Indian Forest Service (Mains), Chemistry optional · 2022 · Paper I
Q1(c). MgO impurity in alumina
Question
Mention point defects which can arise for MgO as an impurity in Al₂O₃. How many Mg²⁺ ions must be added to form each of these defects?
Answer in brief
Each substitutional Mg²⁺ on an Al³⁺ site has one negative effective charge; compensation needs one oxygen vacancy per two substitutional Mg or one Al interstitial per three.
Key points
- Write the substitutional defect as Mg′Al; the prime denotes −1 charge relative to the normal Al site.
- An oxygen vacancy VO•• has effective charge +2; the neutral combination 2Mg′Al + VO•• needs 2 Mg ions per oxygen vacancy.
- An Al interstitial Ali••• compensates 3Mg′Al: three substitutional Mg ions per Al interstitial.
- A Mg interstitial Mgi•• could compensate two substitutional Mg ions, requiring three added Mg ions in total for that neutral cluster.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coverspoint defectsKröger–Vink notationaliovalent impurityoxygen vacancyinterstitialMgO in Al2O3
All parts of Question 1
- 1(a) Continuity of the wavefunction8 marks
- 1(b) Dipole moments of ammonia and nitrogen trifluoride8 marks
- 1(c) MgO impurity in alumina8 marks
- 1(d) Neutral pH at body temperature8 marks
- 1(e) Activity coefficient of potassium ferricyanide8 marks