AP Chemistry - Medium Difficulty Concepts

Front

Define Formal Charge and the calculation formula.

Back

Formal Charge (FC) is the theoretical charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms. Formula: FC = (Valence e-) - (Non-bonding e- + 1/2 Bonding e-). It helps determine the most stable Lewis structure when the octet rule is satisfied for multiple structures.

Front

What are the Geometry and Bond Angles of a molecule with 4 bonding domains and 1 lone pair (See-saw shape)?

Back

Parent structure: Trigonal Bipyramidal. Molecular shape: See-saw. Bond angles: <90° and <120°. The lone pair occupies an equatorial position to minimize 90° repulsions, causing the axial bonds to bend down slightly, reducing the ideal angles.

Front

Explain the relationship between Bond Strength, Bond Length, and Bond Order (Energy).

Back

Bond Order relates directly to strength and inversely to length. Higher bond order (e.g., triple bond) means more electrons are shared, resulting in a shorter bond length and higher bond energy (stronger). Lower bond order (single bond) results in longer length and lower energy.

Front

Compare Lattice Energy in NaCl vs. MgO.

Back

MgO has a significantly higher lattice energy than NaCl. Lattice Energy is proportional to (Q1 * Q2) / r. MgO involves +2 and -2 ions, while NaCl involves +1 and -1 ions. Since 2*2 > 1*1, the attraction in MgO is much stronger (Coulomb's Law), requiring more energy to break.

Front

Define the Enthalpy of Hydration (Hydration Energy).

Back

The enthalpy change associated with dissolving gaseous ions in water to form aqueous ions. It is always exothermic (negative ΔH). It depends on ion charge density: smaller, highly charged ions (e.g., Li+, Al3+) have more negative (more exothermic) hydration enthalpies because they attract water dipoles more strongly.