Required Practical 3

• describe a method to measure rate of reaction

  1. Add about 10 cm of 1 moldm-3 hydrochloric acid to the 'acid' tube. Place this tube into a plastic container (without the cross under it).

  2. Use a measuring cylinder to add 10.0 cm of 0.05 moldm-3 sodium thiosulfate solution to the second tube. Place this tube into the plastic container with the cross under it and carefully place a thermometer in this tube. 3. Record the start temperature and then add 1 cm of the acid to the thiosulfate solution and start timing.

  4. Look down through the tube from above and record the time for the cross to disappear from view.

  5. Record the final temperature of the reaction mixture, then pour the cloudy contents of the tube into the sodium carbonate solution.

  6. Now add water from a very hot water tap (or kettle) to the plastic container. The water should be no hotter than 55 °C. Add cold water if necessary.

  7. Measure another 10.0 cm of 0.05 mol dm sodium thiosulfate solution into a clean tube. Insert this tube into the correct hole in the plastic container (ie the one with the cross under it).

  8. Leave the tube to warm up for about 3 minutes.

  9. Repeat steps (3) to (6) in order to obtain results for at least 5 different temperatures in total.
• what are some safety precautions to be taken?

  ● To minimise the escape of toxic sulfur dioxide during the experiment a lid is advised. 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. Could also perform the experiment in a fume cupboard.

  ● Wear eye protection, a lab coat and gloves as HCl is an irritant.

  ● Ensure that the investigation is carried out in a well-ventilated room and that appropriate measures are taken to dispose of waste solutions.
• why is a stop bath used?

  ● Containers of sodium carbonate solution and phenolphthalein (stop baths) should be available to students so that the acid and sulphur dioxide can be neutralised at any point during the experiment.

  ● 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.
• how do you analyse the date produced?

  ● In these experiments at different temperatures, the concentrations of all the reactants are the same. ● The time taken to produce the same amount of sulfur at different temperatures is an indication of rate of the reaction.

  ● A graph of the amount of sulfur produced against time can be plotted.

  ● The initial rate of reaction = (amount of sulfur)/time so the initial rate of reaction is proportional to 1/time.

  ● This is an approximation for rate of reaction as it does not include concentration. This can be used because it is assumed that the amount of sulphur produced is fixed and constant.
• What are some issue with with the disappearing cross method?

  - the disappeared cross is judged by eye and can therefore be unreliable

  - there may be a delayed reaction time with the stopwatch

  - at low temperature the reaction may be too slow

  - there may not be enough precipitate for the cross to disappear
• In an experiment investigating the effect of temperature on rate of reaction, what variables will you control?

  - Concentrations of reactants and products.

  - Volumes of reactants and products.

  - Use the same beaker and cross underneath.

  - Use the same person to decide when the cross has disappeared.
• How can we keep a substance at a constant temperature?

  Using a water bath keeps the temperature constant. For this experiment you can set up water baths of different temperatures to see how it affects the rate of reaction.