Kinetics and Redox
Kinetics and Redox
This domain combines rate measurement and rate equations with oxidation numbers, half-equations and volumetric redox calculations, which are frequently examined together in titration contexts.
Rates and orders
Rate = k[A]m[B]n; orders m and n are found experimentally, never from the equation. Compare experiments: if doubling [A] doubles the rate, the order in A is 1; if it quadruples, 2; if nothing changes, 0. Overall order is m + n, and the units of k depend on it. The rate-determining step is the slowest step, and only species in or before it appear in the rate equation. Arrhenius: k = Ae(-Ea/RT), so a graph of ln k against 1/T has gradient -Ea/R and intercept ln A.
Oxidation numbers and half-equations
Rules: elements 0; monatomic ion equals its charge; oxygen usually -2 (but -1 in peroxides); hydrogen +1 (but -1 in hydrides); sum equals the overall charge. Balance half-equations by atoms, then oxygen with water, then hydrogen with H+, then charge with electrons. In alkaline solution, add OH- to both sides to remove H+.
Worked example
A 25.0 cm3 sample of iron(II) solution required 24.0 cm3 of 0.0200 mol dm-3 KMnO4 in acid. MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O and Fe2+ -> Fe3+ + e-. Combining gives MnO4- + 5Fe2+ + 8H+ -> Mn2+ + 5Fe3+ + 4H2O. Moles MnO4- = 0.0200 x 24.0/1000 = 4.80 x 10-4 mol, so moles Fe2+ = 5 x 4.80 x 10-4 = 2.40 x 10-3 mol. [Fe2+] = 2.40 x 10-3 / 0.0250 = 0.0960 mol dm-3. The factor of 5 is the whole point of the question.
Traps
Not scaling by the electron ratio; forgetting H+ and water when balancing; assigning oxygen -2 in H2O2; confusing rate with rate constant; omitting units for k; and assuming the stoichiometric equation gives the orders.
Key ideas
- Orders come from experiment; rate equation follows the rate-determining step.
- ln k = ln A - Ea/RT gives activation energy from a graph.
- Balance redox half-equations in the order atoms, O, H, charge.
- Multiply moles by the electron ratio in redox titrations.
- KMnO4 is its own indicator: purple to colourless.