Vidaara.orgClass 12 · Chemistry
CodeVID-C12-08-T2-01
Assignment — Properties of Transition Metals
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
Colour in transition-metal ions is due to:
- A.s-s transitions
- B.d-d transitions
- C.nuclear changes
- D.isotopes
2.
The spin-only formula for magnetic moment is:
- A.$\mu=n(n+2)$
- B.$\mu=\sqrt{n(n+2)}$
- C.$\mu=\sqrt{n+2}$
- D.$\mu=n^2$
3.
A catalyst used in the Haber process is:
- A.$\text{V}_2\text{O}_5$
- B.Fe
- C.Ni
- D.Pt
4.
Brass is an alloy of:
- A.Cu and Sn
- B.Cu and Zn
- C.Fe and C
- D.Cu and Ni
5.
The oxidation state of Cr in $\text{K}_2\text{Cr}_2\text{O}_7$ is:
- A.+3
- B.+6
- C.+7
- D.+2
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Calculate the spin-only magnetic moment of $\text{Cr}^{3+}$ ($3d^3$).
7.
Why does scandium(III) appear colourless?
8.
Write the half-reaction for dichromate acting as an oxidant in acid.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Explain why transition metals and their compounds are good catalysts.
10.
Write the equations for the reaction of acidified $\text{KMnO}_4$ with (i) $\text{Fe}^{2+}$ and (ii) $\text{I}^-$.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Describe the preparation of potassium dichromate from chromite ore and explain its behaviour with change in pH and as an oxidising agent in acidic medium.
Answer Key
Section A — Multiple Choice Questions
- (B) d-d transitions
- (B) $\mu=\sqrt{n(n+2)}$
- (B) Fe
- (B) Cu and Zn
- (B) +6
Section B — Short Answer (2 marks)
- $n=3$, so $\mu=\sqrt{3(3+2)}=\sqrt{15}=3.87$ BM.
- $\text{Sc}^{3+}$ is $3d^0$; with no d electrons, no d-d transition is possible.
- $\text{Cr}_2\text{O}_7^{2-}+14\text{H}^++6e^-\rightarrow2\text{Cr}^{3+}+7\text{H}_2\text{O}$.
Section C — Short Answer (3 marks)
- They show variable oxidation states (forming intermediates) and provide surfaces that adsorb reactants, both lowering the activation energy; e.g. Fe (Haber) and $\text{V}_2\text{O}_5$ (Contact).
- (i) $\text{MnO}_4^-+5\text{Fe}^{2+}+8\text{H}^+\rightarrow\text{Mn}^{2+}+5\text{Fe}^{3+}+4\text{H}_2\text{O}$; (ii) $2\text{MnO}_4^-+10\text{I}^-+16\text{H}^+\rightarrow2\text{Mn}^{2+}+5\text{I}_2+8\text{H}_2\text{O}$.
Section D — Long Answer (5 marks)
- Chromite ($\text{FeCr}_2\text{O}_4$) is fused with $\text{Na}_2\text{CO}_3$ in air to give $\text{Na}_2\text{CrO}_4$, acidified to $\text{Na}_2\text{Cr}_2\text{O}_7$, then treated with KCl to crystallise $\text{K}_2\text{Cr}_2\text{O}_7$. With pH: $\text{Cr}_2\text{O}_7^{2-}$ (orange) $+2\text{OH}^-\rightleftharpoons2\text{CrO}_4^{2-}$ (yellow) $+\text{H}_2\text{O}$. As an oxidant in acid: $\text{Cr}_2\text{O}_7^{2-}+14\text{H}^++6e^-\rightarrow2\text{Cr}^{3+}+7\text{H}_2\text{O}$, e.g. it oxidises $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$ and $\text{I}^-$ to $\text{I}_2$.
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