201. Why are many transition-metal ions paramagnetic?
ⓐ. Their nuclei rotate rapidly in a magnetic field.
ⓑ. They always contain completely filled \(d\)-subshells.
ⓒ. They contain only paired electrons.
ⓓ. They often have unpaired \(d\)-electrons.
Correct Answer: They often have unpaired \(d\)-electrons.
Explanation: Paramagnetism arises from the presence of unpaired electrons. Many transition-metal ions have partially filled \(d\)-subshells, and these contain unpaired electrons. Because of that, such ions are attracted by an external magnetic field. This is one of the characteristic physical properties of transition-metal species.
202. Which pair of ions is diamagnetic in the usual free-ion picture?
ⓐ. \(Sc^{3+}\) and \(Cu^+\)
ⓑ. \(Ti^{3+}\) and \(Zn^{2+}\)
ⓒ. \(Cr^{3+}\) and \(Cu^+\)
ⓓ. \(Mn^{2+}\) and \(Sc^{3+}\)
Correct Answer: \(Sc^{3+}\) and \(Cu^+\)
Explanation: \(Sc^{3+}\) is \(d^0\), so it has no \(d\)-electrons, while \(Cu^+\) is \(d^{10}\), so all its \(d\)-electrons are paired. Both cases therefore lack unpaired electrons and are diamagnetic. The other pairs each include at least one ion with a partially filled \(d\)-subshell. Hence only \(Sc^{3+}\) and \(Cu^+\) form a diamagnetic pair.
203. The spin-only magnetic moment of \(Cr^{3+}\) is approximately
ⓐ. \(2.83\ \text{B.M.}\)
ⓑ. \(3.00\ \text{B.M.}\)
ⓒ. \(3.87\ \text{B.M.}\)
ⓓ. \(4.90\ \text{B.M.}\)
Correct Answer: \(3.87\ \text{B.M.}\)
Explanation: \(\textbf{Given:}\)
Ion \(= Cr^{3+}\)
Electronic configuration of \(Cr^{3+} = [Ar] 3d^3\)
Number of unpaired electrons, \(n = 3\)
\(\textbf{Required:}\)
Spin-only magnetic moment
\(\textbf{Relevant Formula:}\)
\[\mu = \sqrt{n(n+2)}\ \text{B.M.}\]
\(\textbf{Why this formula applies:}\)
For a transition-metal ion, the spin-only moment is calculated from the number of unpaired electrons.
\(\textbf{Identify known values:}\)
\(n = 3\)
\(\textbf{Substitution:}\)
\[\mu = \sqrt{3(3+2)}\]
\(\textbf{Intermediate Simplification:}\)
\[\mu = \sqrt{15}\]
\(\textbf{Final Simplification:}\)
\[\mu \approx 3.87\ \text{B.M.}\]
The unit is \(\text{B.M.}\)
\(\textbf{Final Answer:}\)
\[3.87\ \text{B.M.}\]
204. Which statement correctly describes magnetic behaviour of transition-metal ions?
ⓐ. Every coloured transition-metal ion must be diamagnetic.
ⓑ. Every colourless transition-metal ion must be strongly paramagnetic.
ⓒ. It depends on the number of unpaired electrons.
ⓓ. Colour in transition-metal ions depends only on atomic mass.
Correct Answer: It depends on the number of unpaired electrons.
Explanation: Paramagnetism and diamagnetism are determined by whether unpaired electrons are present. The greater the number of unpaired electrons, the stronger the paramagnetic behaviour in general. Colour is also related to electronic arrangement, but it is not controlled by atomic mass and does not simply determine magnetic behaviour by itself. So the number of unpaired electrons is the key factor for magnetism.
205. Which statement best explains why many transition elements and their compounds act as catalysts?
ⓐ. They show variable oxidation states and form intermediates.
ⓑ. They are always colourless in all oxidation states.
ⓒ. They never adsorb reacting molecules on their surface.
ⓓ. They remain completely unchanged and do not interact with reactants at all.
Correct Answer: They show variable oxidation states and form intermediates.
Explanation: Catalytic action in transition elements is often linked to their ability to adopt more than one oxidation state. This allows them to participate in reaction pathways without being consumed in the net process. Many of them can also form intermediate compounds or species with reactants. These features help lower the activation barrier and increase the reaction rate.
206. In heterogeneous catalysis by a transition metal, a major role is played by
ⓐ. complete disappearance of the catalyst during reaction
ⓑ. adsorption of reactant molecules on the catalyst surface
ⓒ. conversion of the catalyst into a permanent product
ⓓ. inability of the metal to interact with reactants
Correct Answer: adsorption of reactant molecules on the catalyst surface
Explanation: In heterogeneous catalysis, the catalyst and reactants are present in different phases, often with the catalyst as a solid. Reactant molecules are first adsorbed onto the catalyst surface, where bonds can be weakened or properly oriented for reaction. This surface interaction makes the reaction pathway easier. That is why adsorption is one of the most important steps in such catalysis.
