Determining Molecular Polarity and Net Dipole Moments
Picture raindrops clinging to a windowpane or forming puddles on the ground. Have you ever wondered why water holds together so well, while carbon dioxide, present in the air, remains invisible and dispersed?
The secret lies in molecular polarity.
Molecular Geometry: The Deciding Factor for Molecular Polarity
How Geometry Influences Polarity
Even if a molecule contains polar bonds, its overall polarity depends on its molecular geometry—the 3D arrangement of atoms.
Molecular geometry
Molecular geometry is the three-dimensional arrangement of atoms in a molecule, determined by the repulsion between electron pairs around the central atom.
Geometry determines whether the individual bond dipoles cancel out (non-polar molecule) or add up to form a net dipole moment (polar molecule).
- Symmetrical Geometry:
- In symmetrical molecules (e.g., CO₂, BF₃), the bond dipoles cancel each other out, leading to a non-polar molecule.
Example
In CO₂, the two polar C=O bonds are arranged linearly at 180°, so their dipoles point in opposite directions and cancel.
- Asymmetrical Geometry:
- In asymmetrical molecules (e.g., H₂O, NH₃), the bond dipoles do not cancel, resulting in a polar molecule.
Example
In H₂O, the bent shape means the O–H bond dipoles add up, creating a net dipole moment.
Using VSEPR Theory to Predict Geometry
The Valence Shell Electron Pair Repulsion (VSEPR) model predicts molecular shapes by considering the repulsion between electron domains (bonding and lone pairs). Key geometries include:
- Linear (e.g., CO₂): Bond angle = 180°.
- Trigonal planar (e.g., BF₃): Bond angle = 120°.
- Tetrahedral (e.g., CH₄): Bond angle = 109.5°.
- Bent (e.g., H₂O): Bond angle < 109.5° due to lone pairs.
Tip
Lone pairs exert stronger repulsion than bonding pairs, reducing bond angles and often resulting in asymmetrical shapes.
Net Dipole Moment: The Measure of Molecular Polarity
Dipole moment
A dipole moment describes the separation of electrical charge in a bond or molecule due to differences in electronegativity.
A dipole moment (μ) quantifies the separation of charge in a molecule. It is expressed in debye units (D), where 1 D =
Note
You do not have to know how dipole moment is quantified un debye units. The main focus should be on the geometrical interpretation.
Determining the Net Dipole Moment
To evaluate a molecule's overall polarity:
- Assess Bond Dipoles: Determine the direction and magnitude of dipoles for each bond.
- Consider Geometry: Add the bond dipole vectors. If they cancel out, the molecule is non-polar; if they add up, the molecule is polar.
Example
H₂O vs. CO₂
- H₂O: The O–H dipoles point toward oxygen and do not cancel due to the bent geometry. Result: Net dipole moment ≈ 1.85 D (polar).
- CO₂: The C=O dipoles cancel due to the linear geometry. Result: Net dipole moment = 0 D (non-polar).
Common Mistake
It’s a common mistake to assume that all molecules with polar bonds are polar. Always consider the molecular geometry to determine if dipoles cancel.
Applications of Molecular Polarity
1.Solubility: "Like Dissolves Like"
- Polar molecules (e.g., H₂O) dissolve polar substances (e.g., NaCl, ethanol).
- Non-polar molecules (e.g., hexane) dissolve non-polar substances (e.g., oils, fats).
2.Boiling and Melting Points:
- Polar molecules have higher boiling and melting points due to stronger intermolecular forces (e.g., hydrogen bonding in H₂O).
3.Biological Relevance:
- Polarity governs molecular interactions in biological systems. For instance, water’s polarity makes it an excellent solvent for biochemical reactions.
Reflection
Self review
- Why is H₂O polar while CO₂ is non-polar?
- How does molecular geometry influence dipole cancellation?
Theory of Knowledge
How does molecular polarity blur the line between "polar" and "non-polar"? Consider molecules like ethanol, which have both polar and non-polar regions.
