1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Energies of interaction

  1. Feb 24, 2015 #1
    1. The problem statement, all variables and given/known data
    Neglecting hydrogen-bonding, calculate the interaction energy between
    (i) H3O+ and H2O and
    (ii) H3O+ and H3O+,
    if each pair is separated by 0.3 nm and assuming that the aqueous solvent can be treated as a medium with constant relative permittivity. Using your result, comment on the likelihood of ideal behaviour by H3O+ at high molality

    2. Relevant equations
    Ion-dipole interaction
    r is the distance of separation.
    q is the charge of the ion ( only the magnitude of the charge is shown here.)
    μ is the permanent dipole moment of the polar molecule.

    Ion-ion interaction
    U(r)= (q1q2)/(4*π*E0)*(1/r)

    3. The attempt at a solution
    r is the distance of separation so 0.3nm
    q is the charge of the ion so 1 here
    μ is the permanent dipole moment of the polar molecule 6.2 x 10-30 C m
    So if I can treat it as having constant relative permittivity then I just need the permittivity of a vacuum.
    (-1 x 6.2 x 10-30)/(4π8.85 x 10-12x(0.3x10-9)2
    And I got 0.619 from this which doesn't sound right?

    U(r)= (q1q2)/(4*π*E0)*(1/r)
    (1x1)/(4π8.85 x 10-12x(0.3x10-9) x (1/0.3x10-9)
    =2.70 x 1018 which sounds ridiculous
  2. jcsd
  3. Feb 25, 2015 #2


    User Avatar

    Staff: Mentor

    You definitely need to put units in there.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted