The activation energy, $$\(E_{A}\)$$, for the reaction between dilute hydrochloric acid, $$\(\mathrm{HCl}(\mathrm{aq})\)$$, and aqueous sodium thiosulfate, $$\(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})\)$$, can be determined by an initial rates method. $$\[ 2 \mathrm{HCl}(\mathrm{aq})+\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq}) \rightarrow 2 \mathrm{NaCl}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{S}(\mathrm{s})+\mathrm{SO}_{2}(\mathrm{~g}) \]$$ The solid sulfur formed is seen as a white suspension in the reaction mixture. The reactants are mixed and the time, $$\(t\)$$, for a fixed quantity of sulfur to be formed is recorded. A measure of the initial rate of the reaction is $$\(\frac{1}{t}\)$$. Standard solutions of $$\(0.100 \mathrm{moldm}^{-3} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})\)$$ and $$\(0.500 \mathrm{moldm}^{-3} \mathrm{HCl}(\mathrm{aq})\)$$ are supplied. Measurements are taken for a series of temperatures using the following procedure. step 1 A thermostatically controlled water bath is set up. step 2 A $$\(100 \mathrm{~cm}^{3}\)$$ conical flask is labelled $$\(\mathbf{A}\)$$ and a second $$\(100 \mathrm{~cm}^{3}\)$$ conical flask is labelled B. step $$\(310.00 \mathrm{~cm}^{3}\)$$ of $$\(0.100 \mathrm{moldm}^{-3} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}(\mathrm{aq})\)$$ is added to flask $$\(\mathbf{A}\)$$. Flask $$\(\boldsymbol{A}\)$$ is placed in the water bath. step $$\(410 \mathrm{~cm}^{3}\)$$ of $$\(0.500 \mathrm{moldm}^{-3} \mathrm{HCl}(\mathrm{aq})\)$$ is added to flask $$\(\mathbf{B}\)$$. Flask $$\(\mathbf{B}\)$$ is placed in the same water bath. step 5 Wait for 10 minutes. step 6 Flask $$\(\mathbf{A}\)$$ is removed from the water bath and placed on a tile marked with a black cross. step 7 The contents of flask B are added to flask $$\(\mathbf{A}\)$$ and a timer started. step 8 The timer is stopped when the black cross is no longer visible. The time is recorded. Suggest a change to one controlled variable that the student could make so that the time measured for a given temperature is shorter. ............................................................................................................................................... . ......................................................................................................................................... [Total: 14$$\(]\)$$

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