higher dipole moment sapling #10

[Agustina Santa Cruz 2F]

Sapling #10

Which molecule should have higher dipole moment, and why?

a. COF2 because it contains more F atoms.

b. COFH because the polar bonds in COF 2 nearly cancel each other out.

c. COF2 because it contains more lone pairs.

I'm confused as to why the answer is B. You'd think that COF2 has a higher dipole moment because there are more F's pulling on the O. How do the polar bonds cancel each other out in COF2 if they're competing with a single O atom on the other side. Not only is it 2 versus 1 molecule pulling, but also, F has a higher electronegativity.

[Austin Aldujaili 2D]

I believe the question is trying to get you to look at the molecular shapes of each molecule where in answer b, COFH has higher polarity (meaning higher dipole moment) because there is no F on the other side of the molecule to cancel out the electronegativity of the other F. In the other two molecules, the Fs would be positioned on opposite sides of the molecule, meaning they would pull on the central atom equally. In COFH, the F is able to pull more than the H, creating a stronger dipole moment.

[Katie Nye 2F]

Dipole moments are based on electron density and with COF2, the two F's are balanced but there are far more electrons in an F atom than an H atom so the electron density is less balanced than if there are two of the same atoms on either side.

[Serena Song 1A]

As said above, the two fluorines in COF2 cancel each other out somewhat. Oxygen is very electronegative as well, so overall the polarity in COF2 isn't as strong (relatively speaking). In comparison, the hydrogen in COHF is much less electronegative than the oxygen or fluorine, leading to the dipoles not canceling each other out as much. As a result, COHF has a stronger dipole moment than COF2.

[Jaden Haskins 2E]

Also for this question, how do you determine which atom is the "negative pole"? I don't get how the negative pole can be between two atoms. I thought it would be towards O since O has two lone pairs, meaning more negative charge.

[Gillian Gabrielsen2F]

The answer is COFH because in COF2, the dipole moment of O almost cancels out the one of the fluorine's. First of all, we know each of these molecules are trigonal planar, with bond angles ~120 degrees. It's easiest to think of dipole moments as vectors. In a bond between two atoms, the vector will point towards the more electronegative atom, with the magnitude of the vector proportional to the electronegativity of the atom. The dipole moment of each fluorine is equal in magnitude, so you think of them interacting with each other and forming a single net pull. So a single vector is formed. Since the vector of Oxygen points in the opposite direction and has lower Electronegativity than fluorine, you subtract it from the vector of Fluorine. Since Oxygen has high electronegativity, this leads to a lower dipole moment.

However when you switch a fluorine with a Hydrogen to form COFH, since H has a slightly lower electronegativity than carbon so the vector points to carbon. Because of this the dipole moment forms between O and F. This results in a higher dipole moment since unlike in COF2, the vectors are generally pointing in the same direction.

(in this image O should have 2 lone pairs, not 3)
Hope this helped!