 0$), representing an increase in the disorder or randomness of the system.
• In standard conditions, these criteria are often expressed as $\Delta G^\circ < 0$ and $\Delta S > 0$.
• Evaluating the given conditions:
• If $\Delta G^\circ < 0$, the reaction is spontaneous in the forward direction.
• If $\Delta S > 0$, the disorder of the system increases, which contributes to making the Gibbs Free Energy change more negative (as seen in the equation $\Delta G = \Delta H - T\Delta S$).

Therefore, for a spontaneous process at constant pressure and temperature, the conditions are $\Delta G^\circ < 0$ and $\Delta S > 0$.

CONCEPT:

• Bond Length: It is defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule.
• Atomic Size: In the periodic table, as we move down a group, the atomic size (radius) increases because the number of electron shells increases.
• Relationship: For a bond between a fixed atom (Carbon) and different atoms of the same group (Halogens), the bond length increases as the size of the halogen atom increases.

EXPLANATION:

• The halogens involved in the C-X bond are Fluorine (F), Chlorine (Cl), Bromine (Br), and Iodine (I).
• These elements belong to Group 17 of the periodic table. The order of their atomic size is:

F < Cl < Br < I

• As the size of the halogen atom (X) increases, the distance between the nucleus of the Carbon atom and the nucleus of the Halogen atom also increases.
• Consequently, the bond length of the C-X bond increases in the order:

C-F < C-Cl < C-Br < C-I

• The numerical values for bond lengths in methyl halides are approximately:
• C-F: 139 pm
• C-Cl: 178 pm
• C-Br: 193 pm
• C-I: 214 pm
• Therefore, the decreasing order of C-X bond length is:

CH₃-I > CH₃-Br > CH₃-Cl > CH₃-F

Therefore, the decreasing order of C-X bond length is CH₃I > CH₃Br > CH₃Cl > CH₃F.

CONCEPT:

Transition Elements

• According to the IUPAC definition, a transition element is an element that has an incompletely filled d-subshell in its ground state or in any of its common oxidation states.
• The d-block elements belong to groups 3 to 12 of the periodic table. While all transition elements are d-block elements, not all d-block elements (like Zinc, Cadmium, and Mercury) are transition elements because they do not have partially filled d-orbitals.

EXPLANATION:

• Zirconium ($Zr$):
• The atomic number of Zirconium is 40.
• Its ground state electronic configuration is $[Kr] 4d^2 5s^2$.
• Since the 4d-orbital is partially filled (it contains 2 electrons out of a possible 10), Zirconium satisfies the criteria for being a transition element.
• Zinc ($Zn$):
• The atomic number of Zinc is 30.
• Its ground state electronic configuration is $[Ar] 3d^{10} 4s^2$.
• In its most common oxidation state ($Zn^{2+}$), the electronic configuration is $[Ar] 3d^{10}$.
• In both its neutral atomic form and its stable ionic form, the 3d-subshell is completely filled. Therefore, Zinc is not classified as a transition element.
• The defining difference between a transition element (like Scandium or Zirconium) and Zinc is the presence of partially filled d-orbitals in the former and completely filled d-orbitals in the latter.

Therefore, Zirconium is a transition element because it has a partially filled d-subshell, whereas Zinc is not because its d-subshell is completely filled in both its ground and ionic states.