Bonding, structure, properties and energy changes
This standard tests your ability to link electronic structure to bonding, to relate structure and intermolecular forces to physical properties, and to calculate and interpret energy changes. Excellence answers name the specific particles and forces broken and formed rather than saying vaguely that 'bonds are strong'.
Bonding and shape
- Ionic: electrons transferred, giving a lattice of ions; conducts when molten or aqueous, not when solid; melting point rises with ionic charge and falls with ion size.
- Covalent molecular: discrete molecules held by dispersion forces, plus dipole–dipole if polar, plus hydrogen bonding when H is bonded to N, O or F.
- Covalent network (SiO₂, diamond): very high melting point, no conduction.
- Metallic: delocalised electrons; conducts as a solid and as a liquid.
- VSEPR: electron pairs repel, lone pairs more strongly. Four regions give tetrahedral (109.5°), three trigonal planar (120°), two linear (180°); one lone pair on four regions gives trigonal pyramidal (~107°).
Energy changes
- ΔrH = Σ(bonds broken) − Σ(bonds formed); breaking is endothermic, forming is exothermic, so exothermic reactions have negative ΔH.
- Calorimetry: q = mcΔT, with m the mass of solution (assume 1.00 g mL⁻¹) and c = 4.18 J g⁻¹ °C⁻¹ for water.
- Molar enthalpy: ΔH = −q ÷ n, per mole of the specified reactant.
Worked example
Burning 0.0500 mol ethanol heats 100 g of water by 6.50 °C. q = 100 × 4.18 × 6.50 = 2720 J = 2.72 kJ ΔH = −2.72 ÷ 0.0500 = −54.4 kJ mol⁻¹ The negative sign shows heat is released. The literature value is more negative because heat is lost to the surroundings and combustion is incomplete.
Traps
- Giving a magnitude with the wrong sign, or quoting energy per gram instead of per mole.
- Describing hydrogen bonds as intramolecular, or omitting dispersion forces from a molecule that also has them.
- Quoting bond angles without saying they are approximate.
- Confusing what is overcome on boiling (intermolecular forces) with what is overcome on decomposition (covalent bonds).