Chemistry Notes

Chemistry 12th Important questions and answers 2019

1. Describe how electrical neutrality is maintained in compounds showing Frenkel and Schottky defect?
Ans: In compound showing Frenkel defect, ions just get displaced within the lattice. While incompounds showing Schottky defect, equal number of anions andcations are removed from the lattice. Thus, electrical neutrality ismaintained in both cases.

2. Calculate the number of atoms in a cubic unit cell having one atom on each corner and two atoms on each body diagonal?
Ans: 8 corner atoms × 1/8 atom per unit cell = 1 atom There are four body diagonals in a cubic unit cell and each has two body centre atoms. So 4×2=8 atoms therefore total number of atoms per unit cell = 1+8=9 atoms.

3. The electrical conductivity of a metal decreases with rise in temperature while that of a semi-conductor increases.Explain?
Ans: In metals with increase of temperature, the kernels start vibrating and thus offer resistance to the flow of electrons.Hence conductivity decreases. In case of semiconductors, with increase of temperature, more electrons can shift from valence band to conduction band. Hence conductivity increases.

4. What type of substances would make better permanent magnets, ferromagnetic or ferromagnetic,Why?
Ans: Ferromagnetic substances make better permanent magnets. This is because the metal ions of a ferromagnetic substance are grouped into small regions called domains. Each domain acts as tiny magnet and get oriented in the direction of magnetic field in which it is placed. This persists even in the absence of magnetic field.

5. In a crystalline solid, the atoms A and B are arranged as follows:
a. atoms A are arranged in ccp array
b. atoms B occupy all the octahedral voids and half of the tetrahedral voids. What is the
formula of the compound?
Ans: Let no. of atoms of A be N, No. of octahedral voids = N, No. of tetrahedral voids = 2N,
i)There will be one atom of B in the octahedral void,
ii)There will be one atom of B in tetrahedral void (1/2 X 2N), Therefore, total 2 atoms of B for each atom of A Therefore formula of the compound = AB2.

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6. What type of substances would make better permanent magnets ferromagnetic or ferromagnetic ? Why ?
Ans: Ferromagnetic substances make better permanent magnets. This is because the metal ions of a ferromagnetic substance are grouped into small regions called domains. Each domain acts as tiny magnet and get oriented in the direction of magnetic eld in which it is placed. This persists even in the absence of magnetic field.

7. The electrical conductivity of a metal decreases with rise in temperaturewhile that of a semi-conductor increases. Explain?
Ans: In metals with increase of temperature, the kernels start vibrating at faster rate and thus offer resistance to the ow of electrons. Hence, conductivity decreases. In case of semi-conductors, with increase of temperature, more electrons can shift from valence band to conduction band. Hence conductivity increases.

8. Explain how electrical neutrality is maintained in compounds showingFrenkel and Schottky defect?
Ans: In compound showing Frenkel defect, ions just get displaced within the lattice, while in compounds showing Schottky defect, equal number of anions and cations are removed from the lattice. Thus, electrical neutrality is maintained in both cases.

9. Calculate the number of atoms in a cubic unit cell having one atom on each corner and two atoms on each body diagonal?
Ans: 8 corner × 1/8 atom per unit cell = 1 atom ,there are four body diagonals in a cubic unit cell and each has two body centre atoms.
So, 4 × 2 = 8 atoms therefore, total number of atoms per unit cell = 1 + 8 = 9.

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