Plus Two Chemistry Notes Chapter-01 Part-02

10. Solid A is a very hard electrical insulator in solid as well as in molten state and melts at extremely high temperature. What type of solid is it?  

It is covalent or network solid  

12.Ionic solids conduct electricity in molten state but not in solid state. Explain.       

In the ionic solids, electrical conductivity is due to the movement of ions. Since the ionic mobility is negligible in solid state, these are non- conducting in this state.  Upon melting , the ions present acquire mobility so the ionic solids become conducting  

13.What type of solids are electrical conductors, malleable and ductile?  

Metallic solids    

14. Explain  unit cells  

• Unit cells can be broadly divided into two categories, primitive and centred unit cells. 

(a) Primitive Unit Cells When constituent particles are present only on the corner positions of a unit cell, it is called as primitive unit cell.              

(b) Centred Unit Cells When a unit     cell contains one or more constituent particles present at positions other than corners in addition to those at corners, it is called centred unit cell. Centred unit cells are of three types: 

(i) Body-Centred Unit Cells: Such a unit cell contains one constituent  particle (atom, molecule or ion) at  its body-centre besides the ones that are at its corners. 

(ii) Face-Centred Unit Cells: Such a unit cell contains one constituent particle present at the centre of each face, besides the ones that are at its corners. 

(iii) End-Centred Unit Cells: In such a unit cell, one constituent particle is present at the centre of any two opposite faces besides the ones present at its corners. 

15. Give the significance of a ‘lattice point’              

Lattice point denotes the position  of  particular constituent  in the crystal lattice. It may be atom, ion or molecule. The arrangement of lattice points in space is responsible for shape of particular crystalline solid.  

16. Name the parameters that characterise a unit cell. 

• The dimensions of unit cell along three sides a,b, c. 

• The angles between edges a and b  

17. Distinguish between 

(i) Hexagonal and monoclinic unit cells 

(ii) Face-centred and end-centred unit cells.  

( Answer) 

• hexagonal unit cell       A = b #c , α = β = 90°γ = 120° 

• Monoclinic unit cell       a ≠ b ≠ c , α = γ = 90°β ≠ 90 

• Face centred -Such a unit cell contains one constituent particle present at the centre of each face, besides the ones that are at its corners. 

• End centred - In such a unit cell, one constituent particle is present at the centre of any two opposite faces besides the ones present at its corners. 

18. Explain how much portion of an atom located at 

(i) corner and 

(ii) bodycentre of a cubic unit cell is part of its neighbouring unit cell.   

1. An atom at the corner is shared by eight adjacent unit cell. So the atom at the corner that belongs to one unit cell = 1/8. 

2. An atom at the body centre is not shared by any other unit cell. So it is belongs full unit cell .  

19. What is octahedral voids?             

One of them has the apex of the triangle point in upwards and the other downwards. These voids have been marked as‘O’ in  Such voids are surrounded by six spheres and are called octahedral voids. 

20. What is the two dimensional coordination number of a molecule in square close-packed layer?       

In the 2D square close packed layer a particular molecule is in contact with  four molecules. So coordination number is four.  

21. A compound forms hexagonal closepacked structure. What is the total number of voids in 0.5 mole of it? How many of these are tetrahedral voids? 

22. A compound is formed by two elements M and N. The element N forms ccp and atoms of M occupy 1/3rd of tetrahedral voids. What is the formula of the compound? 

23. Which of the following lattices has the highest packing efficiency  

(i) simple cubic

(ii) body-centred cubic 

(iii)hexagonal close-packed lattice? 

24. An element with molar mass 2.7×10-2 kg mol-1 forms a cubic unit cell with edge length 405 pm. If its density is 2.7×103  kg m-3, what is the nature of the cubic unit cell? 

25. What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way? 

When solid is heated, vacancy defect produced in the crystal. On heating some atoms or ions leave the lattice site completely. As a result density of substance decreases.  

26. What type of stoichiometric defect is shown by: 

(i) ZnS  

(ii) AgBr 

1. ZnS show Frenkel defect since  the cationic size is smaller compared to anionic size. 

2. AgBr - it shows both Frankel and schottky defects. 

27. Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.       

For example NaCl is doped with Srcl, impurity when srcl to to is added to NaCl solid as an impurity,  two Na+ ions will be replaced and one of the sites is occupied by  Sr21- while the other will remain vacant. So when cation of higher valence is added as an impurity to ionic solid. Two or more cations of lower valency are replaced by higher valency to maintain electrical neutrality. Show the cationic vacancies are created.  

28. Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.

For example NaCl crystal is heated in presence of a Na vapour, some Cl- ions leave their lattice site to combine with Na  form NaCl. The electrons absorb energy from visible light get excited to higher energy level and they fall back to ground state the import yellow colour to NaCl crystal.

 29. A group 14 element is to be converted into n-type semiconductor by doping it with a suitable impurity. To which group should this impurity belong?           

The Imperial tea from group 15 should be added to get n type of semiconductor. 

30. What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic. Justify your answer           

Ferromagnetic substances have better permanent magnets. This is because when placed in magnetic field the domains get oriented in the direction of magnetic field so strong  magnetic field is produced. So this ordering  of   persists even when external magnetic field is removed. Ferromagnetic substances become permanent magnet. 

Plus Two Chemistry Chapter -01 Notes Part-03 Click Here