Question

Which molecules are polar? For each that is polar, specify the direction of its dipole moment. (a) \(\mathrm{CH}_{2} \mathrm{Cl}\), (b) \(\mathrm{HCN}\) (c) \(\mathrm{H}_{7} \mathrm{O}\),

Short answer

Question: Determine if the following molecules are polar and identify the direction of their dipole moments: (a) CH2Cl (b) HCN (c) H7O Answer: (a) CH2Cl is polar, and the dipole moment is directed from the carbon atom to the chlorine atom. (b) HCN is polar, and the overall dipole moment is directed from the hydrogen atom towards the nitrogen atom. (c) H7O is non-polar, as the polar bonds are symmetrically distributed and cancel each other out, with no overall dipole moment.

Step-by-step solution

Identify Polar Bonds

First, identify if there are any polar bonds in the molecules, which means the electrons are not shared equally between the two atoms. To do this, examine the difference in electronegativity of the atoms: (a) \(\mathrm{CH}_{2} \mathrm{Cl}\): When comparing the electronegativity values, C-H bonds have a small difference, while C-Cl bonds have a larger difference, making the C-Cl bond polar. (b) \(\mathrm{HCN}\): In this molecule, both the H-C and C-N bonds have a significant electronegative difference, making them polar. (c) \(\mathrm{H}_{7} \mathrm{O}\): The H-O bonds have a significant difference in electronegativity values, making the bonds polar.

Analyze Molecular Structure and Symmetry

Next, analyze the molecular structure and symmetry to determine if the polar bonds are evenly distributed across the molecule, effectively canceling each other out. (a) \(\mathrm{CH}_{2} \mathrm{Cl}\): The molecule has a planar structure with the chlorine atom on one side. The two C-H bonds are symmetrical; however, the C-Cl bond is polar and not symmetrical with respect to the other two bonds. (b) \(\mathrm{HCN}\): The molecule has a linear structure, and the H-C and C-N bonds are not symmetrically distributed, which makes the molecule polar. (c) \(\mathrm{H}_{7} \mathrm{O}\): This molecule has an octahedral structure, with O in the center, surrounded by 6 hydrogen atoms in every corner of an octahedron and one more hydrogen atom on the opposite side of the oxygen atom. In this arrangement, the polar H-O bonds are symmetrically distributed.

Determine Polar Molecules and Dipole Direction

By examining the presence of polar bonds and their distribution in the molecular structure, we can identify if the molecule is polar and determine the direction of the dipole moment. (a) \(\mathrm{CH}_{2} \mathrm{Cl}\): The molecule is polar due to the presence of a polar C-Cl bond and asymmetrical structure. The dipole direction is from the carbon atom to the chlorine atom. (b) \(\mathrm{HCN}\): The molecule is polar due to the presence of polar H-C and C-N bonds and linear structure. The overall dipole moment is directed from the hydrogen atom towards the nitrogen atom. (c) \(\mathrm{H}_{7} \mathrm{O}\): The molecule is non-polar, even though it has polar H-O bonds, because the bonds are symmetrically distributed and cancel each other out. There is no overall dipole moment in the molecule.