207. Which substance is commonly used as a catalyst in the Contact process?
ⓐ. \(Fe_2O_3\)
ⓑ. \(NiO\)
ⓒ. \(V_2O_5\)
ⓓ. \(PtCl_2\)
Correct Answer: \(V_2O_5\)
Explanation: The Contact process involves oxidation of \(SO_2\) to \(SO_3\), and \(V_2O_5\) is the standard catalyst used for this purpose. It is effective because transition-metal oxides can participate in electron-transfer processes and help speed up oxidation reactions. This is a classic example of catalytic behaviour of a transition-metal compound. The catalyst provides an alternative reaction pathway and improves the rate.
208. Which metal is commonly used as a catalyst for the hydrogenation of vegetable oils?
ⓐ. \(Fe\)
ⓑ. \(Cu\)
ⓒ. \(Ag\)
ⓓ. \(Ni\)
Correct Answer: \(Ni\)
Explanation: Finely divided nickel is widely used to catalyse the hydrogenation of vegetable oils. In this process, hydrogen and the unsaturated oil molecules are adsorbed on the metal surface. That makes addition of hydrogen easier and speeds up conversion to a more saturated product. This is a standard example of heterogeneous catalysis by a transition metal.
209. In the Haber process, the catalyst commonly used is
ⓐ. \(Fe\)
ⓑ. \(Ni\)
ⓒ. \(MnO_2\)
ⓓ. \(V_2O_5\)
Correct Answer: \(Fe\)
Explanation: The Haber process for ammonia manufacture commonly uses finely divided iron as the catalyst. Nitrogen and hydrogen are adsorbed on the iron surface, which helps their reaction proceed more readily. Because nitrogen has a very strong triple bond, a catalyst is needed to make the reaction sufficiently fast. Iron provides this catalytic assistance without being used up overall.
210. Why are transition metals often effective as catalysts in redox reactions?
ⓐ. They can only remain in one fixed oxidation state.
ⓑ. They can easily change oxidation state during the reaction cycle.
ⓒ. They never form any temporary species with reactants.
ⓓ. They always act only as reducing agents and never as oxidising agents.
Correct Answer: They can easily change oxidation state during the reaction cycle.
Explanation: Many catalytic reactions involve transfer of electrons from one substance to another. Transition metals are well suited for such processes because they can move between different oxidation states fairly easily. This allows them to accept electrons in one step and donate them in another. Such reversible behaviour helps them act as catalysts in many redox systems.
211. Which statement best distinguishes a homogeneous catalyst from a heterogeneous catalyst?
ⓐ. A homogeneous catalyst always has a solid surface, while a heterogeneous catalyst must be a gas.
ⓑ. A homogeneous catalyst is consumed, while a heterogeneous catalyst is not.
ⓒ. Homogeneous means same phase; heterogeneous means different phase.
ⓓ. A homogeneous catalyst always increases the enthalpy change of the reaction.
Correct Answer: Homogeneous means same phase; heterogeneous means different phase.
Explanation: The distinction between homogeneous and heterogeneous catalysis is based on physical phase. In homogeneous catalysis, catalyst and reactants are present in the same phase, such as all in solution. In heterogeneous catalysis, they are in different phases, often with a solid catalyst and gaseous or liquid reactants. This classification is about the reaction environment, not whether the catalyst is consumed.
212. Which statement about catalytic behaviour of transition elements is false?
ⓐ. Their catalytic action may involve adsorption of reactants.
ⓑ. Their catalytic action may involve formation of intermediate species.
ⓒ. Their catalytic activity is often related to variable oxidation states.
ⓓ. Their catalytic activity depends only on atomic mass.
Correct Answer: Their catalytic activity depends only on atomic mass.
Explanation: Catalytic behaviour of transition elements is mainly linked to chemical and electronic factors, not to atomic mass alone. Important reasons include variable oxidation states, adsorption ability, and the tendency to form intermediate compounds or complexes. These properties help provide lower-energy pathways for reactions. Atomic mass by itself does not explain catalytic efficiency.
213. Which oxide is commonly used as a catalyst for the decomposition of \(KClO_3\)?
ⓐ. \(MnO_2\)
ⓑ. \(ZnO\)
ⓒ. \(TiO_2\)
ⓓ. \(CuO\)
Correct Answer: \(MnO_2\)
Explanation: \(MnO_2\) is a familiar catalyst used in the decomposition of \(KClO_3\) to produce oxygen. The catalyst speeds up the reaction without being consumed in the net process. This is a familiar example of catalytic behaviour of a transition-metal compound. It shows that not only metals but also their oxides can act as catalysts.
