Question

Which of the following has the largest dipole moment? A. HCN B. H2O C. CCl4 D. SO2

Short answer

H2O has the largest dipole moment.

Step-by-step solution

Understand Dipole Moment

A dipole moment arises due to the difference in electronegativity between bonded atoms, creating a polar bond. The dipole moment is a measure of the separation of positive and negative charges in a molecule.

Analyze Each Molecule

Look at each molecule's structure and determine the orientation and magnitude of its dipole moments.

Evaluate HCN

HCN (Hydrogen Cyanide) has a linear structure with a significant difference in electronegativity between H, C, and N. This results in a moderate dipole moment along the molecule.

Evaluate H2O

H2O (Water) has a bent structure (104.5°) and a large difference in electronegativity between O and H, creating strong dipole moments that do not cancel out. This results in a large dipole moment.

Evaluate CCl4

CCl4 (Carbon Tetrachloride) has a tetrahedral structure with symmetrical polar C-Cl bonds. The dipoles cancel each other due to symmetry, resulting in a net dipole moment of zero.

Evaluate SO2

SO2 (Sulfur Dioxide) has a bent structure (~120°) and strong dipole moments due to the difference in electronegativity between S and O. The dipoles do not cancel out, resulting in a significant dipole moment.

Compare All Dipole Moments

Compare the dipole moments calculated or deduced from the structures. HCN has a moderate dipole moment, H2O has a large dipole moment, CCl4 has zero dipole moment, and SO2 has a significant dipole moment.

Determine the Largest Dipole Moment

Based on the comparisons, the molecule with the largest dipole moment is H2O due to its bent structure and strong O-H dipole moments.

Key concepts

Molecular Geometry

Understanding molecular geometry is key. The shape of a molecule influences its physical and chemical properties.
For example, H2O (Water) has a bent geometry with an angle of 104.5°. This shape means the molecule is not symmetrical, leading to a net dipole moment.
HCN (Hydrogen Cyanide) is linear, influencing its dipole direction and magnitude.
CCl4 (Carbon Tetrachloride) has a tetrahedral shape, but due to its symmetry, the dipole moments cancel out.
SO2 (Sulfur Dioxide) is bent (~120°), similar to H2O, which affects its overall dipole moment.
Geometries like linear, tetrahedral, and bent directly impact how dipole moments add up or cancel out.

Electronegativity

Electronegativity measures an atom's ability to attract electrons in a bond. The larger the difference in electronegativity between atoms, the more polar the bond.
Oxygen is more electronegative than hydrogen, causing strong dipole moments in H2O molecules.
In HCN, nitrogen is more electronegative than carbon and hydrogen, thus creating a measurable dipole moment.
Carbon and chlorine in CCl4 have significant electronegativity differences, but the symmetry of the molecule cancels out the dipoles.
Sulfur and oxygen have different electronegativities in SO2, leading to a significant dipole moment.
The varying electronegativities are key in determining the magnitude and direction of dipole moments.

Polar Bonds

Polar bonds are formed when atoms with different electronegativities bond together, creating positive and negative poles.
H2O has polar O-H bonds because oxygen is much more electronegative than hydrogen.
HCN exhibits a polar bond primarily between C and N, and H and C.
CCl4 has polar C-Cl bonds, but because of its symmetrical arrangement, the polarities cancel out.
In SO2, the S=O bonds are polar, not canceling out due to the molecule's bent shape.
Recognizing polar bonds helps explain the existence and magnitude of dipole moments in molecules.

Symmetry in Molecules

Symmetry in a molecule can determine whether dipole moments cancel out or add up.
CCl4 is symmetrical (tetrahedral), thus its dipole moments cancel out, resulting in a non-polar molecule.
H2O and SO2 have less symmetry (both are bent), causing their dipole moments to add up, leading to a net dipole moment.
HCN, being linear, shows a clear direction for the dipole moment between hydrogen and nitrogen.
Symmetry or lack thereof is crucial. It helps decide if a molecule will have a net dipole moment or if the individual dipole moments will cancel out.