The activation energy, EA, for the reaction between dilute hydrochloric acid, HCl(aq), and aqueous sodium thiosulfate, Na₂S₂O₃(aq), can be determined by an initial rates method. 2HCl(aq) + Na₂S₂O₃(aq) → 2NaCl(aq) + H₂O(l) + S(s) + SO₂(g) The solid sulfur formed is seen as a white suspension in the reaction mixture. The reactants are mixed and the time, t, for a fixed quantity of sulfur to be formed is recorded. A measure of the initial rate of the reaction is 1/t. Standard solutions of 0.100 mol dm⁻³ Na₂S₂O₃(aq) and 0.500 mol dm⁻³ HCl(aq) are supplied. Measurements are taken for a series of temperatures using the following procedure. step 1 A thermostatically controlled water bath is set up. step 2 A 100 cm³ conical flask is labelled A and a second 100 cm³ conical flask is labelled B. step 3 10.00 cm³ of 0.100 mol dm⁻³ Na₂S₂O₃(aq) is added to flask A. Flask A is placed in the water bath. step 4 10 cm³ of 0.500 mol dm⁻³ HCl(aq) is added to flask B. Flask B is placed in the same water bath. step 5 Wait for 10 minutes. step 6 Flask A is removed from the water bath and placed on a tile marked with a black cross. step 7 The contents of flask B are added to flask A and a timer started. step 8 The timer is stopped when the black cross is no longer visible. The time is recorded.

1. (a)Suggest a reason why it is necessary to wait for 10 minutes in step 5. [1][1]
2. (b)The procedure does not mention how a value for the temperature of the mixture during the reaction is obtained.
3. (b)(i)State the temperature measurements that should be taken and at which stage in the procedure they should be taken. [1][1]
4. (b)(ii)State how to use the temperature measurements to determine an accurate value for the temperature of the mixture during the reaction. [1][1]
5. (c)A student carries out the procedure at three different temperatures and records the measurements in Table 2.1. Complete Table 2.1. Record values for temperature to the nearest whole number and the values for 1/t to four decimal places. Table 2.1 | temperature, T/°C | time, t/s | temperature, T/K | 1/t /s⁻¹ | |:---|:---|:---|:---| | 15 | 176 | | | | 24 | 92 | | | | 32 | 62 | | | [2][2]
6. (d)A second student carries out the procedure at six different temperatures and analyses their data to give the results in Table 2.2. Table 2.2 | 1/T /K⁻¹ | log (1/t) | |:---|:---| | 0.00353 | -2.43 | | 0.00336 | -1.99 | | 0.00325 | -1.68 | | 0.00314 | -1.47 | | 0.00302 | -1.21 | | 0.00287 | -0.82 |
7. (d)(i)Use the results from Table 2.2 to plot a graph on the grid in [Figure 2.1] to show the relationship between log (1/t) and 1/T. Use a cross (x) to plot each data point. Draw a line of best fit. [2][2]
8. (d)(ii)Determine the gradient of your line of best fit in [Figure 2.1]. State the coordinates of both points you use in your calculation. These must be selected from your line of best fit. Give the gradient to three significant figures. coordinates 1 coordinates 2 gradient = ... K [2][2]
9. (d)(iii)An equation relating time and temperature variables is shown. log(1/t) = – 0.434EA / RT + constant Determine the activation energy, EA, of this reaction using this equation and your answer to (d)(ii). (If you were unable to find the gradient in (d)(ii), then use the value –3.21 × 10³ K. This is not the correct answer.) Include units in your answer. Show your working. EA= units [3][3]
10. (d)(iv)Use your graph to state whether the results from the experiment are reliable. Justify your answer. [1][1]
11. (e)Suggest a change to one controlled variable that the student could make so that the time measured for a given temperature is shorter. [1][1]

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