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Homework Help: Dipole Moment, what is Q? and other questions

  1. Jul 14, 2012 #1
    Hello, I have some questions about Dipole Moment, taken from a General Chemistry textbook, in which they treat Dipole Moment as a scalar. Here is a worked example from this textbook:

    1. The problem statement, all variables and given/known data
    The dipole moment of HCl is 1.11 D, and the distance between atoms is 127 pm. What is the percent ionic character of the Hcl bond?

    2. Relevant equations

    3. The given solution
    μ=(1.60*10^-19 C)*(127*10^-12 m)*(1D/3.336*10^-30 C*m) = 6.09D

    The observed dipole moment of HCl (1.11D) implies that the H-Cl bond is only about 18% ionic:
    (1.11 D)/(6.09 D) = 18.2%


    Now, here are my questions, along with my train of thought. Please correct me if my understanding is incorrect.

    1. We used Q = 1.60*10^-19 C, which is the charge of an electron, and we used it to calculate μ in the scenario when a proton and an electron is at a distance of r apart. But I'm thinking, since the proton has a charge of +1.60*10^-19 C also, shouldn't we use for the value of Q the product of the charges of a proton and an electron, (1.60*10^-19 C)^2 ? I don't understand why we used the value Q = 1.60*10^-19 C in this example problem.

    2. I understand the use of μ=Q*r equation in this problem is treating as if we have an ionic compound, where an electron is completely stripped from one atom and transferred to another item. Thus, the actual Dipole Moment of the covalent-bonded molecule in this problem is much lower, because the electron is shared and only partially spends more time around the Cl atom.

    Now, suppose a problem asks to find the Dipole Moment in an ionic compound, Calcium Chloride. Between a Ca and a Cl atom, there's a difference of 1 electron lost and 1 electron gained. But wait, the Ca item actually loses 2 electrons, so its charge is even less. Now, taking into account this difference in number of electrons between Ca and Cl, to calculate the μ between Ca and just one Cl atom, what would we use for the Q value?

    3. Similarly, suppose a problem asks to find the Dipole Moment in a covalent molecule, Carbon Dioxide. (The molecule has net Dipole Moment of 0, because it's linear and the 2 vectors cancel each other out. But let's look at the Dipole Moment between C and just one O molecule anyway.) The C and O atoms are double-bonded, sharing 2 pairs of electrons. So sometimes, the O atom can have not one more, but up to 2 more electrons spending time around it than around the C atom. Again, taking this into account, what would we use for the Q value?
  2. jcsd
  3. Jul 14, 2012 #2
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