Which of the following has the highest vapor pressure?

SiH4

HF

PH3

H2Se

GeH4

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  • 8 months ago

    The substance in the list with highest vapor pressure is the one with the weakest intermolecular attractions.  It's the one with only London dispersion forces because if it had hydrogen bonding it would have the strongest intermolecular attraction and the lowest vapor pressure.  Likewise, if it were highly polar it would also have strong-ish intermolecular attraction: dipole-dipole attraction (called Keesom forces).

    SiH4 is nonpolar ... tetrahedral ... London forces only

    HF is polar and exhibits hydrogen bonding predominate

    PH3 is polar ... trigonal pyramidal ... Keesom forces predominate

    H2Se is not very polar at all ... London forces predominate

    GeH4 is nonpolar ... tetrahedral ... London forces only

    Our three candidates are SiH4, GeH4 and H2Se.  A quick search for the dipole moment of H2Se turns up 0.24D... not very high but still not zero as for SiH4 and GeH4.  So we can eliminate H2Se.

    The strength of London dispersion forces depends on the total number of electrons and the area over which they are spread.

    SiH4 ... 18 electrons

    GeH4 ... 36 electrons

    Since GeH4 has more electrons and a larger area, it will have stronger London forces and its vapor pressure will be slightly less than for SiH4.

    Therefore, SiH4 will have the highest vapor pressure of the compounds in your list.

    As a double check we can look at the boiling points.  The compound with the lowest boiling point will have the highest vapor pressure.

    H2Se ....... BP = -42C

    GeH4 ....... BP = -88C

    SiH4 ........ BP = -112C

    And this is consistent with our earlier analysis.

  • Anonymous
    8 months ago

    it's your job to find that out

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