Discuss the following: Delta_1H degree becomes less negative along the series Li

Question

Discuss the following: Delta_1H degree becomes less negative along the series LiF, NaF, KF, RbF, and CsF, but more negative along the series Lil, Nal, KI, Rbl, and CsI. Raising the temperature of a sample of a-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. Although a non-metal, graphite is often used as an electrode material. The seminconducting properties of silicon are improved by adding minute amounts of boron. The Cr and Ni content of stainless steels used to make knife blades is different from that used in the manufacturing of spoons.

Explanation / Answer

5. a. Fluoride salts of LiF to CsF are smaller in size when compared to the iodide salts of the same metals. The lattice energy which is the energy evolved when opposite ions come together to form bond, decreases down the group and is more prominent for the fluoride ions owing to its smaller size. Thus, the deltaHf is less negative for the fluorides in the series. In case of iodides which are larger in size, the ionization potential plays a major role, which decreases down the group for the iodide salts, which gives higher deltaHf values to the salts formed.

b. When a sample of alpha-Fe is heated from 298 K to 1200 K (at 1 bat pressure) results in the change in the coordination number of Fe from 8 to 12. This is due to the fact that at low temperature Fe remains as alpha-Fe which is body centered structure with corrdination number of 8. When heated from 298 K to 1200 K, Fe changes from alpha-Fe to gamma-Fe phase with a face centered cubis closed packed structure with a coordination number of 12.

c. Graphite is an inert metal. It does not react with acids or bases and is a very good conductor of electricity. It is also very inexpensive material. It is resistant to thermal shock and does not melt but directly goes from solid to gaseous state allows for less wear. It is much lighter (low density). All these properties makes it a very good material for electrode making.

d. Adding a trivalent metal as impurity such as boron to silicon is a method called p-type doping, which introduces a hole is the silicon crystal or is not electron deficient. the material thus formed is referred to as the p-type semiconductor. This allows for a better e- movement and is a better semiconductor.

e. The stainless steel containing high content of Cr and Ni non-magnetic with low carbon content cannot be hardened by heat treament are used for forks and spoon manufacturing. On the other hand, the low Cr and Ni content stainless steel are magnetic and are hardened by heat are used for knife blade manufacturing.

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