Flashcards · 7405 · A Level Topic 9

Rate equations (A Level) flashcards

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Rate equation: Rate = k[A]^m [B]^n, an experimentally determined relationship.
Order of reaction with respect to a reactant: The power to which its concentration is raised in the rate equation. The overall order is the sum of the orders.
Rate-determining step: The slowest step in a mechanism; it limits the overall rate.
Units of k for overall orders 1, 2 and 3: First: s⁻¹. Second: mol⁻¹ dm³ s⁻¹. Third: mol⁻² dm⁶ s⁻¹.
Rate = k[A][B]². [A] is halved and [B] is tripled. What happens to the rate?: ½ × 3² = 4.5, so the rate increases by a factor of 4.5.
Shapes of rate against [A] for orders 0, 1 and 2: Order 0: horizontal line. Order 1: straight line through the origin. Order 2: upward curve through the origin.
The Arrhenius equation and its straight-line form: k = A e^(−Ea/RT). ln k = −Ea/(RT) + ln A: plot ln k against 1/T; gradient = −Ea/R, intercept = ln A.
What can change the value of k?: Temperature (k rises as T rises) or adding a catalyst. Never a change in concentration.
What do the orders tell you about the rate-determining step?: Species in the rate equation take part in the rate-determining step (or a step before it). A zero-order reactant reacts after it.