Topic 2 of 6 · Form 5
Stoichiometry, energetics and electrochemistry
Mole calculations, empirical and molecular formulae, titrations, percentage yield, atom economy, gas volumes.
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Stoichiometry
Mole calculations, empirical and molecular formulae, titrations, percentage yield, atom economy, gas volumes.
Chemical energetics
Enthalpy changes (formation, combustion, neutralisation, atomisation, hydration, solution); Hess's law cycles; bond energies; lattice energy and Born–Haber cycles; entropy (ΔS) and Gibbs free energy ΔG = ΔH − TΔS (feasibility).
Worked example: For a reaction ΔH = +180 kJ mol⁻¹ and ΔS = +200 J K⁻¹ mol⁻¹. Above what temperature is it feasible?
ΔG < 0 when T > ΔH/ΔS = 180 000 ÷ 200 = 900 K
Electrochemistry
Redox and oxidation numbers; standard electrode potentials E°; cell potentials E°cell = E°(reduction) − E°(oxidation); predicting feasibility; the Nernst equation E = E° + (0.059/z) log([oxidised]/[reduced]); electrolysis and the Faraday constant (F = Le).
Worked example: Find the mass of silver deposited by 0.50 A for 30 minutes (Ar Ag = 108).
Q = 900 C → n(e⁻) = 900 ÷ 96 500 = 9.33 × 10⁻³ mol = n(Ag) → mass = 1.01 g
Check yourself
- State Hess's law.
- E°(Cu²⁺/Cu) = +0.34 V; E°(Zn²⁺/Zn) = −0.76 V. Find E°cell.
- Why does lattice energy become more exothermic for smaller, more highly charged ions?
Show answers
- The enthalpy change of a reaction is independent of the route taken.
- +1.10 V
- Stronger electrostatic attraction between ions that are closer and more highly charged.
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