Lesson 2. Molecular Polarity SUMMARY.docx

**properties and structure of polar and non-polar molecules**: **1.The primary difference between polar and non-polar molecules.** **2. The molecular geometry affect polarity.** - Molecular geometry plays a crucial role in determining polarity. Even if a molecule has polar bonds, if the sha...

**properties and structure of polar and non-polar molecules**: **1.The primary difference between polar and non-polar molecules.** **2. The molecular geometry affect polarity.** - Molecular geometry plays a crucial role in determining polarity. Even if a molecule has polar bonds, if the shape is symmetrical, the dipoles may cancel out, making the molecule non-polar. For example: - **Symmetrical shapes** (e.g., linear, tetrahedral) can make a molecule non-polar if the bond dipoles cancel. - **Asymmetrical shapes** (e.g., bent, trigonal pyramidal) usually lead to a polar molecule because the dipoles do not cancel. **3. The properties of polar molecules.** - **Dipole Moment**: Polar molecules have a measurable dipole moment due to the unequal sharing of electrons. - **Solubility**: Polar molecules tend to dissolve well in polar solvents like water (\"like dissolves like\"). - **Boiling/Melting Points**: Polar molecules usually have higher boiling and melting points compared to non-polar molecules due to stronger intermolecular forces like dipole-dipole interactions and hydrogen bonding. - **Examples**: Water (H₂O), ammonia (NH₃), and hydrogen chloride (HCl). **4. The properties of non-polar molecules.** - Non-polar molecules have the following properties: - **No Dipole Moment**: Non-polar molecules have no permanent dipole moment since the electron distribution is even. - **Solubility**: Non-polar molecules dissolve well in non-polar solvents like oil or hexane but not in polar solvents like water. - **Boiling/Melting Points**: Non-polar molecules typically have lower boiling and melting points because they only experience weak London dispersion forces. - **Examples**: Methane (CH₄), carbon dioxide (CO₂), and diatomic molecules like nitrogen (N₂) and oxygen (O₂). **5. Polar molecules have higher boiling points than non-polar molecules.** - Polar molecules have stronger intermolecular forces, such as dipole-dipole interactions and hydrogen bonds, which require more energy (in the form of heat) to break. Non-polar molecules only experience weaker van der Waals (London dispersion) forces, which means they require less energy to transition from liquid to gas, resulting in lower boiling points. **6. Molecule have polar bonds but still be non-polar.** - Yes, a molecule can have polar bonds but still be non-polar if the molecular geometry is symmetrical. For instance, carbon dioxide (CO₂) has polar C=O bonds, but because the molecule is linear, the dipoles cancel out, making CO₂ non-polar overall. **7. Example of a polar molecule and a non-polar molecule with similar atoms.** - **Polar Molecule**: H₂O (water). Even though both hydrogen and oxygen are present in water, the bent shape causes an uneven distribution of charge, making the molecule polar. - **Non-polar Molecule**: O₂ (oxygen). The molecule consists of two oxygen atoms with equal electronegativities and a linear shape, resulting in an even distribution of charge, making it non-polar. **8. Polarity affect the solubility of a molecule in water.** - Polarity greatly influences solubility. Polar molecules dissolve well in water, a polar solvent, because of their ability to form hydrogen bonds or dipole-dipole interactions with water molecules. Non-polar molecules, however, do not dissolve well in water, as they cannot form these interactions and tend to separate from water, forming immiscible layers. **9. Why is water considered a polar molecule?** - Water is considered polar because of its bent structure and the significant difference in electronegativity between hydrogen and oxygen. The oxygen atom pulls the shared electrons closer to itself, giving it a partial negative charge, while the hydrogens have a partial positive charge. This creates a dipole moment, making the molecule polar. **10. How do intermolecular forces differ between polar and non-polar molecules?** - **Polar molecules** experience dipole-dipole interactions and, in some cases, hydrogen bonding (if H is bonded to N, O, or F), which are stronger forces. - **Non-polar molecules** only experience London dispersion forces (van der Waals forces), which are weaker than dipole-dipole interactions. This results in generally lower boiling and melting points for non-polar molecules compared to polar ones. **Here are examples of substances that exhibit each of the following intermolecular forces:** **1. Dipole-Dipole Interaction:** - **Substance Example**: **Hydrogen chloride (HCl)** - **Explanation**: Dipole-dipole interactions occur between molecules that have permanent dipoles (polar molecules). In HCl, chlorine is more electronegative than hydrogen, leading to a partial negative charge on Cl and a partial positive charge on H. The positive end of one HCl molecule is attracted to the negative end of another HCl molecule. **2. Hydrogen Bonding:** - **Substance Example**: **Water (H₂O)** - **Explanation**: Hydrogen bonding is a specific type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. In water, hydrogen bonds form between the hydrogen of one molecule and the oxygen of another due to the polarity of the O-H bond, giving water its unique properties such as high boiling point and surface tension. **3. Dispersion Forces (London Dispersion Forces):** - **Substance Example**: **Helium (He)** - **Explanation**: Dispersion forces are the weakest of all intermolecular forces and are present in all molecules, but they are the only forces present in non-polar molecules and noble gases. For example, helium atoms experience momentary dipoles due to fluctuations in electron distribution, creating weak attractions between them. **4. Ion-Dipole Interaction:** - **Substance Example**: **Sodium chloride (NaCl) in water (H₂O)** - **Explanation**: Ion-dipole interactions occur between an ion and a polar molecule. In an aqueous solution of NaCl, the Na⁺ ions are attracted to the partial negative oxygen atoms in water molecules, while Cl⁻ ions are attracted to the partial positive hydrogen atoms. This interaction allows ionic substances to dissolve in polar solvents like water.