The activation energy, EA, for the reaction between dilute hydrochloric acid, HCl(aq), and aqueous sodium thiosulfate, Na₂S₂O₃(aq), can be determined by an initial rates method. 2HCl(aq) + Na₂S₂O₃(aq) → 2NaCl(aq) + H₂O(l) + S(s) + SO₂(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 1/t. Standard solutions of 0.100 mol dm⁻³ Na₂S₂O₃(aq) and 0.500 mol dm⁻³ HCl(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 cm³ conical flask is labelled A and a second 100 cm³ conical flask is labelled B. step 3 10.00 cm³ of 0.100 mol dm⁻³ Na₂S₂O₃(aq) is added to flask A. Flask A is placed in the water bath. step 4 10 cm³ of 0.500 mol dm⁻³ HCl(aq) is added to flask B. Flask B is placed in the same water bath. step 5 Wait for 10 minutes. step 6 Flask 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 A and a timer started. step 8 The timer is stopped when the black cross is no longer visible. The time is recorded.