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. The procedure does not mention how a value for the temperature of the mixture during the reaction is obtained. (i) State the temperature measurements that should be taken and at which stage in the procedure they should be taken. ....................................................................................................................................... . ................................................................................................................................. (ii) State how to use the temperature measurements to determine an accurate value for the temperature of the mixture during the reaction. ....................................................................................................................................... . .................................................................................................................................

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