Flashcards · 9620 · A Level Topic 11

Rate equations (A Level) flashcards

13 cards on A Level Topic 11, subtopics 3.1.11.1 to 3.1.11.2. Pick a subtopic or study them all.

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Rate equation: Rate = k[A]^m [B]^n. It is found by experiment.
Order with respect to a reactant: The power of that reactant’s concentration in the rate equation.
Overall order: The sum of all the individual orders.
Units of k for overall orders 0, 1, 2 and 3: 0: mol dm⁻³ s⁻¹ 1: s⁻¹ 2: mol⁻¹ dm³ s⁻¹ 3: mol⁻² dm⁶ s⁻¹
The Arrhenius equation and its straight-line form: k = Ae^(−Ea/RT) ln k = −Ea/RT + ln A: plot ln k against 1/T; gradient = −Ea/R, intercept = ln A. Ea in J mol⁻¹, T in K.
Effect of temperature and of concentration on k: Higher temperature → larger k. Changing a concentration changes the rate, not k.
Rate = k[V]²[L]. [V] is halved and [L] tripled. Factor change in rate?: 0.5² × 3 = 0.75

Rate-determining (rate-limiting) step: The slowest step of a mechanism.
Rate–concentration graph shapes for orders 0, 1 and 2: Order 0: flat horizontal line. Order 1: straight line through the origin. Order 2: curve through the origin, getting steeper.
Finding an order from initial-rate data: Compare two runs where only one concentration changes. Rate unchanged → order 0; same factor → order 1; factor squared → order 2.
What does a zero order with respect to a reactant tell you about the mechanism?: That reactant is not in the rate equation: it takes part after the rate-determining step. It is still consumed.
Initial rate from a concentration–time graph: Draw a tangent to the curve at t = 0 and take its gradient (ignoring the minus sign).
Required practical 8: in a clock reaction, how is rate related to time?: Rate ∝ 1/t, where t is the time taken for a fixed small amount of reaction (the sudden colour change).