Polarity and Properties
Learn
The type of bonding in a substance determines its physical properties. Understanding polarity helps predict melting points, solubility, and conductivity.
Determining Molecular Polarity
- Identify if bonds are polar (different electronegativities)
- Determine molecular geometry
- Check if polar bonds cancel out (symmetric) or add up (asymmetric)
Properties of Ionic vs. Covalent Compounds
| Property | Ionic Compounds | Covalent Compounds |
|---|---|---|
| Melting Point | High (usually > 300C) | Low (usually < 300C) |
| State at Room Temp | Solid | Solid, liquid, or gas |
| Conductivity (solid) | No | No |
| Conductivity (liquid/solution) | Yes | No (usually) |
| Solubility in water | Often soluble | Polar: soluble; Nonpolar: insoluble |
Like Dissolves Like
Polar substances dissolve in polar solvents (like water). Nonpolar substances dissolve in nonpolar solvents (like oil).
Practice Problems
Problem 1: Is H2O a polar or nonpolar molecule? Explain.
Answer: Polar
Water has polar O-H bonds (O is more electronegative). The bent geometry means the polar bonds don't cancel, creating a net dipole.
Problem 2: Why does NaCl have a high melting point (801C)?
Answer: NaCl is ionic. Strong electrostatic attractions between Na+ and Cl- ions in the crystal lattice require a lot of energy to break.
Problem 3: Will hexane (C6H14) dissolve in water? Explain.
Answer: No
Hexane is nonpolar (C-H bonds are essentially nonpolar). Water is polar. Like dissolves like, so nonpolar hexane won't dissolve in polar water.
Problem 4: Why does solid NaCl not conduct electricity, but molten NaCl does?
Answer: In solid NaCl, ions are locked in fixed positions in the crystal lattice and cannot move. When melted, ions are free to move and carry charge.
Problem 5: Is CCl4 polar or nonpolar?
Answer: Nonpolar
Although C-Cl bonds are polar, CCl4 has tetrahedral geometry with symmetric bond arrangement. The polar bonds cancel out.
Problem 6: Predict whether methanol (CH3OH) is soluble in water.
Answer: Yes, soluble
Methanol has an O-H group that can form hydrogen bonds with water. It's polar like water.
Problem 7: Why does CO2 have a lower boiling point (-78C) than H2O (100C)?
Answer: CO2 is nonpolar (linear, symmetric) with only weak London dispersion forces between molecules. H2O is polar with strong hydrogen bonds, requiring more energy to boil.
Problem 8: Will oil and water mix? Why or why not?
Answer: No
Oil is nonpolar; water is polar. Like dissolves like, so they don't mix. Oil floats on top because it's less dense.
Problem 9: Is NH3 polar or nonpolar?
Answer: Polar
NH3 has polar N-H bonds and a trigonal pyramidal shape (asymmetric due to lone pair). The dipoles don't cancel.
Problem 10: Which would you expect to have a higher melting point: CaCl2 or molecular chlorine (Cl2)? Explain.
Answer: CaCl2 has a much higher melting point.
CaCl2 is ionic with strong attractions between Ca2+ and Cl- ions. Cl2 is a nonpolar covalent molecule with only weak London dispersion forces.
Check Your Understanding
- What two factors determine if a molecule is polar?
- Why do ionic compounds conduct electricity when dissolved but not when solid?
- What does "like dissolves like" mean?
Next Steps
- Review intermolecular forces if needed
- Move on to Common Mistakes to avoid errors on tests