Kinetics and Reaction Mechanisms
What this domain tests
Rate equations from experimental data, orders of reaction, the effect of temperature, and deducing mechanisms.
Core rules
- Rate = k[A]m[B]n; orders m and n come from experiment, never from the stoichiometric equation.
- Units of k follow from the overall order: for rate = k[A][B]2 (third order), k has units mol−2 dm6 s−1.
- First order: constant half-life, and a plot of ln[A] against t is linear. Zero order: a plot of [A] against t is linear.
- Arrhenius: k = Ae−Ea/RT, so a plot of ln k against 1/T has gradient −Ea/R.
- Raising temperature increases both the collision frequency and, far more importantly, the fraction of collisions exceeding Ea.
- In a mechanism, the rate-determining step fixes the rate equation; species in the mechanism but not the overall equation may be intermediates or catalysts.
Worked example
From initial rates, doubling [A] at constant [B] doubles the rate, while doubling [B] at constant [A] quadruples it. Therefore rate = k[A][B]2, overall order 3. Rearranging gives k = rate/([A][B]2), so the units are mol dm−3 s−1 ÷ (mol dm−3)3 = mol−2 dm6 s−1.
Traps
Deriving orders from coefficients; saying a catalyst lowers Ea for the forward reaction only (it lowers both); confusing rate with the rate constant; reading a gradient without checking the axes; proposing a rate-determining step that omits species appearing in the rate equation.