6 Required practical 3: Investigation of how the rate of a reaction changes with temperature

A-level Chemistry exemplar for required practical No. 3 Investigation of how the rate of a reaction changes with temperature: To investigate how the rate of the reaction of sodium thiosulfate with hydrochloric acid changes as the temperature of the reaction is changed. Introduction Sodium thiosulfate reacts with hydrochloric acid according to the equation Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(g) + S(s) The reaction produces a precipitate of sulfur. The rate of this reaction can be monitored by measuring the time taken for a fixed amount of sulfur to be produced. An easy method to do this is by timing how long it takes for a cross, marked on the bottom of the reaction vessel, to disappear as it is obscured by the sulfur precipitate. Dilute hydrochloric acid will be added to sodium thiosulfate solution at different temperatures in a series of experiments. This table shows the approximate temperatures for five experiments. Experiment 1 2 3 4 5 Approximate temperature / °C room* ~25 ~35 ~45 ~55** [* The temperature of the room is likely to be 15 to 18 °C] [** The temperature must not exceed 55 °C] It is not necessary for these exact temperatures to be used although the temperature used must not exceed 55 °C. However, the actual temperature at which each experiment is carried out must be known as accurately as possible. One way that this can be achieved is to measure both the initial temperature and the final temperature and then use a mean temperature when plotting your graph. Requirements In addition to general laboratory apparatus, each student needs:  thermometer (–10 °C to 110 °C)  400 cm3 beaker (for use as a water bath)  plastic container with lid  2 glass tubes to hold 12–14 cm3 of liquid  0.05 mol dm–3 (or 40 g dm –3 ) sodium thiosulfate solution  1.0 mol dm–3 hydrochloric acid (or 0.5 mol dm–3 sulfuric acid)  10 cm3 measuring cylinder  plastic graduated pipette  stopwatch  graph paper. A lid is advised in this experiment. Two holes should be made in the lid using a hot wide cork borer. These holes should securely hold the glass tubes and vertically in the plastic container. A cross should be marked on the inside base of the plastic container below one of the larger holes using a permanent black marker pen. Alternatives include using clear A4 plastic wallets to use as a lid over the plastic container. Also, rather than marking the bottom of the plastic container directly, a laminated sheet marked with a black cross could be used Caution: the CLEAPSS Hazcard states ‘sulfur dioxide is produced in this reaction’ and ‘known sufferers of asthma should be closely monitored’. Centres are advised to ensure that the investigation is carried out in a well-ventilated room and that appropriate measures are taken to dispose of waste solutions. Stop baths – containers of sodium carbonate solution and phenolphthalein should be available to students so that the acid and sulfur dioxide can be neutralised (immediately, if required, during the practical and) after the experiment has finished. Once the colour of the solution in the stop bath changes, the sodium carbonate has been used up and the stop bath will need to be replenished. The stop bath should be placed in a fume cupboard, if available. Analysing the data In these experiments at different temperatures, the concentrations of all the reactants are the same. You are investigating the time taken to produce the same amount of sulfur at different temperatures. If you were to plot a graph of the amount of sulfur produced against time, it would initially be a straight line because the reaction has only just started. Therefore, the initial rate of reaction = (amount of sulfur)/time so the initial rate of reaction is proportional to 1/time ( 1 ). AS analysis  calculate the mean temperature of each reaction mixture  for each of the five temperatures, calculate 1 to 3 significant figures, where t is the time taken for the cross to be obscured  plot a graph of 1 on the y-axis against average temperature  the plotting of the points may be more straightforward if you multiply all of the values for 1 t by a common factor (eg 104 ).