CUET UG Chemistry Booster Test – 3 Structural Concepts

Transition metals form a variety of oxides because they show variable oxidation states. NCERT explains that the highest oxidation number in oxides often corresponds to the group number of the element. For example, chromium in CrO₃ shows +6 oxidation state and manganese in Mn₂O₇ shows +7 oxidation state. In the first transition series, manganese reaches the highest oxidation state in Mn₂O₇. Also, as the oxidation state of the metal increases, the oxide becomes more covalent and acidic in nature. Hence, statements 2, 3 and 4 are correct.

The highest oxide of a transition metal generally reflects the maximum oxidation state possible for that metal. Scandium belongs to Group 3 and forms Sc₂O₃, where scandium is in the +3 oxidation state. Titanium belongs to Group 4 and forms TiO₂, where titanium is +4. Vanadium belongs to Group 5 and forms V₂O₅, where vanadium is +5. Chromium belongs to Group 6 and forms CrO₃, where chromium is +6. Thus, the correct matching is 1-b, 2-d, 3-c and 4-a.

The basic character of transition metal oxides decreases as the oxidation state of the metal increases. V₂O₃ contains vanadium in the +3 oxidation state and is the most basic among the given oxides. V₂O₄ contains vanadium in the +4 oxidation state and is less basic than V₂O₃. V₂O₅ contains vanadium in the +5 oxidation state and is amphoteric though mainly acidic. Therefore, the decreasing order of basic character is V₂O₃ > V₂O₄ > V₂O₅, which corresponds to 2, 3 and 1.

Vanadium pentoxide, V₂O₅, is amphoteric though mainly acidic. NCERT explains that V₂O₅ reacts with acids to form the oxocation VO₂⁺. This behaviour shows that although V₂O₅ mainly shows acidic character, it can also react with acids under suitable conditions. The formation of VO₂⁺ is associated with vanadium in a high oxidation state. In transition metal chemistry, oxocations are common for metals in higher oxidation states because oxygen remains strongly bonded to the metal centre. Therefore, the oxocation formed from V₂O₅ with acids is VO₂⁺.

Chromate and dichromate ions are interconvertible in aqueous solution depending on pH. Chromate ion, CrO₄²⁻, is yellow and exists predominantly in basic medium. When acid is added and the pH is lowered, chromate ions are converted into orange dichromate ions, Cr₂O₇²⁻. The oxidation state of chromium remains +6 in both chromate and dichromate, so the change is not oxidation or reduction. Structurally, chromate is tetrahedral, while dichromate consists of two tetrahedra sharing one corner. Therefore, the transformation occurs under acidification.

The dichromate ion, Cr₂O₇²⁻, consists of two tetrahedral CrO₄ units sharing one corner oxygen atom. The Cr-O-Cr bond angle at the bridging oxygen is 126°, as described in NCERT. In Cr₂O₇²⁻, chromium has an oxidation state of +6. This can be calculated by taking oxygen as -2: 2x + 7(-2) = -2, so 2x = +12 and x = +6. Dichromates are strong oxidising agents, not weak reducing agents, especially in acidic medium and in organic chemistry. Thus, statements 1, 2 and 3 are correct.

Potassium dichromate is prepared from chromite ore, FeCr₂O₄. The ore is fused with sodium carbonate in the presence of air. During this process, chromium is oxidised to sodium chromate, Na₂CrO₄. The roasted mass is then extracted with water and filtered, giving a yellow solution of sodium chromate. On acidification, sodium chromate is converted into sodium dichromate, and finally potassium dichromate can be obtained by adding potassium chloride. Therefore, the compound present in the yellow solution is Na₂CrO₄.

Potassium dichromate is an important oxidising agent used in chemical industries. NCERT mentions that dichromates are used in the preparation of many organic compounds and dyes. Azo compounds are a class of highly coloured organic compounds widely used as dyes because of their characteristic azo group, -N=N-. Potassium dichromate is valued in such chemical applications because of its oxidising nature. While aldehydes, ketones and carboxylic acids are important organic compounds, the specific highly coloured compounds used widely as dyes are azo compounds. Therefore, the correct answer is azo compounds.

In acidic solution, dichromate ion acts as a strong oxidising agent. The half-reaction is Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. In Cr₂O₇²⁻, chromium is in the +6 oxidation state. In Cr³⁺, chromium is in the +3 oxidation state. Thus, each chromium atom gains 3 electrons. Since one dichromate ion contains two chromium atoms, the total number of electrons gained is 6. This electron unit value is also used in calculating the equivalent weight of potassium dichromate in acidic redox reactions.

Chromate and dichromate ions are interconvertible depending on the pH of the solution. In acidic medium, orange dichromate ion, Cr₂O₇²⁻, is favoured. When a strong base like NaOH is added, the medium becomes basic and dichromate ions convert into yellow chromate ions, CrO₄²⁻. The oxidation state of chromium remains +6 in both species, so the colour change is due to pH-controlled equilibrium and not a redox reaction. Therefore, the yellow colour after adding excess base is due to the formation of chromate ion, CrO₄²⁻.

