Experimental methods for rate dataEdexcel A-Level Chemistry: Mind map

Exam questions on Experimental methods for rate data

1. A student investigates the rate of reaction between magnesium ribbon and an excess of dilute hydrochloric acid at constant temperature: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). The student wants to follow the reaction continuously from the moment the magnesium is added.
   Explain why measuring the mass lost by the reaction mixture would be an unsuitable way of following this reaction.2 marks
2. A student follows the reaction between bromine and methanoic acid, Br₂(aq) + HCOOH(aq) → 2Br⁻(aq) + 2H⁺(aq) + CO₂(g), at constant temperature. Bromine solution is orange-brown and the products are colourless. The student uses a colorimeter.
   Describe how the student could use the absorbance readings to decide whether the reaction is first order or zero order with respect to bromine.2 marks
3. In an iodine clock reaction, hydrogen peroxide oxidises iodide ions in acid solution: H₂O₂(aq) + 2I⁻(aq) + 2H⁺(aq) → I₂(aq) + 2H₂O(l). A small, fixed amount of sodium thiosulfate and some starch are added to each mixture. The thiosulfate reduces the iodine back to iodide ions as soon as it forms, until the thiosulfate has been used up, when the solution suddenly turns blue-black. The student varies only the concentration of iodide ions, keeping the total volume constant. When [I⁻] = 0.010 mol dm⁻³ the colour change takes 80 s, when [I⁻] = 0.020 mol dm⁻³ it takes 40 s and when [I⁻] = 0.030 mol dm⁻³ it takes 27 s.
   Explain why 1/t can be used as a measure of the initial rate, and deduce the order of reaction with respect to iodide ions.3 marks