CUET UG Chemistry Booster Test – 3 Chemical Properties

Transition elements exhibit a wide range of oxidation states because the energies of the ns and (n−1)d orbitals are very close. Consequently, electrons from both orbitals can participate in chemical bonding. This allows transition elements to display successive oxidation states that generally differ by one unit, such as +2, +3, +4 and +5. This is a characteristic feature of d-block elements discussed in NCERT. In contrast, representative (non-transition) elements usually involve only their outermost electrons in bonding. Therefore, their common oxidation states generally differ by two units. The availability of d-electrons for bonding is the primary reason for the greater variability in oxidation states among transition elements.

Potassium manganate(VI), K₂MnO₄, is the dark green intermediate formed during the industrial preparation of potassium permanganate. It is obtained by heating manganese dioxide with potassium hydroxide in the presence of an oxidising agent such as oxygen or potassium nitrate. In this compound, manganese exists in the +6 oxidation state. The manganate(VI) ion is stable only in alkaline solution. Upon acidification, manganate(VI) undergoes disproportionation to produce permanganate ions, manganese dioxide and water. This reaction forms the basis of the commercial preparation of potassium permanganate and is explained in NCERT.

Across the first transition series, the third ionisation enthalpy generally increases because the effective nuclear charge increases. However, manganese and zinc show exceptionally high values because the removal of the third electron disturbs particularly stable electronic configurations. Manganese possesses a half-filled d⁵ configuration, whereas zinc has a completely filled d¹⁰ configuration. These arrangements are especially stable due to exchange energy and symmetrical distribution of electrons. Removing an electron from these stable configurations requires additional energy, resulting in unusually high third ionisation enthalpies. NCERT highlights this behaviour while discussing the ionisation enthalpy trends of transition elements.

The second ionisation enthalpy is the energy required to remove an electron from a singly charged gaseous ion. Across the first transition series, this value generally increases because of increasing effective nuclear charge. However, the electronic configuration of each element also influences the trend. Copper exhibits the highest second ionisation enthalpy among the given elements because the electron removed belongs to a relatively stable electronic arrangement. Zinc also has a high value due to its filled d-subshell, whereas chromium and manganese have comparatively lower values. Thus, the decreasing order is Cu > Zn > Cr > Mn, which agrees with the thermodynamic trends described in NCERT.

Enthalpy of atomisation is the energy required to convert one mole of a metallic solid into isolated gaseous atoms. The 4d and 5d transition metals generally possess higher enthalpies of atomisation than the corresponding 3d elements because they exhibit stronger metal–metal bonding. The stronger metallic bonding results from better overlap of d-orbitals and greater participation of d-electrons in bonding. Since the metallic lattice is held together more strongly, greater energy is needed to separate individual atoms. Consequently, these heavier transition metals show higher melting points, greater hardness and larger enthalpies of atomisation, as discussed in NCERT.

Density is defined as the mass contained in a unit volume of a substance. In the first transition series, density generally increases from titanium to copper because atomic masses increase while atomic volumes do not increase proportionately. NCERT discusses density as one of the important physical properties of transition metals. The commonly used unit for density in chemistry is g cm⁻³. This unit provides a convenient way to compare the compactness of metallic structures and is widely used for transition elements.

The standard electrode potentials for the M²⁺/M couples are Ti²⁺/Ti = –1.63 V, Mn²⁺/Mn = –1.18 V, Fe²⁺/Fe = –0.44 V and Cu²⁺/Cu = +0.34 V. These values are listed in NCERT and help explain the chemical reactivity of transition metals. The highly negative value for titanium indicates strong reducing tendency, while the positive value for copper reflects its relatively noble character. Matching these values correctly gives the sequence 1-b, 2-a, 3-d and 4-c.

The V³⁺ ion has a d² configuration, while V²⁺ has a d³ configuration. In an octahedral field, the d³ arrangement gives a particularly stable t₂g³ configuration. This additional stability reduces the tendency of V²⁺ to lose an electron and form V³⁺, leading to a comparatively low E° value for the V³⁺/V²⁺ couple. NCERT highlights the role of electronic stability in explaining unusual electrode potential values among transition elements.

In MnO₄²⁻, manganese exists in the +6 oxidation state. During the reaction, part of the manganese is oxidised to +7 in MnO₄⁻ and another part is reduced to +4 in MnO₂. Since the same species simultaneously undergoes oxidation and reduction, the process is known as disproportionation. NCERT discusses this reaction while describing the preparation and chemistry of permanganate ions. It is a classic example of disproportionation involving manganese in an intermediate oxidation state.

Mn³⁺ and Co³⁺ possess high positive reduction potentials and therefore act as strong oxidising agents in aqueous solution. Ti²⁺, V²⁺ and Cr²⁺ readily lose electrons and behave as strong reducing agents. Cr²⁺ can reduce H⁺ ions from dilute acids, liberating hydrogen gas. However, Fe³⁺ is generally an oxidising agent rather than a reducing agent. Therefore, statements 1, 2 and 3 are correct while statement 4 is incorrect. This conclusion follows directly from the NCERT discussion of standard electrode potentials and reactivity trends.

