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 alte