RP 3 Flashcards
What’s the point of RP3?
To investigate how the rate of readtion of sodium thiosulfate with HCl changes as the temperature of the reaction is changed.
Equation of the reaction taking place
Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(g) + S(s)
How can we monitor this reaction?
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 under the bottom of the reaction vessel, to disappear as it is obscured by the sulfur
precipitate.
What is the general method of this experiment?
Dilute hydrochloric acid will be added to sodium thiosulfate solution at different temperatures in
a series of experiments. And the time taken for the cross to dissapear will be measured.
OFFICIAL METHOD (AQA)
a) Add about 10 cm3 of 1 mol dm-3 hydrochloric acid (or 0.5 mol dm-3 sulfuric(VI) acid) to the ‘acid’ tube. Place this tube into the correct hole in the plastic container (ie the one without the cross under it).
b) Use a measuring cylinder to add 10.0 cm3 of 0.05 mol dm-3 sodium thiosulfate solution to the second tube. Place this tube into the correct hole in the plastic container (ie the one with the cross under it) and carefully place a thermometer in this tube.
c) Note the start temperature and then add 1 cm3 of the acid to the thiosulfate solution and start timing.
d) Look down through the tube from above and record the time for the cross to disappear from view.
e) Record the temperature of the reaction mixture. Pour the cloudy contents of the vial into the sodium carbonate solution (the ‘stop bath’).
f) 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. g) Measure another 10.0 cm3 of 0.05 mol dm-3 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). h) Leave the tube to warm up for about 3 minutes.
i) Repeat steps (c) to (e) above.
j) Repeat to obtain results for at least 5 different temperatures in total.
What are the stop baths for?
Help minimise the release of toxic sulfur dioxide fumes.
How are the stop baths used?
● Containers of sodium carbonate solution and phenolphthalein (stop baths) should be available to students so that the acid and sulfur 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.
Safety
● 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.
Data analysis A1
- in this experiement the concentrations of the reactants remain the same
- time taken for same amound of sulfur to be produced is indication of rate
- because the amount of sulfur producded is assumed fixed and constant rate = 1/time (would be g/s)
- so rate = s-1
Data analysis A2
- using Arrhenius equation we can find Ea with our results
- natural log both sides
- ln k = 1/time (rate) because k simply gives a proportionality between the rate and some power of the concentration(s) of the reactant(s) and the conc of reactants is constant.
- so plot graph of 1/t (y) against (1/T) x
- gradient is -Ea/R
Why does rate of reaction increase with increasing temperature?
● As temperature increases, the kinetic energy of the molecules increases. This means more molecules have energy greater than the activation energy and can therefore react.
● Also as kinetic energy of molecules increases, their velocities increase resulting in more frequent and successful collisions.
What are issues with this 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 temperatures the reaction may be too slow.
● There may not be enough precipitate for the cross to disappear.
What are the control variables?
● 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.
