Flashcards · 9701 · AS Topic 8

Reaction kinetics (AS Level) flashcards

13 cards on AS Topic 8, subtopics 8.1 to 8.3. Pick a subtopic or study them all.

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Two conditions for a successful collision: The particles collide with energy greater than or equal to the activation energy, Eₐ, and with the correct orientation.
Explain why increasing concentration increases the rate: More particles per unit volume → more frequent collisions → more effective collisions per second. The proportion of collisions that are effective is unchanged.
Why does increasing surface area or gas pressure increase the rate?: Surface area: more particles exposed, so more frequent collisions. Pressure: particles closer together, the same as raising concentration.
Gas volume rises from 24 cm³ to 39 cm³ between 20 s and 30 s. Average rate?: Rate = (39 − 24) ÷ (30 − 20) = 15 ÷ 10 = 1.5 cm³ s⁻¹
Activation energy, Eₐ: The minimum energy that colliding particles must have for a reaction to occur.

Key features of a Boltzmann distribution curve: Starts at the origin; rises to a peak (most probable energy); never touches the x-axis at high energy; area under the curve = total number of particles. The area to the right of Eₐ is the number that can react.
How does the Boltzmann distribution change at a higher temperature?: The peak moves right and becomes lower and broader; the total area is unchanged; the area beyond Eₐ increases significantly.
Why does a small temperature rise cause a large increase in rate?: A much greater proportion of particles have energy ≥ Eₐ. This matters far more than the modest increase in collision frequency.
Which factor increases the energy of collisions?: Only temperature. Concentration, pressure and surface area change the collision frequency only.

How does a catalyst increase the rate?: It provides an alternative reaction pathway with a lower activation energy. It is chemically unchanged at the end and does not change ΔH, the yield or the position of equilibrium.
Homogeneous catalyst, with an example: A catalyst in the same phase as the reactants, e.g. aqueous iodide ions catalysing the decomposition of aqueous hydrogen peroxide.
Heterogeneous catalyst, with an example: A catalyst in a different phase from the reactants; it works by adsorption onto its surface, reaction, then desorption. e.g. solid iron in the gas-phase Haber process.
How does a catalyst show on a Boltzmann distribution and on a reaction-pathway diagram?: Boltzmann: the curve is unchanged, the Eₐ line moves left, so more particles lie beyond it. Pathway diagram: same start and end enthalpy levels (ΔH unchanged), lower peak.