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 same volume initial gradient should be steeper to show that rate is faster at higher temperature. maximum volume of oxygen gas produced should be the same but the time taken to reach the maximum volume should be shorter. (c) The experiment is repeated at an increased temperature. All other conditions stay the same. Draw labelled Boltzmann distribution curve(s) to explain why the rate of reaction increases at a higher temperature. [3]

● When temperature increases, there is an increase in the number of particles with kinetic energy  Ea (larger shaded area under the curve at T2 than T1). ● This leads to an increase in the frequency of effective collisions between reactant particles. Hence the rate of reaction increases. (d) Manganese(IV) oxide is added to 20 cm3 of aqueous hydrogen peroxide. The total volume of oxygen gas produced is 72 cm3 at r.t.p. 2H2O2(aq) → 2H2O(l) + O2(g) (i) Calculate the number of moles of hydrogen peroxide that reacted. [1] Amt of O2 = 72 / 24000 = 0.003 Amt of H2O2 = 0.003 x 2 = 0.00600 mol Some students wrongly assume that H2O2 is a gas. r.t.p. 1 mol of gas occupies 24,000 cm3 (ii) Calculate the concentration of aqueous hydrogen peroxide in g/ dm3. [1] [H2O2] = 0.006 x 1000 / 20 = 0.3 mol/ dm3 [H2O2] = 34 x 0.3 = 10.2 g/ dm3 [Total: 14] 2 Methane reacts with steam to produce hydrogen gas. equation 2.1 CH4(g) + H2O(g) CO(g) + 3H2(g) ΔH = + 200 kJ mol–1 The reaction takes place at 1000 °C and 100 kPa pressure. (a) The reaction is reversible and reaches dynamic equilibrium in a closed system. (i) State two features of a dynamic equilibrium. [2] The rate of forward reaction is equal to the rate of the backward reaction. The concentrations of reactants and products remain constant over time. There is no net change in the concentration of reactants and products over time even though the forward and backward reaction is still ongoing. (ii) Sketch a labelled graph to show how the rates of the forward and reverse reactions change from the start of the reaction to the time the reaction reaches equilibrium. [2]

the graphs meet at t when the reaction reaches dynamic equilibrium ● Rf decreases with time since [A] and [B] decreases ● Rb increases with time since [C] and [D] increases (b) State and explain, the effect on the amount of hydrogen when: (i) the pressure is increased [2] When the pressure is increased, POE will shift to the LHS as there are fewer gaseous molecules on the LHS. Amount of hydrogen decreases. Students are to explain why POE shift to the LHS. Key word: gaseous (ii) the temperature is increased [2] When the temperature is increased, POE shifts to the RHS to absorb the excess heat and favour the forward endothermic reaction. Amount of hydrogen increases. Students are to explain why favour endothermic reaction. (c) (i) Write the expression for the equilibrium constant, Kc, for equation 2.1. [1] 3 2 42 [ ][ ] [ ][ ] c CO HK CH H O= (ii) State the effect of decreasing temperature on the equilibrium constant, Kc. [1] Kc value will decrease. When temperature decreases, POE will shift left and Kc will decrease. [Total: 10] 3 Perspex, a transparent thermoplastic, is commonly utilised in sheet form as a lightweight, shatter-resistant substitute for glass. It also finds application as a casting resin, inks, and coatings. Moreover, Perspex, poly(methyl 2-methylpropenoate), is formed by polymerising methyl 2-methylpropenoate, a single monomer.

(a) (i) Identify all the functional groups in methyl 2-methylpropenoate. [1] ester and alkene note: ketone is NOT present Students should avoid drawing the functional group C=C but rather name the functional group as alkene. (ii) State the functional group that reacts with bromine in CC l4 and the expected observation. [1] Alkene. Reddish brown bromine decolourises. (b) (i) Describe the term polymer. [1] A polymer is a macromolecule built up from monomers, with average molar mass of at least 1000 or at least 100 repeat units. (ii) State the type of polymerisation the monomer undergoes to form Perspex. Draw the