An experiment is carried out to determine the rate constant, k, for the hydrolysis of ethyl ethanoate, CH3COOC2H5, using a hydrochloric acid, HCl(aq), catalyst. The equation for the reaction is shown. H⁺ CH3COOC2H5 + H2O → CH3COOH + C2H5OH The rate equation for this reaction is shown. rate of reaction = k [CH3COOC2H5] The progress of the reaction is followed by determining how the concentration of acid changes with time. A portion of the reaction mixture is removed every 5 minutes and titrated with sodium hydroxide, NaOH(aq). A final titration is carried out after 180 minutes. A student carries out the following steps. step 1 Add 70cm³ of iced water to seven separate small conical flasks. Add a few drops of phenolphthalein indicator to each flask. Phenolphthalein is pink in alkaline conditions and colourless in acidic conditions. step 2 Use a measuring cylinder to transfer 100cm³ of 0.200 moldm⁻³ HCl(aq) into a large conical flask. step 3 Add 5.00 cm³ of CH3COOC2H5 to the large conical flask and swirl the flask to mix the contents. Start a stopwatch. step 4 Transfer 10.00cm³ of reaction mixture to one of the small conical flasks containing the iced water and indicator. Record the time. Shake the small flask. step 5 Carry out a single titration of the mixture in the small conical flask using 0.15moldm⁻³ NaOH(aq). step 6 Repeat steps 4 and 5 at the times shown in Table 2.1 using a different small conical flask for each titration.

1. (a)Give two reasons that explain why the use of iced water in step 4 decreases the rate of reaction. reason 1 ................................................................................................................................................. reason 2 .................................................................................................................................................[2]
2. (b)Describe the observation that is used to determine the end-point of the titrations.[1]
3. (c)(i)The results of the experiment are shown in Table 2.1. Vₜ is the titre at a given time, t. V∞ is the final titre at t = 180 min when the reaction is assumed to be complete. Table 2.1 1 2 3 4 time, t/min titre, Vₜ/cm³ (V∞ – Vₜ)/cm³ log[(V∞ – Vₜ)/cm³] 0 12.00 5 16.40 10 22.15 15 23.45 20 26.30 25 28.70 180 45.70 Complete Table 2.1. Give your answers in column 3 to two decimal places and your answers in column 4 to four significant figures.[2]
4. (c)(ii)Plot a graph on the grid in [Figure 2.1] to show the relationship between log[(V∞ – Vₜ)/cm³] and time, t. Use a cross (x) to plot each data point. Draw a straight line of best fit.[2]
5. (c)(iii)Circle the one point on the graph that you consider to be most anomalous. Suggest one reason to explain the anomalous point you have circled. Assume no error was made in the experimental value of the titre.[1]
6. (c)(iv)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 = ...................................... min⁻¹[2]
7. (d)The equation of the straight line plotted in [Figure 2.1] is shown. log[(V∞ – Vₜ)/cm³] = – kt / 2.303 + constant Use the gradient determined in (c)(iv) to calculate a value for k in s⁻¹. [If you were unable to determine an answer to (c)(iv), then use the value –0.0312 min⁻¹ for the gradient. This is not the correct answer.] k = ...................................... s⁻¹[2]
8. (e)Use your graph in [Figure 2.1] to state whether you consider the results to be reliable. Give a reason for your answer.[1]
9. (f)k increases with temperature. A second experiment is carried out at a higher temperature. Sketch a suggested line of best fit on [Figure 2.1] for the second experiment.[1]

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