HCI 2021 Prelim Paper 3 Solutions

Updated 27 Sep 2021 2021 HCI C2 H2 Chemistry Prelim / Paper 3 1 1 (a) [1] – correctly drawn curve When temperature is increased, the reactant particles have greater average kinetic energy. They move more quickly and so collide more frequently. The proportion of particles that have kinetic energies greater than or equal to the activation energy increases. [1] Frequency of effective collisions hence rate of reaction and rate constant increases. [1] The question requires that you use the Boltzmann distribution to explain why an increase in temperature will affect the rate constant of a reaction. In order to do this, you must sketch the Boltzmann distribution curve and show how the kinetic energy of the particles in the reaction mixture changes with an increase in temperature. When drawing the graph, please ensure that the axes label are correct. Also, ensure that the shape of the curve is correct where it flattens out at higher kinetic energy. It is not sufficient to state in your answer that the average kinetic energy of the reactant particles increase. In order for the rate of the reaction to increase, the particles need to have kinetic energy greater than or equal to activation energy. This phrase is essential. Because of this, we can then conclude that the frequency of effective collision between the reactant particles increase and hence the rate of reaction would increase. For this question, we ask how temperature would affect the rate constant of the reaction. You need to explicitly tell us that the rate constant will increase with increasing temperature. HWA CHONG INSTITUTION 2021 C2 H2 CHEMISTRY PRELIM PAPER 3 SUGGESTED SOLUTIONS

Updated 23 Sep 2021 2021 HCI C2 H2 Chemistry Prelim / Paper 3 2 (b) (i) 1 mark for each step (deduct ½ for missing arrows, partial charges, lone pairs, slow fast step) The question states that Reaction 1 follows the SN1 mechanism. As such, it is not necessary to name the mechanism. Please ensure that the arrows that represent the movement of electrons start from a bond or a lone pair, and end at the atom where the new bond is formed. (ii) Rate = k[C6H5CHBrCH3] [1] For the SN1 mechanism, the only reactant in the rate determining step is the halogen derivative. Please ensure that the name of the compound is correct if you choose to use the name of the compound when writing the rate equation. (iii) Order of reaction with respect to a reactant is the power on its concentration term in the experimentally determined rate equation. [1] This is a definition question. Please study your definitions. (iv) [1] Show constant half-life on the graph [1]

Updated 27 Sep 2021 2021 HCI C2 H2 Chemistry Prelim / Paper 3 3 Based on the design of the experiment given in the question, the variables being monitored are the concentration of the bromoalkane at various times. As such