Case Study Questions for Class 12 Chemistry Chapter 1 The Solid State

There is Case Study Questions in class 12 Chemistry in session 2020-21. The first two questions in the board exam question paper will be based on Case Study and Assertion & Reason. The first question will have 5 MCQs out of which students will have to attempt any 4 questions. The second question will carry 5 Assertion & Reason type questions with the choice to attempt any four. Here are the questions based on case study.

Case Study Question 1:

Read the passage given below and answer the following questions:

The adjective, ‘crystalline’ when applied to solids, implies an ideal crystal in which the structural units, termed as unit cells, are repeated regularly and indefinitely in three dimensions in space. The unit cell, containing at least one molecule has definite orientation and shape defined by the translational vectors, a, b and c. The unit cell therefore has a definite volume, V that contains the atoms and molecules necessary for generating the crystal. Every crystal can be classified as a member of one of the seven possible crystal systems or crystal classes that are defined by the relationship between the individual dimensions, a, b and c of the unit cell and between the individual angles, α, β, and γ of the unit cell. The structure of the given crystal may be assigned to one of the 7 crystal systems, to one of the 14 Brevais lattices, and to one of the 230 space groups. This uniquely define the possible ways of rearranging atoms in a three-dimensional solid, Based on these observations, seven crystal systems were identified: triclinic, monoclinic, trigonal or rhombohedral, tetragonal, hexagonal, rhombic or orthorhombic and cubic.

The following questions are multiple choice questions. Choose the most appropriate answer:

(1) The crystal system of a compound with unit cell dimensions, a=0.387 nm, b=0.387 nm and c=0.504 nm and α = β = 900 and γ=1200 is
(a) cubic
(b) hexagonal
(c) orthorhombic
(d) rhombohedral

(2) The unit cell with the structure given below represents ________crystal system.

(a) cubic
(b) orthorhombic
(c) tetragonal
(d) trigonal

(3) In a triclinic crystal
(a) a = b = c, α = β = γ ≠ 900 
(b) a ≠ b = c, α = β = γ = 900 
(c) a ≠ b ≠ c, α ≠ β ≠ γ ≠ 900 
(d) a ≠ b ≠ c, α = γ = 900 , β ≠ 900 

(4) The unit cell with dimensions α = β = γ = 900 , a = b ≠ c is
(a) cubic
(b) triclinic
(c) hexagonal
(d) tetragonal

OR

An example of orthorhombic crystal system is
(a) SnO2
(b) KNO3
(c) ZnO
(d) K2Cr2O7

Answer Answers: (1) b
(2) a
(3) c
(4) d
OR
b

Case Study Question 2:

Read the passage given below and answer the following questions:

In ideally ionic structures, the coordination numbers of the ions are determined by electrostatic considerations. Cations surround themselves with as many anions as possible and vice versa. This maximizes the attractions between neighbouring ions of opposite charge and hence maximizes the lattice energy of the crystal. This requirement led to the formulation of the radius ratio rule for ionic structures in which the ions and the structure adopted for a particular compound depend on the relative sizes of the ions. Thus, for the stable ionic crystalline structures, there is definite radius ratio limit for a cation to fit perfectly in the lattice of
anions, called radius ratio rule. This depends upon the ratio of radii of two types of ions, r+/r.

This ratio for coordination numbers 3, 4, 6 and 8 are respectively 0.155 – 0.225, 0.225 -0.414, 0.414 -0.732 and 0.732 – 1.000. The coordination number of ionic solids also depends upon temperature and pressure. On applying high pressure, coordination number increases. On the other hand, on applying high temperature, it decreases.

The following questions are multiple choice questions. Choose the most appropriate answer:

(1) The ionic radii of K+, Rb+ and Br are 137, 148 and 195 pm. The coordination number of cation in RbBr and KBr structures are respectively
(a) 8,6
(b) 6,4
(c) 6,8
(d) 4,6

OR

For a coordination number 4, the maximum limiting radius ratio is
(a) 0.414
(b) 0.732
(c) 0.225
(d) 0.155

(2) If the radius of Nation is 95 pm and that of Cl ion is 181 pm, the coordination number of Na+ ion is
(a) 6
(b) 4
(c) 8
(d) 12

(3) Which is not the correct statement for ionic solids in which positive and negative ions are held by strong electrostatic attractive forces?
(a) The radius ratio r+/r increases as coordination number increases.
(b) As the difference in size of ions increases, coordination number increases.
(c) When coordination number is eight, r+/r ratio lies between 0.225 to 0.414.
(d) In ionic solid of the type AX (ZnS, wurtzite), the coordination number of Zn2+ and S2- respectively
are 4 and 4.

(4) If the pressure of CsCl is increased, then its coordination number will
(a) increase
(b) remain the same
(c) decrease
(d) none of these

Answer Answers: (1) a
Or
a (2) a
(3) c
(4) a

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