A student investigates the effect of concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid at 20∘C20^\circ\text{C}20∘C using the "disappearing cross" method. The equation for the 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 trial, the student mixes different volumes of 0.15 mol/dm3 sodium thiosulfate solution, distilled water, and 10 cm3 of 2.0 mol/dm3 hydrochloric acid. They add water to ensure the total volume of the reaction mixture is kept constant at 60 cm3.
| Trial | Volume of 0.15 mol/dm3 Na2S2O3\text{Na}_2\text{S}_2\text{O}_3Na2S2O3 solution (cm3\text{cm}^3cm3) | Volume of distilled water (cm3\text{cm}^3cm3) | Volume of 2.0 mol/dm3 HCl\text{HCl}HCl (cm3\text{cm}^3cm3) | Time taken for cross to disappear (s) |
|---|---|---|---|---|
| 1 | 50 | 0 | 10 | 40 |
| 2 | 30 | 20 | 10 | 72 |
| 3 | 20 | 30 | 10 | 110 |
Explain how keeping the total volume of the reaction mixture constant makes each experiment a fair test.
Calculate the concentration of sodium thiosulfate, in mol/dm3\text{mol/dm}^3mol/dm3, in the mixture of Trial 2 before the reaction starts. Show your working.
Suggest a safety hazard associated with the gaseous product in this reaction, and name one precaution needed to address this hazard.
