The questions are on the reaction mechanism test. Take it under real conditions before reading the mark scheme.
Quick answer: A reaction mechanism is a sequence of elementary steps whose sum is the overall equation. This mark scheme gives the model answers and the marks for the 8 questions of the AP Chemistry test on reaction mechanism, from elementary steps to temperature and collisions, with the equivalent answers to accept.
Model answers and a marking guide follow. Give credit for correct reasoning carried forward from an earlier mistake, and accept the rate law in any equivalent form, for example with the product of constants written as a single k.
Level: AP Chemistry · Curriculum: AP Chem Unit 5
Question 1 Elementary steps (2 marks)
(a) Bimolecular.
(b) Rate = k[NO₂]².
Marking: 1 mark each.
Question 2 Testing a mechanism (3 marks)
(a) NO₂Cl + NO₂Cl + Cl → NO₂ + Cl + NO₂ + Cl₂; cancel Cl: 2 NO₂Cl → 2 NO₂ + Cl₂.
(b) The intermediate is Cl.
(c) Rate = k[NO₂Cl] (slow step unimolecular).
Marking: 1 mark for each part.
Question 3 Energy profile (3 marks)
(a) 95 − 20 = 75 kJ.
(b) ΔH = 55 − 20 = +35 kJ, endothermic.
(c) 95 − 55 = 40 kJ.
Marking: 1 mark each.
Question 4 Role of a catalyst (2 marks)
A catalyst provides an alternative pathway with a lower activation energy, so a larger fraction of collisions are successful and the rate increases. It does not change ΔH.
Marking: 1 for lower Ea or alternative path with effect on rate; 1 for ΔH unchanged.
Question 5 Activation energy (3 marks)
Eₐ = R ln(k₂/k₁)/(1/T₁ − 1/T₂) = 8.314 × ln 4.0/(1/350 − 1/370) = 8.314 × 1.386/(1.544 × 10⁻⁴) = 7.46 × 10⁴ J/mol = 75 kJ/mol.
Marking: 1 for ln 4.0, 1 for the temperature term, 1 for the answer in kJ/mol.
Question 6 Fast equilibrium (3 marks)
(a) Rate = k₂[N₂O₂][O₂]; K = [N₂O₂]/[NO]², so rate = k₂K[NO]²[O₂] = k[NO]²[O₂].
(b) Third order.
Marking: 1 for rate of the slow step, 1 for substituting [N₂O₂], 1 for the order.
Question 7 Catalyst or intermediate (2 marks)
Catalysts: Br⁻ (and H⁺); intermediate: HOBr.
Marking: 1 for the catalyst, 1 for the intermediate.
Question 8 Temperature and collisions (2 marks)
At higher temperature the energy distribution shifts and widens, so a larger fraction of collisions has energy of at least Ea; the number of successful collisions per second grows, and k increases.
Marking: 1 for the larger fraction above Ea; 1 for linking it to more successful collisions and larger k.
