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Flashcards · 9701 · A Level Topic 26

Reaction kinetics (A Level) flashcards

15 cards on A Level Topic 26, subtopics 26.1 to 26.2. Pick a subtopic or study them all.

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26.1 Simple rate equations, orders of reaction and rate constants

Rate equation
rate = k[A]ᵐ[B]ⁿ: links the rate to the concentrations of reactants. The orders are found by experiment, never from the balanced equation.
Order of reaction (with respect to a reactant) and overall order
Order: the power to which that concentration is raised in the rate equation. Overall order: the sum of the orders, m + n.
Units of k for zero, first, second and third overall order
0: mol dm⁻³ s⁻¹ 1: s⁻¹ 2: dm³ mol⁻¹ s⁻¹ 3: dm⁶ mol⁻² s⁻¹
Concentration–time and rate–concentration graphs for zero, first and second order
Zero: straight line falling; rate graph horizontal. First: curve with constant half-life; rate graph straight line through the origin. Second: steeper curve, half-life increases; rate graph an upward curve.
Half-life of a first-order reaction
Constant, independent of starting concentration. k = 0.693 ÷ t½. e.g. t½ = 40 s: k = 0.693 ÷ 40 = 1.7 × 10⁻² s⁻¹.
Rate-determining step
The slowest step in a multi-step mechanism. The rate equation contains the species in this step (and steps before it); species reacting only afterwards are zero order.
Intermediate vs catalyst in a mechanism
Intermediate: formed in one step and used up in a later step. Catalyst: used in one step and reformed in a later step; it may appear in the rate equation but not in the overall equation.
Rate of hydrolysis of 2-bromo-2-methylpropane = k[(CH₃)₃CBr]. What does this show?
The slow step involves only the halogenoalkane, (CH₃)₃CBr → (CH₃)₃C⁺ + Br⁻ (SN1). OH⁻ reacts in a fast later step, so it is zero order.
Effect of temperature on the rate constant
Raising the temperature increases k: a larger proportion of molecules have energy ≥ Eₐ, so there are more successful collisions per unit time. Quote k with its temperature.

26.2 Homogeneous and heterogeneous catalysts

Three stages of heterogeneous catalysis
1 Adsorption of reactants onto active sites on the surface. 2 Bonds in the reactants are weakened (and they are held in a suitable orientation), so they react. 3 Desorption of the products, freeing the active sites.
How does iron catalyse the Haber process?
N₂ and H₂ adsorb on the iron surface, the strong N≡N bond is weakened, they react, and NH₃ desorbs.
Catalytic converter: catalyst and reactions
Platinum, palladium and rhodium on a ceramic support (heterogeneous). 2CO + 2NO → 2CO₂ + N₂ 4CO + 2NO₂ → 4CO₂ + N₂
How does NO₂ catalyse the oxidation of atmospheric SO₂?
SO₂ + NO₂ → SO₃ + NO NO + ½O₂ → NO₂ NO₂ is reformed (homogeneous catalyst); NO is an intermediate.
How do iron ions catalyse the reaction of S₂O₈²⁻ with I⁻?
S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺ 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ Each step is between oppositely charged ions, unlike the slow direct reaction between two negative ions.
Homogeneous catalyst: mode of action
In the same phase as the reactants; used in one step and reformed in a later step, giving a route of lower activation energy.

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