5. Discuss the following: a. Delta ,H becomes less negative along the series LiF

Question

5. Discuss the following: a. Delta ,H becomes less negative along the series LiF, NaF, KF, RbF, and CsF, but more negative along the series Lil, Nal, Kl, RbI, and Csl. b. Raising the temperature of a sample of alpha-Fe from 298 K to 1200 K (at one bar pressure) results in a change of coordination number of each Fe atom from 8 to 12. c. Although a non-metal, graphite s often used as an electrode material. d. The semiconducting properties of silicon are improved by adding minute amounts of boron. e. The Cr and NI content of stainless steels used to make knife blades Is different from that used In the manufacturing of spoons. 6. Why is graphite a good conductor where a diamond is not? (Both contain infinity lattices of covalently bound carbon atoms.)

Explanation / Answer

5 a. The high stability of fluoride ion than iodide ion is the reason for less negative value for enthalpy of formation of alkali metal fluorides.

b. At 298 K Fe is in alpha phase (BCC), where CN is 8. At 1200 K, Fe is in gamma phase (FCC), where CN is 12.

c. Due to semiconducting properties of graphite, it can act as electrode.

d. Silicon is an intrinsic semiconductor, so has poor conductivity. So, boron is added to produce holes on silicon and improve its conductivity.

e. The corrosion resistance of knife blades steel is somewhat less than that of spoon steel. This is done to obtain good cutting properties. So, Cr/Ni content is different.

6. Diamond has sp3 hybridization, tetrahedral structure and strong covalent bonds. Due to hardness of structure, electrons cannot move freely and thus it acts as insulator. Graphite has sp2 hybridization, so planar structure and thus electrons can move easily between the planes. So, graphite conducts electricity.

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