Half-life and experimental techniquesEdexcel International A Level Chemistry: Subtopic test

1. 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.

   (a)

   Which technique allows the reaction to be monitored continuously without removing samples from the mixture?

   [1 mark]

   • ATitrating samples with sodium thiosulfate solution
   • BColorimetry
   • CMeasuring the volume of gas given off
   • DMeasuring the loss in mass of the mixture

   (b)

   In a titrimetric method, a sample (aliquot) is removed from the mixture at intervals and added to excess sodium hydrogencarbonate solution before it is titrated with sodium thiosulfate solution. Why is this done?

   [1 mark]

   • ATo dissolve the iodine so that it can be titrated
   • BTo oxidise iodide ions to iodine
   • CTo increase the rate so that the reaction goes to completion
   • DTo neutralise the acid catalyst, which stops the reaction

   (c)

   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]

   Total for question 1: 4 marks

2. 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.

   (a)

   What is the half-life of Q?

   [1 mark]

   • A90 s
   • B180 s
   • C45 s
   • D270 s

   (b)

   What is the concentration of Q at 360 s, assuming the pattern continues?

   [1 mark]

   • A0.020 mol dm⁻³
   • B0.030 mol dm⁻³
   • C0.040 mol dm⁻³
   • D0.060 mol dm⁻³

   (c)

   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]

   Total for question 2: 4 marks

3. 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.

   (a)

   Explain how this clock reaction works and why 1/t can be used as a measure of the initial rate.

   [3 marks]

   (b)

   Calculate 1/t for each experiment, deduce the order of reaction with respect to I⁻, and state the shape of a graph of 1/t against [I⁻] that would confirm your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student must choose a method to follow the rate of each of three reactions. Reaction 1: magnesium with dilute sulfuric acid, which gives off hydrogen gas. Reaction 2: acidified potassium manganate(VII) with ethanedioic acid, in which the purple solution slowly becomes colourless. Reaction 3: alkaline hydrolysis of ethyl ethanoate, CH₃COOC₂H₅ + OH⁻ → CH₃COO⁻ + C₂H₅OH, in which hydroxide ions are replaced by ethanoate ions.

   (a)

   For each of the three reactions, suggest a suitable technique for following its rate continuously and justify your choice.

   [6 marks]

   (b)

   In a further experiment on the iodination of propanone in acid, 10.0 cm³ samples were removed at 4-minute intervals, quenched with sodium hydrogencarbonate solution and titrated with 0.0100 mol dm⁻³ sodium thiosulfate solution. The titres were 24.0 cm³ at 0 min, 19.2 cm³ at 4 min, 14.4 cm³ at 8 min and 9.6 cm³ at 12 min. Calculate the initial concentration of iodine, calculate the rate of reaction in mol dm⁻³ s⁻¹, and deduce the order of reaction with respect to iodine, justifying your answer.

   [6 marks]

   Total for question 4: 12 marks

End of questions