Transition metals form a large number of complex compounds because their ions are comparatively small in size, possess high ionic charges, and have available d orbitals for accepting electron pairs from ligands. According to NCERT, ligands donate electron pairs to the central metal ion, forming coordinate bonds. Transition metal ions such as Fe²⁺, Co³⁺, Ni²⁺ and Cu²⁺ readily form complexes because their charge density is high and they can accommodate electron pairs from ligands like NH₃, CN⁻ and H₂O. Hence, smaller ion size, high ionic charge and available d orbitals are the key factors responsible for complex formation.

The species [Cu(NH₃)₄]²⁺ is a complex ion because it contains a central transition metal ion, Cu²⁺, bonded to four ammonia molecules. In NCERT terminology, NH₃ acts as a ligand because it donates a lone pair of electrons to the central metal ion through coordinate bonding. Such species are called coordination complexes or complex ions. Transition metals readily form these complexes due to their small size, high charge and available d orbitals. Therefore, [Cu(NH₃)₄]²⁺ is the correct example of a complex species.

The spin-only magnetic moment is given by the formula μ = √n(n + 2) BM, where n is the number of unpaired electrons. If n = 4, then μ = √4(4 + 2) = √24 = 4.90 BM. Chromium has atomic number 24. Its electronic configuration is [Ar] 3d⁵ 4s¹. In the Cr²⁺ ion, two electrons are removed, giving the configuration 3d⁴. This gives four unpaired electrons in the gaseous or high-spin case. Therefore, a divalent ion with magnetic moment 4.90 BM corresponds to Cr²⁺, and the atomic number is 24.

The manganate ion, MnO₄²⁻, is green and contains manganese in the +6 oxidation state. Since manganese has atomic number 25, Mn⁶⁺ has a 3d¹ configuration, giving one unpaired electron. Therefore, manganate is paramagnetic. The permanganate ion, MnO₄⁻, is purple and contains manganese in the +7 oxidation state. Mn⁷⁺ has a 3d⁰ configuration, so it has no unpaired electrons and is diamagnetic. Both manganate and permanganate ions have tetrahedral geometry according to NCERT discussion of manganese oxoanions. Hence statements 1, 2 and 3 are correct.

The spin-only magnetic moment is calculated by μ = √n(n + 2) BM, where n is the number of unpaired electrons. Mn²⁺ has 3d⁵ configuration with 5 unpaired electrons, so μ = √5(7) = √35 = 5.92 BM. Fe²⁺ has 3d⁶ configuration with 4 unpaired electrons, so μ = √24 = 4.90 BM. Co²⁺ has 3d⁷ configuration with 3 unpaired electrons, so μ = √15 = 3.87 BM. Ni²⁺ has 3d⁸ configuration with 2 unpaired electrons, so μ = √8 = 2.83 BM. Thus the decreasing order is Mn²⁺ > Fe²⁺ > Co²⁺ > Ni²⁺.

Iron has atomic number 26 and its ground state electronic configuration is [Ar] 3d⁶ 4s². During ion formation, electrons are removed first from the 4s orbital and then from the 3d orbital. For Fe³⁺, two 4s electrons and one 3d electron are removed, giving the configuration [Ar] 3d⁵. According to Hund's rule, the five d electrons occupy the five d orbitals singly before pairing. Therefore, Fe³⁺ contains five unpaired electrons. This number is required before applying the spin-only magnetic moment formula μ = √n(n + 2) BM.

Lanthanoid ions are generally coloured due to electronic transitions within the 4f orbitals, known as f-f transitions. However, La³⁺ and Lu³⁺ are colourless because these transitions are not possible in them. La³⁺ has the 4f⁰ configuration, meaning there are no f electrons available for excitation. Lu³⁺ has the 4f¹⁴ configuration, meaning the f subshell is completely filled. Since there is no suitable intra-f electronic excitation in either case, these ions do not absorb visible light in the manner required to produce colour. Therefore, both La³⁺ and Lu³⁺ remain colourless.

The colour of aquated transition metal ions is generally due to d-d transitions between split d orbitals in the presence of ligands such as water. Ti³⁺ has the electronic configuration 3d¹ and is observed as purple. Cr³⁺ has 3d³ configuration and is violet. Mn³⁺ has 3d⁴ configuration and is also associated with violet colour in the given NCERT-based data. Fe²⁺ has 3d⁶ configuration and appears green. Therefore, the correct matching is 1-c, 2-d, 3-b and 4-a.

Transition metals and their compounds are effective catalysts because they can use their 3d and 4s electrons to form temporary bonds with reacting molecules. On a solid catalyst surface, reactant molecules are adsorbed, which increases their concentration at the surface. This adsorption also weakens the bonds present in the reacting molecules, allowing the reaction to proceed through a pathway of lower activation energy. According to NCERT, transition metals show catalytic activity due to their ability to adopt variable oxidation states and form complexes or surface bonds. Thus, the main outcome is increased surface concentration of reactants and reduced activation energy.

Ziegler catalysts are important transition metal catalysts used in the polymerisation of alkenes such as ethene to produce polyethylene. These catalysts generally involve transition metal compounds, commonly titanium compounds, along with organoaluminium compounds. NCERT discusses the catalytic role of transition metals in several industrial reactions because they can form coordination complexes and provide suitable reaction pathways. In the manufacture of polyethylene, Ziegler-type catalysts help coordinate the alkene molecules and promote chain growth under controlled conditions. Therefore, the catalyst type associated with the manufacture of polyethylene is Ziegler catalysts.