2024 ACJC Prelim H1 Chem Paper 2 (Solutions)

Section A Answer all the questions in this section in the spaces provided. 1 Aqueous hydrogen peroxide, H2O2, slowly forms water and oxygen at room temperature and pressure, r.t.p. This rate of reaction is increased by the addition of a small amount of solid manganese(IV) oxide. 2H2O2(aq) → 2H2O(l) + O2(g) (a) (i) Define the term, rate of reaction. [1] The rate of a reaction is defined as the change of amount or concentration of a reactant or product per unit time. key word: change of concentration & per unit time (ii) State the role of manganese( IV) oxide and explain the reason. [2] It is a heterogeneous catalyst as it is a catalyst which is in different phase as the reactant, aq H2O2. note: different phase ≠ different state (example: organic vs aqueous) (iii) State the test for oxygen gas. [1] The test is to relight a glowing splint. rekindle = relight = reignite glowing splint ≠ burning splint (iv) State the effect on the mass of oxygen gas produced if the mass of manganese(IV) oxide is increased. [1] No effect A catalyst only speeds up the rate of reaction of the oxygen produced but it does not increase the mass of oxygen gas produced. (b) A student investigates the rate of formation of oxygen gas when manganese( IV) oxide is added to aqueous hydrogen peroxide. The volume of oxygen gas formed is measured at regular time intervals at r.t.p. The results are plotted onto the graph in Fig. 1.1. (i) State how the graph in Fig. 1.1 shows the rate of reaction at time t2, is lower than at time t1. [1] The gradient is gentler OR lower OR less steep at t2 compared to t1. (ii) Explain, using collision theory, why the rate of reaction at time t2 is lower than at time t1. [2] ● When concentration of the H 2O2 decreases, the number of molecules per unit volume having energy ≥ Ea decreases.

There will be greater surface area on the catalyst for the adsorption of the reactant molecules. ● This leads to an decrease in the frequency of effective collisions between reactant particles. (iii) On Fig. 1.1, sketch the graph obtained when the experiment is repeated using aqueous hydrogen peroxide at a higher temperature. All other conditions remain the same. [1] steeper curve which does not cross original curve and levels off before the original curve and finishes at