Intermolecular forces, electronegativity and bond properties — common mistakes

When asked to explain boiling point differences, always state the specific types of intermolecular forces present in each substance being compared. A complete answer will mention all forces present (e.g., 'water has hydrogen bonds and van der Waals forces'), compare their relative strengths, and link this to the energy required to overcome them.

A hydrogen bond is an intermolecular force, an attraction between molecules. It's about 5-10% of the strength of a typical covalent bond. A covalent bond is an intramolecular force, involving the sharing of electrons within a molecule, and is much stronger.

Yes, absolutely. Van der Waals forces exist between all molecules because all molecules have electrons that are in constant motion. Polar molecules have both van der Waals forces and permanent dipole-dipole forces (and sometimes hydrogen bonds). The total intermolecular attraction is the sum of all these forces.

First, check for polar bonds by looking for atoms with different electronegativities. Second, consider the molecule's shape (from VSEPR theory). If the molecule is symmetrical and the polar bonds are arranged in a way that their dipoles cancel out (e.g., linear $CO_2$, tetrahedral $CCl_4$), the molecule is non-polar. If the shape is asymmetrical (e.g., bent $H_2O$, trigonal pyramidal $NH_3$), the dipoles won't cancel, and the molecule will be polar.