Disproportionation is a redox reaction in which the same species is simultaneously oxidised and reduced. In manganate ion, MnO₄²⁻, manganese is in the +6 oxidation state. In neutral or acidic solution, manganate undergoes disproportionation to form permanganate, MnO₄⁻, where manganese is +7, and manganese dioxide, MnO₂, where manganese is +4. Thus, one part of manganese is oxidised from +6 to +7, while another part is reduced from +6 to +4. Since oxidation and reduction occur in the same species, the reaction is disproportionation.

The manganate ion, MnO₄²⁻, contains manganese in the +6 oxidation state. Mn⁶⁺ has d¹ configuration, so it has one unpaired electron and is paramagnetic. Permanganate ion, MnO₄⁻, contains manganese in the +7 oxidation state. Mn⁷⁺ has d⁰ configuration, so it has no unpaired electrons and is diamagnetic. La³⁺ has f⁰ configuration and is diamagnetic. Mn²⁺ has d⁵ configuration and contains five unpaired electrons, making it strongly paramagnetic. Therefore, the correct matching is 1-d, 2-c, 3-a and 4-b.

To arrange electrode potentials in decreasing order, compare the numerical values of E°. The reduction of MnO₄⁻ to MnO₂ has E° = +1.69 V, which is the highest among the given values. The reduction of MnO₄⁻ to Mn²⁺ has E° = +1.52 V, which comes next. The reduction of MnO₄⁻ to MnO₄²⁻ has E° = +0.56 V, which is the lowest. Since all values are in volts, direct comparison is valid. Therefore, the decreasing order is +1.69 V > +1.52 V > +0.56 V, corresponding to 2, 3 and 1.

Potassium permanganate shows strong oxidising action in acidic, neutral and alkaline media. In acidic solution, iodide ions are oxidised to free iodine, I₂. In neutral or faintly alkaline solution, iodide may be oxidised further to iodate, IO₃⁻. In neutral solution, thiosulphate is oxidised to sulphate. These examples show the strong oxidising behaviour of KMnO₄. Statement 4 is incorrect because nitrite is not reduced back to nitrate. In fact, conversion of nitrite to nitrate is oxidation because nitrogen increases its oxidation state. Therefore, statements 1, 2 and 3 are correct.

The passage states that interstitial compounds are formed when small atoms such as hydrogen, carbon or nitrogen are trapped in the crystal lattices of transition metals. They are usually non-stoichiometric, meaning their formulas do not follow simple whole-number ratios. Examples like VH₀.₅₆ and TiH₁.₇ clearly show fractional composition. NCERT explains that such formulas do not correspond to any normal oxidation state of the metal. This is because the small atoms occupy interstitial spaces rather than forming ordinary ionic or covalent compounds with fixed valencies. Therefore, the correct implication is that these formulas do not correspond to normal integral oxidation states.

The passage clearly defines interstitial compounds as substances formed when small atoms like H, C or N are trapped inside the crystal lattices of metals. These small atoms occupy the interstitial spaces or holes present in the metal lattice. Because the atoms are fitted into these spaces without forming ordinary ionic or covalent structures, the compounds often become non-stoichiometric. NCERT uses examples such as TiC, Mn₄N, Fe₃H, VH₀.₅₆ and TiH₁.₇ to show this behaviour. Therefore, the term interstitial compound is based on the physical trapping of small atoms inside metal crystal lattices.

Transition metals readily form alloys because their metallic radii are similar, often within about 15 percent of one another. This allows atoms of one metal to be randomly distributed among atoms of another metal in the crystal lattice. Such atomic-level mixing produces homogeneous solid solutions rather than ordinary mechanical mixtures. These alloys often show useful properties such as hardness, strength and high melting points. NCERT mentions that transition metal alloys are important industrial materials, especially ferrous alloys containing elements such as chromium, manganese, vanadium and tungsten. Therefore, transition metal alloys are best described as homogeneous solid solutions.

A non-ferrous alloy is an alloy in which iron is not the chief constituent. Brass is an important non-ferrous alloy made mainly of copper and zinc. Both copper and zinc belong to the d-block region, and brass has wide industrial use due to its strength, corrosion resistance and workability. Stainless steel and tungsten steel are ferrous alloys because they are based mainly on iron. Mischmetall is a lanthanoid alloy and is discussed separately in the f-block context. Therefore, among the given options, brass is the important non-ferrous alloy containing transition metals.

Steel is essentially an alloy of iron with carbon, along with other elements added to obtain specific properties. Carbon is the key non-metal element in steel production because it strongly influences hardness, tensile strength and microstructure. Transition metals such as chromium, manganese and nickel are also added to produce different alloy steels with improved corrosion resistance, toughness and strength. However, without controlled carbon content, steel cannot be properly distinguished from very pure iron or cast iron categories. Hence, the essential non-metal element added in steel production is carbon.

Transition metals and their compounds are important industrial catalysts because they can show variable oxidation states and form temporary bonds with reactants. Iron catalyst is used in the Haber process for the synthesis of ammonia from N₂ and H₂. Palladium(II) chloride is used in the Wacker process, where ethene is oxidised to ethanal. Nickel catalyst is used in catalytic hydrogenation of fats. Vanadium pentoxide, V₂O₅, is used in the Contact process for oxidation of SO₂ to SO₃ during sulphuric acid manufacture. Therefore, the correct matching is 1-b, 2-d, 3-c and 4-a.