Chemistry XI Chapter 4 Descriptive Questions

Crystallographic elements: A unit cell of a crystal can be completely described by six parameters. These six parameters are three edge lengths i.e a, b, c and three angles i.e α, β, γ. These three angles and three lengths are called crystallographic elements. Edge lengths a, b, and c are length along x, y and z axises, respectively. Angle ‘α’ is between length b and c, the angle ‘β’ is between the side ‘a’ and ‘c’ and angle ‘γ’ is between sides ‘a’ and ‘b’. 

Figure: Six crystallographic elements and cleavage point

Question 4.41: How cell unit is defined by unit cell dimension? 

Answer: A unit cell of a crystal can be completely described by six parameters. These six parameters are three edge lengths i.e a, b, c and three angles i.e α, β, γ. These three angles and three lengths are called crystallographic elements. Edge lengths a, b, and c are length along x, y and z axises, respectively. Angle ‘α’ is between length b and c, the angle ‘β’ is between the side ‘a’ and ‘c’ and angle ‘γ’ is between sides ‘a’ and ‘b’. 

Figure: Six crystallographic elements, lattic point and unit cellQuestion 4.42: What is lattice energy? Which factor affect lattice energy.

Answer: Amount of energy released when one mole of ionic crystal is formed from the gaseous ions. Or amount of energy required to break the one mole of ionic crystal into gaseous ions is called lattice energy. The factor which affects the lattice energy is size of either cation or anion. Lattice energy cation decreases with increase in size of cation keeping anion size same. It also decreases with increase in size of anion keeping cation size same. Reason is that with increase in size of either cation or anion, the packing of oppositely charged ions becomes loose. 

Question 4.43: Table given below shows the lattice energies of various ionic compounds. Ionic compounds 

  

Why is there a decrease in lattice energies of KCl and KBr as compared to NaCl and NaBr respectively? 

Answer: Lattice energy of cation decreases with increase in size of cation keeping anion size same. It also decreases with increase in size of anion keeping cation size same. Reason is that with increase in size of either cation or anion, the packing of oppositely charged ions becomes loose. In case of NaCl and KCl, although size of anions (Cl-) are same but size of Na is small as compared to size of K. Therefore, lattice energy of NaCl is greater than those of KCl. Similarly, In case of KBr and NaBr , although the size of (Br-) is same but size of Na+ is smaller than that of K+. Thus, lattice of energy of NaBr is greater than that of KBr. 

Question 4.44: Why are the ionic crystals hard, low volatile and show high melting points? 

 Answer: In ionic crystals, there are strong electrostatic force of attractions between cations and anions. So, they are tightly held with each other and very high energy is required to separate the cations and anions from each other. So ionic crystals are hard, low volatile and show high melting point. 

Question 4.45: How can you prove that there are 4NaCl units per unit cell? 

Answer: When we look at cubic crystal lattice of NaCl, we see that there eight Cl- at corners of the cube, and each is being shared amongst eight cubes. 1/8 part of each Cl- ion is considered for this unit cell. Hence, one complete Cl- is contributed by eight corners. Likewise, six Cl- ions are present at the face centers and each is being shared between two cells. 1/2 part of each Cl- ions is considered for this unit cell. Thus, 3 Cl- are being shared by face centers of the cube. Total number of Cl- ions are 4. Also, we can justify the presence of 4Na+ if we take a unit cell with 8Na+ at eight corners and 6Na+ at faces. 

Figure: Unit cell of NaCl showing only Chloride ions
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