Zadanie 3b, Paper 2, maj 2025, TZ2, Chemia IB HL
Wzorowane na: Chemistry HL Paper 2, May 2025, TZ2, pytanie 3(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
Carbonyl dibromide, Br2CO, is a reactive compound used as a brominating agent in organic synthesis.
Polecenie
In moist air carbonyl dibromide is slowly hydrolyzed:
Br2CO(g) + H2O(g) → CO2(g) + 2 HBr(g)
(i) Suggest a method of monitoring the progress of this reaction at constant temperature so that its rate can be determined.
(ii) Outline two reasons why a collision between a carbonyl dibromide molecule and a water molecule may not produce a reaction.
(iii) The reaction is first order with respect to each reactant. Write the rate equation.
(iv) In a dilute solution of carbonyl dibromide in water, the hydrolysis behaves as a first-order reaction overall. Outline why this observation does not contradict your rate equation in (b)(iii).
(v) To find the activation energy, a student measured the rate constant at several absolute temperatures, T, and obtained the straight-line graph shown. State the quantities plotted on the two axes and label them on the graph.
Powiększ(vi) The activation energy of the hydrolysis is 15 kJ mol−1. Calculate the gradient of the line in (b)(v). Use sections 1 and 2 of the data booklet.
(vii) The Maxwell–Boltzmann distribution below shows the kinetic energies of the reactant molecules at temperature T1. Sketch a second curve for the same sample at a higher temperature.
Powiększ(viii) Use the two curves, annotated with the activation energy, to explain why raising the temperature increases the rate of the hydrolysis.
(ix) A student claims that a certain solid catalyses the hydrolysis. Describe two observations that would support this claim.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) measure the change in pressure per unit time at constant volume «2 mol of gas become 3 mol»
OR
measure the change in volume per unit time at constant pressure ✔
(ii) the molecules may collide with an unsuitable orientation ✔
the collision energy may be less than the activation energy ✔
(iii) rate = k[Br2CO][H2O] ✔
(iv) water is the solvent, so [H2O] is very much larger than [Br2CO] and remains effectively constant «rate = k′[Br2CO] with k′ = k[H2O]» ✔
(v) x-axis: 1/T «K−1» AND y-axis: ln k ✔
(vi) gradient «» = −1.81 × 103 «K» ✔
(vii) second curve starting at the origin with a lower maximum displaced to higher energy, staying above the axis at high energy «area under both curves the same» ✔
(viii) Ea marked as a vertical line on the energy axis ✔
at the higher temperature a greater proportion of molecules have energy equal to or greater than Ea «larger area under the curve to the right of Ea», so a greater fraction of collisions are successful ✔
(ix) the solid is chemically unchanged / its mass is unchanged at the end of the reaction ✔
the rate of reaction is greater in its presence «under otherwise identical conditions» ✔
Schemat punktowania
(i) [1] a physical change that follows the increase in the amount of gas; do not accept pH measurement or titration. (ii) [2] M1 orientation; M2 energy below Ea. (iii) [1]. (iv) [1] excess water, concentration constant. (v) [1] both axes required; accept lg k on the y-axis. (vi) [1] accept −1.8 × 10^3 K; the negative sign is required. (vii) [1] no mark if the curve does not start at the origin, crosses the axis or has a larger area. (viii) [2] M1 Ea indicated; M2 greater proportion of molecules with E ≥ Ea. (ix) [2] M1 unchanged at the end; M2 rate increased. Answers that only quote the definition of a catalyst without an observation do not score.
Komentarz. In (iv) the pseudo-first-order rate constant already contains the constant water concentration; the overall order changes, the mechanism does not.