03.03 Physical Properties of Covalent Compounds
Physical Properties of Covalent Compounds
a. Low Melting and Boiling Points
- Reason:
- Weak Intermolecular Forces: Covalent compounds are composed of molecules held together by weak intermolecular forces such as London dispersion forces, dipole-dipole interactions, and hydrogen bonds.
- Less Energy Required: These weak forces require significantly less energy to overcome compared to the strong ionic bonds found in ionic compounds, resulting in lower melting and boiling points.
- Examples:
- Water (H₂O):
- Melting Point: 0°C
- Boiling Point: 100°C
- Reason: Hydrogen bonding between water molecules.
- Carbon Dioxide (CO₂):
- Sublimation Point: -78°C (directly from solid to gas)
- Reason: Non-polar molecules with London dispersion forces.
- Methane (CH₄):
- Boiling Point: -161°C
- Reason: Non-polar molecule with weak London dispersion forces.
- Water (H₂O):
- State at Room Temperature:
- Typically Liquids or Gases
- Examples:
- Water (H₂O): Liquid
- Carbon Dioxide (CO₂): Gas
- Ethanol (C₂H₅OH): Liquid
- Examples:
- Typically Liquids or Gases
b. Poor Electrical Conductivity
- Reason:
- No Free Charge Carriers: Covalent compounds lack free electrons or ions that can move and carry electrical current.
- Insulating Nature: Without these charge carriers, covalent substances generally do not conduct electricity.
- Examples:
- Sugar (C₁₂H₂₂O₁₁):
- Conductivity: Does not conduct electricity in solid or dissolved state.
- Pure Water (H₂O):
- Conductivity: Poor conductor; however, impurities like salts can increase conductivity.
- Methanol (CH₃OH):
- Conductivity: Poor electrical conductor.
- Sugar (C₁₂H₂₂O₁₁):
- Exception:
- Electrolytes in Solution: Some covalent compounds can conduct electricity when dissolved if they ionize (e.g., acids like HCl).
c. Molecular Structure
- Intramolecular Forces:
- Strong Covalent Bonds: Atoms within a molecule are held together by strong covalent bonds, involving the sharing of electrons.
- Discrete Molecules: These strong bonds result in the formation of distinct molecules rather than an extended network.
- Examples:
- Methane (CH₄): Each carbon atom forms four covalent bonds with hydrogen atoms.
- Glucose (C₆H₁₂O₆): Contains multiple covalent bonds forming a complex molecule.
- Intermolecular Forces:
- Weak Forces Between Molecules: These include:
- London Dispersion Forces: Present in all molecules; stronger in larger, more polarizable molecules.
- Dipole-Dipole Interactions: Occur in polar molecules where positive and negative ends attract.
- Hydrogen Bonds: A special, stronger type of dipole-dipole interaction when hydrogen is bonded to highly electronegative atoms (N, O, F).
- Examples:
- Hydrogen Bonding in Water (H₂O): Leads to higher boiling points compared to similar-sized molecules without hydrogen bonding.
- London Dispersion in Iodine (I₂): Causes iodine to be a solid at room temperature due to stronger dispersion forces.
- Dipole-Dipole in Hydrogen Chloride (HCl): Results in a higher boiling point than non-polar molecules of similar size.
- Weak Forces Between Molecules: These include: