FC
Reactivity 1.2 Flashcards
Active Recall for Energy Cycles
Flashcards: read all 12 questions and answers
- State Hess's Law.The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.
- Give the Hess's Law formula using formation data.ΔH(rxn) = Σ ΔHf(products) − Σ ΔHf(reactants)
- Give the Hess's Law formula using combustion data.ΔH(rxn) = Σ ΔHc(reactants) − Σ ΔHc(products). Note: the subtraction is reversed compared to formation.
- Define lattice enthalpy.The enthalpy change when 1 mole of a solid ionic compound is broken down into its constituent gaseous ions under standard conditions: MX(s) → M⁺(g) + X⁻(g). Always endothermic (positive, ΔH > 0).
- What are the 5 steps in a Born-Haber cycle?1. Atomisation of metal 2. Atomisation of non-metal 3. Ionisation energy 4. Electron affinity 5. Lattice enthalpy
- Which Born-Haber step is always endothermic?Ionisation energy - removing an electron from a gaseous atom always requires energy.
- Define enthalpy of solution.The enthalpy change when 1 mole of a solute dissolves in enough water to form an infinitely dilute solution.
- Define enthalpy of hydration.The enthalpy change when 1 mole of gaseous ions is surrounded by water molecules. Always exothermic.
- State the dissolution cycle equation.ΔH(sol) = ΔH(latt) + Σ ΔH(hyd)
- What factors make hydration enthalpy more exothermic?Smaller ionic radius and higher ionic charge - both increase the charge density, leading to stronger ion-dipole attractions with water.
- Why is the 2nd electron affinity endothermic?Adding an electron to an already negative ion (e.g. O⁻ → O²⁻) requires energy to overcome the repulsion between the electron and the negatively charged ion.
- What makes lattice enthalpy larger / more endothermic?Smaller ions and higher charges - both increase electrostatic attraction between ions (Coulomb's Law: F ∝ q₁q₂/r²).