3.3 Polar Bonds and Polar Molecules

3.3 Polar Bonds and Polar Molecules

Introduction to Polar Bonds

• In Chapter 2, the focus was on chemical bonds between ions or atoms in compounds.

• This section reviews polar bonds and their influence on substance properties.

Bond Polarity

• Bond Polarity Definition: A measure of how unequal the electron sharing is between two atoms in a covalent bond.

• Polar covalent bonds occur between atoms with significantly different electronegativities, leading to unequal sharing of bonding electron pairs.

• Shared electrons in a polar covalent bond cluster closer to the more electronegative atom, resulting in:

  • Negative Pole: The end with greater electron density (more electronegative atom).

  • Positive Pole: The end with less electron density (less electronegative atom).

Summary of Chemical Bonds (Table 1)

• Ionic Bonds: Formed by transfer of valence electrons between ions leading to large crystal lattices.

• Covalent Bonds: Formed by sharing pairs of valence electrons, resulting in individual molecules.

Importance of Understanding Previous Material

• Revisiting previous sections on defining important terminologies like "polar covalent bond," "non-polar covalent bond," and "electronegativity difference (DEN)" is crucial.

• Reference Figure 5 in Section 2.3 for a visual representation of bond types, such as hydrogen chloride (HCl) with a DEN of 1.0 indicating a polar covalent bond.

Extent of Bond Polarity

• The degree of polarity in covalent bonds relies directly on the electronegativity difference:

  • H-H: Non-polar (� 0.0)

  • N-H: Polar (� 0.9)

  • O-H: Very polar (� 1.4)

Phenomenon of Polar Liquids

• Scientists have observed interesting behaviors in various liquids when interacting with charged objects.

• Mini-Possibility: Observe how different liquids behave nearby charged objects using a thin stream.

Mini Investigation (Skills Handbook A6.2)

• Skills Focus: Performing, Observing, Analyzing, Communicating

• Materials Needed:

  • Safety goggles, lab apron, gloves

  • 50 mL glass burette, funnel, beaker

  • Charged acetate and vinyl strips or balloon

  • Liquids: water, ethanol, propanone (acetone), tetrachloroethene

Steps for Investigation:

1. Create a table for recording observations with an additional column.

2. Prepare and set up the burette securely with the assigned liquid.

3. Charge the acetate strip to acquire a positive charge.

4. Observe the liquid stream behavior when the charged strip is applied and note reactions.

5. Repeat the experiment with a negatively charged vinyl strip.

6. Examine results from peer groups with different liquids.

7. Dispose of liquids properly post-experiment.

Observational Behaviour (Table 2)

Analyzing Polar Molecules

• Investigate the correlation between molecular shape, presence of polar bonds, and interactions with charges.

• Polar molecules possess positively and negatively charged ends, while non-polar molecules distribute charges evenly.

• Examples of polar molecules: hydrogen chloride (HCl) and water (H2O).

  • HCl shows a polar bond with a partial charge difference.

  • Water has a unique bent structure contributing to its polar nature.

Molecular Properties and Predictions

• Knowledge of molecular polarity helps predict boiling/melting points.

• Water's exceptional properties can be explained through its molecular structure and polarity.

Explaining the Behaviour of Polar Liquids

• Polar liquids like water display unique behaviors around charged objects:

  • When close to a positively charged object, water's negative poles orient toward it, causing a deflection of the stream.

  • Conversely, a negatively charged object attracts the positive poles of water.

Connecting Polar Molecule Examples

• Hydrogen Chloride (HCl): Contains polar covalent bond classified as polar due to unequal electron sharing.

• Water (H2O): Contains two polar bonds, bent shape; creates a polar molecule overall.

• Non-polar Molecules Examples: Oxygen (O2) and Carbon Dioxide (CO2) - symmetrical arrangements lead to non-polar characterization despite containing polar covalent bonds.

Determining Molecular Polarity

1. Evaluate number and types of atoms in the molecule.

2. Draw the Lewis structure.

3. Identify the covalent bonds and determine the electronegativity differences.

4. If there are polar bonds, denote partial charges on atoms.

5. Assess symmetry/analyze shape for overall polarity.

Rules of Determining Molecular Polarity (Table 3)

Sample Problems for Practice

• Example of non-polar: Fluorine molecules.

• Example of polar: Ammonia molecule.

Summary of Key Concepts (3.3 Summary)

• Bond polarity derived from electronegativity differences.

• Molecular polarity is influenced by bond polarity and overall shape.

• Diatomic molecules polar if covalent bond is polar.

• Polyatomic molecules: Non-polar if all bonds non-polar; polar if asymmetrical.

• Use molecular diagrams for better visualization of symmetry/asymmetry.