An A Level chemistry student investigates the kinetics of the reaction between sodium thiosulfate and hydrochloric acid at a constant temperature of 25∘C25^\circ\text{C}25∘C using the "disappearing cross" technique. The chemical equation representing the precipitation reaction is:
Na2S2O3(aq)+2HCl(aq)→2NaCl(aq)+H2O(l)+SO2(g)+S(s) \text{Na}_2\text{S}_2\text{O}_3(\text{aq}) + 2\text{HCl}(\text{aq}) \rightarrow 2\text{NaCl}(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{SO}_2(\text{g}) + \text{S}(\text{s}) Na2S2O3(aq)+2HCl(aq)→2NaCl(aq)+H2O(l)+SO2(g)+S(s)In each run of the experiment, the student combines variable volumes of 0.20 mol/dm30.20\text{ mol/dm}^30.20 mol/dm3 aqueous sodium thiosulfate and deionised water with a fixed volume of 5 cm35\text{ cm}^35 cm3 of 1.0 mol/dm31.0\text{ mol/dm}^31.0 mol/dm3 hydrochloric acid. The volume of deionised water added is varied to maintain a constant total volume of 50 cm350\text{ cm}^350 cm3 for each reaction mixture.
| Run | Volume of 0.20 mol/dm30.20\text{ mol/dm}^30.20 mol/dm3 Na2S2O3(aq)\text{Na}_2\text{S}_2\text{O}_3\text{(aq)}Na2S2O3(aq) (cm3\text{cm}^3cm3) | Volume of deionised water (cm3\text{cm}^3cm3) | Volume of 1.0 mol/dm31.0\text{ mol/dm}^31.0 mol/dm3 HCl(aq)\text{HCl(aq)}HCl(aq) (cm3\text{cm}^3cm3) | Time taken for cross to be obscured (s) |
|---|---|---|---|---|
| 1 | 45 | 0 | 5 | 28 |
| 2 | 25 | 20 | 5 | 52 |
| 3 | 15 | 30 | 5 | 88 |
Explain how maintaining a constant total volume for the reaction mixture ensures a fair test across all three runs.
Calculate the concentration of sodium thiosulfate, in mol/dm3\text{mol/dm}^3mol/dm3, in the reaction mixture of Run 2 prior to the start of the reaction. Show your working.
Identify one specific safety hazard associated with a gaseous product of this reaction and suggest an appropriate practical precaution to minimise this risk.
