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    Practice Exam - Boron Halides; Radius Ratio Rule; Monomer vs. Dimer; Viscosity; Zeolites

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    3. Explain why:
    a. Lewis acid strengths of boron halides are in the order BBr3 >> BCl3 > BF3 opposite to the order expected based on electronegativities.
    b. Radius ratio rule is correct only 66% of the times in predicting coordination numbers in ionic compounds.
    c. Me3B is a monomer, whereas Me3Al exits as a dimer.
    d. Viscosity of liquid sulfur increases sharply at ~ 159oC
    e. Zeolites are used as shape selective reforming catalysts.

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    Solution Preview

    Boron reacts with all halogen elements to give monomeric, highly reactive trihalides, which readily form complexes with amines, phosphines, ethers, and halide ions. The electron configuration of boron is 1s22s22p. It has only three electrons to work with, so the ion is unpolarizable, and does not hydrate. For this reason, boron is not eager to donate electrons in an electrovalent bond, and can also not accept them easily. Steric factors affect both Lewis acids and bases in similar ways. A trivalent boron compound is triangular planar with an sp2 hybridized boron atom. When a Lewis acid-base adduct is formed, the boron atom is rehybridized to sp3 and the three groups on boron are forced closer together. Large, bulky groups on boron will not favor being forced closer together and this will affect the strength of the interaction.This is sometimes referred to as F-strain. Thus when Boron halides are formed, due to bulkier mass of Br and Cl when compared to F, BBr3 can dissociate more easily that BCl3 which can dissociate more easily than BF3.

    Radius Ratio Rules are used as a scheme for understanding the packing of atoms of unequal sizes.Large ...