Kinetics

🔬 1. Reaction Mechanisms & Intermediates

✅ Tips:

  • Use the slow step to write the rate law.

  • Intermediates = species made in one step, used in another — they don’t appear in the overall equation.

  • If an intermediate is in the rate law but not a reactant, substitute using a fast equilibrium step.

✅ Do:

  • Identify slow step → use its reactants for rate law.

  • Confirm catalysts appear at the start and reappear at the end.

  • Know that intermediates are produced first, consumed later.

❌ Don’t:

  • Don’t use products in the rate law.

  • Don’t assume stoichiometry = order of reaction.

  • Don’t forget to substitute out intermediates if they appear in the rate law.

🔒 Limit:

  • Mechanisms are proposed, not proven — they must match the overall reaction and match the rate law from experiment.

📈 2. Integrated Rate Laws & Graphs

✅ Tips:

  • Zero order → [A] vs time is linear (slope = –k)

  • First order → ln[A] vs time is linear (slope = –k)

  • Second order → 1/[A] vs time is linear (slope = +k)

✅ Do:

  • Memorize the graphs that go with each order.

  • Know that half-life is constant only for first-order.

  • Use units of rate constant (k) to help identify the order.

❌ Don’t:

  • Don’t mix up half-life equations for different orders.

  • Don’t assume straight-line graph = rate always increasing. It just means constant rate change.

🔒 Limit:

  • Only valid when temperature and conditions are constant (i.e., no catalyst added).

🔁 3. Rate Constant vs Reaction Rate

✅ Tips:

  • k (rate constant) is only affected by temperature or catalyst.

  • Rate changes with concentration, temperature, catalyst, etc.

✅ Do:

  • When asked about rate, check how concentration is changing.

  • When asked about k, only change it for temperature or catalysts.

❌ Don’t:

  • Don’t say k changes if only concentration changes — it doesn’t.

  • Don’t confuse k with the rate of reaction — they’re related but not the same.

🔒 Limit:

  • Rate laws apply only to the conditions under which the experiment was performed. You can’t extrapolate k to another temperature without data or the Arrhenius equation.

🧪 4. Stoichiometry vs Rate Law

✅ Tips:

  • In elementary steps, stoichiometric coefficients do tell you the order.

  • In overall reactions, only experimental data reveals the order.

✅ Do:

  • For mechanisms: Use coefficients from the slow step only.

  • For overall reactions: Always use rate data to find order.

❌ Don’t:

  • Don’t assume that “2A + B → C” means second order in A and first in B — check the data.

🔒 Limit:

  • The overall balanced equation may hide complex steps — don’t oversimplify.

⚛ 5. Collision Theory, Catalysts, and Energy

✅ Tips:

  • More collisions = faster rate, but only effective collisions cause reaction.

  • Catalysts lower activation energy (Ea), don’t affect ΔH or equilibrium.

  • Homogeneous catalysts are in the same phase as reactants; heterogeneous are in a different phase.

✅ Do:

  • Discuss rate in terms of collision frequency and activation energy.

  • Know how to label energy diagrams: Ea, activated complex, ΔH.

❌ Don’t:

  • Don’t say catalysts increase the energy of collisions — they lower Ea.

  • Don’t mix up Ea and ΔH — Ea is kinetic, ΔH is thermodynamic.

🔒 Limit:

  • Even if collisions increase, rate won’t increase unless they're effective (i.e., enough energy & correct orientation).