Experimental methods for rate dataEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student investigates the rate of reaction between magnesium ribbon and an excess of dilute hydrochloric acid at constant temperature: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). The student wants to follow the reaction continuously from the moment the magnesium is added.

   (a)

   Which technique is the most suitable for following this reaction continuously?

   [1 mark]

   • AColorimetry, because the reaction mixture is coloured
   • BCollecting the hydrogen in a gas syringe and recording the volume at intervals
   • CMeasuring the mass lost by the reaction mixture on a balance
   • DWithdrawing samples and titrating them without stopping the reaction

   (b)

   The student plots the volume of hydrogen against time. How is the rate of reaction at 30 s found from the plot?

   [1 mark]

   • ADivide the total volume of gas at 30 s by 30
   • BRead the volume of gas at 30 s from the vertical axis
   • CMeasure the area under the curve up to 30 s
   • DDraw a tangent to the curve at 30 s and find its gradient

   (c)

   Explain why measuring the mass lost by the reaction mixture would be an unsuitable way of following this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student follows the reaction between bromine and methanoic acid, Br₂(aq) + HCOOH(aq) → 2Br⁻(aq) + 2H⁺(aq) + CO₂(g), at constant temperature. Bromine solution is orange-brown and the products are colourless. The student uses a colorimeter.

   (a)

   Which colour of filter should be chosen for the colorimeter?

   [1 mark]

   • ABlue
   • BOrange
   • CRed
   • DYellow

   (b)

   What must the student do first so that absorbance readings can be converted into bromine concentrations?

   [1 mark]

   • AZero the colorimeter using the reaction mixture
   • BWeigh the cuvette before and after each reading
   • CPlot a calibration curve of absorbance against concentration using bromine solutions of known concentration
   • DMeasure the pH of the reaction mixture at the start

   (c)

   Describe how the student could use the absorbance readings to decide whether the reaction is first order or zero order with respect to bromine.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In an iodine clock reaction, hydrogen peroxide oxidises iodide ions in acid solution: H₂O₂(aq) + 2I⁻(aq) + 2H⁺(aq) → I₂(aq) + 2H₂O(l). A small, fixed amount of sodium thiosulfate and some starch are added to each mixture. The thiosulfate reduces the iodine back to iodide ions as soon as it forms, until the thiosulfate has been used up, when the solution suddenly turns blue-black. The student varies only the concentration of iodide ions, keeping the total volume constant. When [I⁻] = 0.010 mol dm⁻³ the colour change takes 80 s, when [I⁻] = 0.020 mol dm⁻³ it takes 40 s and when [I⁻] = 0.030 mol dm⁻³ it takes 27 s.

   (a)

   Explain why 1/t can be used as a measure of the initial rate, and deduce the order of reaction with respect to iodide ions.

   [3 marks]

   (b)

   The clock method is only an approximation to the initial-rate method. Explain why, and describe two variables that the student must control.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student follows the reaction CH₃COCH₃(aq) + I₂(aq) → CH₃COCH₂I(aq) + HI(aq), which is catalysed by acid, by a titration method. Propanone, iodine solution and sulfuric acid are mixed in a conical flask and a timer is started. At timed intervals a 10.0 cm³ sample is removed with a pipette and added to excess sodium hydrogencarbonate solution. The iodine in the sample is then titrated with 0.0050 mol dm⁻³ sodium thiosulfate solution. The titres are 20.0 cm³ at time zero, 14.0 cm³ after 6.0 minutes and 8.0 cm³ after 12.0 minutes.

   (a)

   Explain how this method allows the rate to be followed, and how the results would be used to find the order of reaction with respect to iodine.

   [6 marks]

   (b)

   Use the titration data to calculate the concentration of iodine in the 10.0 cm³ sample at each time, deduce the order of reaction with respect to iodine, and calculate the rate of reaction in mol dm⁻³ s⁻¹.

   [6 marks]

   Total for question 4: 12 marks

End of questions