31. In the preparation of potassium dichromate from chromite ore, the oxidation state of chromium changes from:
A) +2 to +6
B) +3 to +6
C) +6 to +3
D) +2 to +3
Answer: B
Explanation:
In the preparation of potassium dichromate (K2Cr2O7) from chromite ore (FeCr2O4), chromium changes its oxidation state from +3 in chromite to +6 in dichromate. This involves an oxidation process in which chromium is converted from its +3 oxidation state to the +6 state.
32. Lanthanide contraction is responsible for:
A) The similarity in the atomic radii of the second and third transition series
B) The higher melting point of lanthanides
C) The color of lanthanide ions
D) The paramagnetic behavior of lanthanides
Answer: A
Explanation:
Lanthanide contraction refers to the steady decrease in the ionic radii of the lanthanides as you move across the series. This is due to the poor shielding effect of the 4f electrons. As a result, the atomic and ionic sizes of the second and third transition series become very similar, which influences their chemical properties.
33. Which of the following is true about actinides compared to lanthanides?
A) Actinides have lower atomic numbers
B) Actinides have more variable oxidation states
C) Actinides are more stable than lanthanides
D) Actinides do not form complexes
Answer: B
Explanation:
Actinides have more variable oxidation states compared to lanthanides. While lanthanides predominantly exhibit a +3 oxidation state, actinides can show a wide range of oxidation states (+3, +4, +5, +6, and even +7), due to the involvement of 5f, 6d, and 7s orbitals.
34. The catalytic properties of transition metals are due to:
A) Their high melting points
B) Their ability to adopt multiple oxidation states
C) Their large atomic radii
D) Their poor conductivity
Answer: B
Explanation:
Transition metals are excellent catalysts because they can adopt multiple oxidation states, which allows them to facilitate oxidation-reduction reactions. Their partially filled d-orbitals can also form temporary bonds with reactants, providing a surface for the reaction to occur more efficiently.
35. Which of the following elements is used as a catalyst in the Haber process for ammonia synthesis?
A) Iron
B) Nickel
C) Zinc
D) Copper
Answer: A
Explanation:
Iron is used as a catalyst in the Haber process for the synthesis of ammonia. The iron catalyst helps to speed up the reaction between nitrogen and hydrogen to form ammonia (NH3NH3) by providing a surface for the reaction and lowering the activation energy.
36. The ionic radii of lanthanides:
A) Increase across the period
B) Decrease across the period
C) Remain constant
D) First increase and then decrease
Answer: B
Explanation:
The ionic radii of lanthanides decrease steadily across the period from lanthanum to lutetium. This decrease is known as the lanthanide contraction and is due to the poor shielding effect of the 4f electrons, which causes the outer electrons to be more strongly attracted to the nucleus.
37. The element with the atomic number 92 belongs to which group of the periodic table?
A) d-block
B) p-block
C) s-block
D) f-block
Answer: D
Explanation:
The element with atomic number 92 is uranium, which belongs to the f-block of the periodic table. Uranium is an actinide and its electrons are filling the 5f orbital.
38. Which of the following shows the least tendency to form complexes?
A) Cu2+
B) Fe3+
C) Zn2+
D) Co3+
Answer: C
Explanation:
Zn2+ shows the least tendency to form complexes because its 3d orbital is completely filled (3d¹⁰). It does not have available d-orbitals to accept electron pairs from ligands, making it less likely to form coordination compounds compared to other transition metals.
39. The metal in [Ni(CN)4]2− has the following hybridization:
A) sp2
B) sp3
C) dsp2
D) d2sp3
Answer: C
Explanation:
In [Ni(CN)4]2−, the nickel ion is in a square planar geometry, which corresponds to dsp2dsp2 hybridization. This hybridization occurs when one d-orbital, one s-orbital, and two p-orbitals are mixed to form four hybrid orbitals for bonding.
40. The most common oxidation state of the first series of transition elements is:
A) +1
B) +2
C) +3
D) +4
Answer: B
Explanation:
The most common oxidation state of the first series of transition elements is +2. This occurs because they tend to lose the two electrons from their outermost s-orbital, leading to a +2 oxidation state in many of their compounds.
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