Question 1

What is a bond dipole moment and how is it calculated?

Accepted Answer

A bond dipole moment (µ) arises from charge separation in polar covalent bonds where electrons shift toward the more electronegative atom. It is calculated using the formula µ = Qr, where Q is the magnitude of partial charges determined by electronegativity difference, and r is the distance between charges. The dipole moment is represented as a vector pointing from the less to the more electronegative atom, with arrow length proportional to the electronegativity difference.

Question 2

Why is carbon dioxide nonpolar while carbonyl sulfide is polar?

Accepted Answer

Both molecules are linear, but their polarity differs due to bond dipole cancellation. In CO2, the two polar C=O bonds point in opposite directions, so their dipole moments cancel completely, making the molecule nonpolar. In carbonyl sulfide (OCS), the C-O and C-S bonds have different electronegativities; oxygen is more electronegative than sulfur, so the dipole moments do not cancel, resulting in a net dipole moment and a polar molecule.

Question 3

How does molecular geometry determine whether a molecule is polar or nonpolar?

Accepted Answer

Molecular geometry determines polarity by controlling whether individual bond dipole moments cancel. When bonds are arranged symmetrically, their vectors sum to zero, producing a nonpolar molecule. When geometry is asymmetrical, bond dipoles do not cancel, yielding a polar molecule. For example, water's bent shape prevents its two O-H bond dipoles from canceling, making it polar, whereas predicting molecular geometry using VSEPR theory helps identify these structural arrangements.

Question 4

What role do lone pairs play in determining molecular polarity?

Accepted Answer

Lone pairs affect molecular geometry, which in turn determines polarity. In water, lone pairs on oxygen create a bent shape rather than linear, preventing bond dipole cancellation and making the molecule polar. In contrast, boron trifluoride has no lone pairs and adopts a trigonal planar geometry where bond dipoles cancel perfectly, making it nonpolar. Lone pairs thus indirectly influence whether a molecule exhibits a net dipole moment.

Question 5

How do polar and nonpolar molecules behave differently in an electric field?

Accepted Answer

Polar molecules align their positive end toward the negative plate and negative end toward the positive plate when placed in an electric field, responding to the external charge. Nonpolar molecules remain unaffected by electric fields because they lack a net dipole moment and cannot orient themselves in response to the field. This difference reflects the presence or absence of charge separation within the molecule.

Question 6

Why does water dissolve sugar but oil does not?

Accepted Answer

Water is a polar solvent that dissolves polar solutes like sucrose (table sugar) because similar types of molecules have more favorable interactions. Oil is nonpolar and remains immiscible in water, following the principle that nonpolar solvents dissolve nonpolar solutes. The polarity of both solvent and solute determines solubility through electrostatic compatibility.

Question 7

What two conditions must a molecule meet to be polar?

Accepted Answer

A molecule must satisfy two conditions to be polar: it must contain at least one polar covalent bond, and its molecular structure must be such that the vector sum of each bond dipole moment does not equal zero. If bond dipoles cancel due to symmetry, the molecule is nonpolar regardless of individual bond polarity. Both structural and electronic factors are essential for net polarity.

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Moment dipolaire d'une molécule

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