214. Which statement best explains the role of adsorption in catalytic activity?
ⓐ. Adsorption permanently converts the catalyst into the product.
ⓑ. Adsorption brings reactants close and weakens bonds.
ⓒ. Adsorption prevents reactants from reacting on the surface.
ⓓ. Adsorption works only for non-transition metals.
Correct Answer: Adsorption brings reactants close and weakens bonds.
Explanation: When reactant molecules are adsorbed on the catalyst surface, they are held in positions that favour interaction. In many cases, this adsorption weakens existing bonds in the reactants and helps new bonds form more easily. That lowers the activation energy for the process. This is one of the main reasons finely divided transition metals can be highly effective catalysts.
215. Assertion: Transition-metal compounds can act as catalysts even when the metal is not in elemental form.
Reason: Transition-metal oxides may participate in reactions through changes in oxidation state.
ⓐ. Both Assertion and Reason are false.
ⓑ. Assertion is true, but Reason is false.
ⓒ. Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
ⓓ. Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
Correct Answer: Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
Explanation: Many important catalysts are compounds of transition metals rather than the pure metals themselves. Oxides such as \(V_2O_5\) and \(MnO_2\) are standard examples. Their effectiveness often comes from the ability of the metal within the compound to participate in electron-transfer steps through variable oxidation states. Therefore the reason directly supports the assertion.
216. Which statement best summarizes the catalytic behaviour of transition elements and their compounds?
ⓐ. They act as catalysts only because they are coloured.
ⓑ. They act as catalysts only when they have a \(d^{10}\) configuration.
ⓒ. Their catalytic action is unrelated to surface effects or oxidation states.
ⓓ. It often involves adsorption, intermediates, and variable oxidation states.
Correct Answer: It often involves adsorption, intermediates, and variable oxidation states.
Explanation: No single reason explains all catalytic behaviour of transition elements. In some cases, surface adsorption is the main factor, especially in heterogeneous catalysis. In others, the ability to change oxidation state or form intermediate species is especially important. Together, these features explain why transition metals and their compounds are so widely used as catalysts.
217. Why do many transition-metal ions readily form complex compounds?
ⓐ. They are small, often highly charged, and can accept electron pairs from ligands.
ⓑ. They always have completely filled \(d\)-subshells and cannot interact with other species.
ⓒ. They form complexes only because they are coloured.
ⓓ. They never use vacant orbitals during bonding.
Correct Answer: They are small, often highly charged, and can accept electron pairs from ligands.
Explanation: Many transition-metal ions have high charge density because their ionic size is small and their positive charge can be significant. Such ions can attract electron-pair donors strongly. They also often possess orbitals that can participate in coordinate bond formation. These features together make complex formation a common property of transition elements.
218. Which property least favours complex formation by a metal ion?
ⓐ. High positive charge
ⓑ. Small ionic size
ⓒ. Very low charge density
ⓓ. Availability of suitable orbitals for bond formation
Correct Answer: Very low charge density
Explanation: Complex formation is favoured when the metal ion can strongly attract electron-pair donors. Small size and high positive charge increase charge density and strengthen this attraction. Availability of suitable orbitals also supports coordinate bond formation. A very low charge density would weaken the tendency to bind ligands strongly.
219. Which ion is most likely to form stable complex compounds?
ⓐ. \(Na^+\)
ⓑ. \(Ca^{2+}\)
ⓒ. \(K^+\)
ⓓ. \(Fe^{3+}\)
Correct Answer: \(Fe^{3+}\)
Explanation: \(Fe^{3+}\) has a high positive charge and relatively small ionic size, so its charge density is high. That makes it strongly attractive toward ligands that donate lone pairs. Transition-metal ions such as \(Fe^{3+}\) therefore form complexes much more readily than simple alkali-metal ions like \(Na^+\) or \(K^+\). This is one reason complex chemistry is so characteristic of transition elements.
220. Which statement best explains the role of ligands in complex formation?
ⓐ. Ligands always remove all \(d\)-electrons from the metal ion.
ⓑ. Ligands donate one or more lone pairs to the metal ion.
ⓒ. Ligands are always negatively charged ions and never neutral molecules.
ⓓ. Ligands prevent the metal ion from accepting electron pairs.
Correct Answer: Ligands donate one or more lone pairs to the metal ion.
Explanation: A ligand is a species that donates an electron pair to a metal ion to form a coordinate bond. The ligand may be a neutral molecule such as \(NH_3\) or a negative ion such as \(CN^-\) or \(Cl^-\). The metal ion acts as the electron-pair acceptor. This donor-acceptor interaction is the basis of complex formation.
