Half-life and experimental techniquesEdexcel International A Level Chemistry: Mind map

Exam questions on Half-life and experimental techniques

1. A student follows the reaction between propanone and iodine in the presence of an acid catalyst: CH₃COCH₃(aq) + I₂(aq) → CH₃COCH₂I(aq) + HI(aq). Iodine solution is brown; the other species are colourless.
   Explain why the volume of sodium thiosulfate solution needed to react with a fixed volume of the quenched mixture is proportional to the concentration of iodine, and state the role of starch in the titration.2 marks
2. The concentration of a reactant Q in solution is measured at intervals at constant temperature. It is 0.640 mol dm⁻³ at 0 s, 0.320 mol dm⁻³ at 90 s, 0.160 mol dm⁻³ at 180 s and 0.080 mol dm⁻³ at 270 s, and the points lie on a smooth curve.
   Deduce the order of reaction with respect to Q, and use your answer to find the time taken for the concentration to fall from 0.320 mol dm⁻³ to 0.040 mol dm⁻³.2 marks
3. A student investigates the reaction H₂O₂(aq) + 2I⁻(aq) + 2H⁺(aq) → I₂(aq) + 2H₂O(l) by an iodine clock method. A small, fixed amount of sodium thiosulfate solution and some starch are added to every mixture. The time t for the blue-black colour of the iodine–starch complex to appear is measured. In experiment 1, [I⁻] = 0.040 mol dm⁻³ and t = 80 s. In experiment 2, [I⁻] = 0.080 mol dm⁻³ and t = 40 s. In experiment 3, [I⁻] = 0.020 mol dm⁻³ and t = 160 s. In all three experiments the concentrations of H₂O₂ and H⁺, the total volume and the temperature are the same.
   Explain how this clock reaction works and why 1/t can be used as a measure of the initial rate.3 marks