Topic 8: Reaction Kinetics Flashcards
Definitions I:
(a) Rate of reaction
(b) Effective collision
(c) Activation energy
Definitions I:
(a) Change in concentration of reactant or product per unit time.
(b) Collision with energy ≥ activation energy and correct orientation.
(c) Minimum energy required for a collision to result in reaction.
Definitions II:
(a) Frequency of collisions
(b) Catalyst
(c) Catalysis
Definitions II:
(a) Number of collisions between particles per unit time.
(b) Substance that increases reaction rate without being consumed.
(c) Process of speeding up a reaction using a catalyst.
In an experiment, [H₂O₂] drops from 0.1 to 0.06 mol/dm³ in 20 s. Calculate the rate. [2 Marks]
Rate = Δ[H₂O₂] / Δt = (0.1 - 0.06) / 20 (1 mark)
Rate = 0.002 mol/dm³/s (1 mark)
For N₂ + 3H₂ → 2NH₃, how does doubling [H₂] affect the rate if collisions increase? [2 Marks]
More H₂ increases collision frequency (1 mark)
Rate increases, more effective collisions (1 mark)
Sketch a Boltzmann distribution for a reaction at 25°C and 50°C with EA labelled. [3 Marks]
Curve shifts right, higher peak at 50°C (1 mark)
EA line, more area under curve at 50°C (1 mark)
Labelled axes: energy vs. number of molecules (1 mark)
Why does raising temperature from 300 K to 310 K increase the rate of decomposition of HI? [2 Marks]
More molecules exceed EA at 310 K (1 mark)
Higher frequency of effective collisions (1 mark)
In a catalysed reaction like 2H₂O₂ → 2H₂O + O₂, how does MnO₂ affect the pathway? [2 Marks]
MnO₂ provides lower EA pathway (1 mark)
Different mechanism, faster reaction (1 mark)
Draw a reaction pathway diagram for CH₄ + Cl₂ with and without a catalyst. [3 Marks]
Uncatalysed: high EA peak (1 mark)
Catalysed: lower EA peak (1 mark)
ΔH same, labelled enthalpy vs. progress (1 mark)
How does a catalyst like Fe in the Haber process affect the Boltzmann distribution? [2 Marks]
Lowers EA, more molecules exceed it (1 mark)
Distribution unchanged, area past EA increases (1 mark)
Does increasing pressure in 2NO(g) + O₂(g) → 2NO₂(g) speed up the reaction? [2 Marks]
Yes, higher pressure increases concentration (1 mark)
More frequent effective collisions (1 mark)
Why is a collision between H₂ and O₂ at low energy non-effective? [2 Marks]
Energy below EA (1 mark)
No reaction, insufficient to break bonds (1 mark)
For a reaction, rate doubles when temperature rises from 20°C to 30°C. Explain using EA. [2 Marks]
Higher T, more molecules exceed EA (1 mark)
Frequency of effective collisions doubles (1 mark)
