5. Aluminium combines readily with both dry fluorine and dry chlorine. Anhydrous

Question

5. Aluminium combines readily with both dry fluorine and dry chlorine. Anhydrous aluminium chloride is a white solid which sublimes at about 200°C, it reacts with water and dissolves in non-polar solvents. Aluminium fluoride is a crystalline solid up to a temperature of 1290°C; it is insoluble in non-polar solvents. a. Explain how the electron-sea model accounts for the high electrical and thermal conductivity of aluminium b. Predict, using the information above only, the name of the bond type present in: c. Give an explanation for the difference in bond type present in the two anhydrous d. Predict whether anhydrous aluminium fluoride would have a low or a high melting anhydrous aluminium chloride, and anhydrous aluminium fluoride. aluminium halides based on your prediction for part b. temperature carefully explaining the reasons for your prediction.

Explanation / Answer

5. Aluminium chloride and aluminium fluride are formed from metalic aluminium and chlorine or fluorine.

a. The metallic aluminium is a having high thermal and electrical property. This can be explained on the basis of electron sea model, which tells us that in metals the valence electrons of atoms (aluminium here) are spread all over like a sea to form metalic bonding. The availability of these electrons all over help in metals to behave like a good conductor. Also the heat can be easily transferred from one end of metal to another, by using these electron clouds. Thus aluminium metal behaves as both a good conductor and have good thermal perperties.

b. In anhydrous AlCl3 (aluminium chloride) the aluminium atom shares its valence electrons with that of Cl valence electrons, therefore, the type of bonding we expect here is covalent. whereas, in aluminium fluoride with F being a highly electronegative element, it forms ionic bond with aluminium. This can also be explained based on fajan's rule, according to which as the size of halogen decreases from Cl to F, the net ionic character of anion (halogen ion) increases in the compound. Therefore, AlF3 is ionic whereas, AlCl3 is covalent.

c. As explained above, the size of anion decided the type and extent of bonding in aluminium halides. So as size decreases from Cl to F, the net ionic character increases.

d. Ionic compounds tend to have very high melting points. Thus, following the similar pattern, AlF3 would have a higher melting point as compared to covalent AlCl3. this directly related to the strength of bonds, that is ionic bonds are much stronger than covalent bonds and require more energy to break this bond.

c.

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