NJC REACTION KINETICS TUTORIAL suggested solution

National Junior College SH1 H2 Chemistry 2023 REACTION KINETICS TUTORIAL A. BASIC CONCEPTS (QUALITATIVE KINETICS) Section A1: Boltzmann distribution 1 Explain, using collision theory, how the following changes can affect the initial reaction rate of a gaseous system. Illustrate your answer with the Boltzmann distribution curve for parts (ii) and (iii). (i) increasing the pressure There is greater no of particle per unit volume leading to greater collision frequency therefore frequency of effective collision increases leading to increase in rate. Note: Assumption made that increase in total pressure comes from smaller volume. (ii) decreasing the temperature As the temperature decreases, the average kinetic energy of the particles decreases. There are less particles with kinetic energy equal or greater than the activation energy as shown in the Boltzmann distribution. This result in lower frequency of effective collisions. Hence rate of reaction decreases.

National Junior College SH1 H2 Chemistry 2023 (iii) addition of a catalyst In the presence of catalyst, the reaction proceeds with an alternative reaction pathway of lower activation energy(Ea’). There are more particles with kinetic energy equal or greater than the lowered activation energy as shown in the Boltzmann distribution curve. This result in greater frequency of effective collisions. Hence rate of reaction increases. 2 Which statement best explains why a small increase in temperature leads to a significant increase in the rate of a gaseous reaction? A The frequency of collisions between the molecules is greater at a higher temperature. B The activation energy of the reaction is lower when the gases are at a higher temperature. C The average kinetic energy of the molecules is greater at a higher temperature. D The frequency of effective collisions between molecules with kinetic energy greater than the activation energy is greater at a higher temperature Ans: D When temperature of the reaction increases, average kinetic energy of the reacting molecules increases. The fraction of molecules with K.E ≥ Ea decreases as shown in the Boltzmann distribution. The frequency of effective collisions increases hence rate of reaction increases.

National Junior College SH1 H2 Chemistry 2023 Section A2: Catalysis 3 Which is a correct statement about the effect of a homogeneous catalyst? A It does not appear in the rate equation for the reaction. B It causes the reaction to proceed via a different mechanism. C It increases the rate constant of the forward reaction but not that of the backward reaction. D It increases the kinetic energy of the reactant particles and therefore the frequency of effective collision. Ans: B Due to the involvement of the homogenous catalyst, the alternative pathway will require one additional step to complete the reaction. This is because there can only be a maximum of two colliding particles in a particular step as probability of effective collision between 3 or more particles are hardly likely. 4 Answer the following questions based on the energy profile diagram shown below: (a) Does the graph represent an endothermic or exothermic reaction? The graph represents an endothermic reaction, ∆H = +ve (b) Label the position of the reactants, products, and transition state. (c) Determine the heat of reaction, ΔH, (enthalpy change) for this reaction. ΔH = +50 kJ. (100 kJ – 50 kJ)

National Junior College SH1 H2 Chemistry 2023 (d) Determine the activation energy, Ea(f) for forward reaction. Ea(f) = +200 kJ. Activation energy is the amount of energy required to go from the energy level of the reactants (50 kJ) to the highest energy point on the graph, the activated complex (250 kJ). (e) Determine the activation energy, Ea(b) for backward reaction. Hence explain why this reaction could be reversible. Ea(b) = 150kJ. The 200kJ of activation energy supplied enable both the forward and backward reaction to take place , hence resulting it potentially a reversible reaction. (f) Sketch how the energy profile will change, if any, if a homogeneous catalyst is added to the reaction system. The reaction would become a 2 steps reaction with lowered Ea. Does not matter step 1 or 2 has the lower Ea. ∆H remains unchanged. (g) Describe how a heterogeneous catalyst can increase effective collision between gaseous reactants by referring to the collision theory. The gaseous reactants are adsorbed on the surface of the solid catalyst. This interaction increases the proximity of the reacting gas particles hence increases frequency of collision. This interaction also weakens the existing inter-atomic bonds within the reacting gas molecules thus reduces the activation energy for the reaction. Products, once formed, leave the active sites (desorption), making the active sites available for other gaseous reactants. Thus, effective collision of this reaction increases, leading to an increase in rate of reaction.

National Junior College SH1 H2 Chemistry 2023 5 The reaction represented by the following equation was carried out. MnO4 (aq) + 5 C2O42 (aq) + 16 H+  Mn2+ (aq) + 10 CO2 (g) + 8 H2O (l) Which graph best shows the relationship between volume of CO2 collected over time from mixing of the reactants? A B C D This is an autocatalysis reaction. (Mn2+ is the catalyst), the gradient of vol vs time graph represents the rate of reaction. (1) At the beginning of the reaction, the rate is slow (gentle gradient) because the collision of two negatively charged reactants experiences repulsion making effective collision between the two reactants difficult. Therefore, volume of CO2 produced is increasing at a slower rate. (2) Once a certain amount of Mn 2+ has been formed, the reacti on rate increases (steeper gradient). More CO2 is being produced. (3) Reaction rate falls (gentle gradient) as the concentration of the reactants decreased to a low level. Therefore, the volume of CO2 will remain constant. t Volume of CO2 t Volume of CO2 1 2 3 t Volume of CO2 t Volume of CO2

National Junior College SH1 H2 Chemistry 2023 6 The rate of decomposition of aqueous H2O2 increases in the presence of the enzyme, catalase. 2H2O2 2H2O + O2 The following graph was obtained in a study to investigate the effect of catalase on the rate of decomposition of H2O2. The steps predicted for this enzymatic decomposition of H2O2 are shown below. catalase + H2O2 catalase–H2O2 complex slow step Catalase–H2O2 complex H2O + O2 + catalase fast step Which statements about the enzymatic decomposition of H2O2 are correct? 1 At low [H 2O2], the rate of decomposition increases with increasing concentration of catalase used. 2 Catalase lowers the activation energy of the decomposition by forming temporary bonds with H2O2 molecules. 3 At high [H 2O2], all the active sites of the catalase enzyme are saturated with H2O2 molecules. A 1, 2 and 3 B 1 and 2 only C 2 and 3 only D 1 only Ans: A Slow step involves the reaction between catalase and H 2O2. At low [H2O2], the rate of reaction increases proportionally with H2O2 and catalase concentration. Hence statement 1 is correct. The attractive forces between the reactant (substrate) molecule and the enzyme weakens the bonds within the substrate molecule, thereby lowerin g the activation energy and increasing the rate of reaction. Hence statement 2 is correct. At high [substrate], all