Unit 5 studies how fast reactions occur and the factors that affect reaction rate — concentration, temperature, and catalysts — with quantitative rate law analysis.
Rate = k[A]^m[B]^n. Order determined experimentally from data (method of initial rates). Zero order: rate = k (constant). First order: rate = k[A], half-life = 0.693/k. Second order: rate = k[A]². Integrated rate laws: [A] vs t for zero (linear), ln[A] vs t for first (linear), 1/[A] vs t for second (linear). Plot that gives straight line reveals order.
Mechanism: sequence of elementary steps. Molecularity: unimolecular, bimolecular. Rate-determining step: slowest step. The rate law for the overall reaction must match the rate law of the rate-determining step. Intermediate: produced in one step, consumed in another. Catalyst: consumed then regenerated. Arrhenius: k = Ae^(-Ea/RT). ln(k₂/k₁) = (Ea/R)(1/T₁ - 1/T₂). Higher T → larger k → faster.
The balanced equation shows the overall reaction (reactants → products), but most reactions occur in multiple steps (a mechanism). The rate depends on the slowest step, not the overall equation. The rate law is determined experimentally because you cannot know the mechanism just by looking at the balanced equation. For example: 2NO₂ + F₂ → 2NO₂F might suggest rate = k[NO₂]²[F₂], but the experimental rate = k[NO₂][F₂], consistent with a two-step mechanism where the first (slow) step involves one molecule of each.
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