Practical 7

• State the observation when excess iodine reacts with starch.

  Blue/ black colour
• Explain why water is added to the potassium iodide solution.

  To ensure the total volume remains constant. Therefore, the volume of potassium iodide is proportional to the concentration of potassium iodide.
• Outline a method to determine the order of reaction with respect to iodide ions.

  > In one conical flask, add 25 cm3 of 0.25 M sulfuric acid, 1 cm3 of starch, 5 cm3 of 0.05 M sodium thiosulfate and 10 cm3 of 0.10 M hydrogen peroxide

  > In another conical flask, add 25 cm3 of potassium iodide

  > Place both conical flasks in a water bath to ensure the temperature is constant

  > Add the contents of one flask to the other and immediately start the timer
• Outline a method to determine the order of reaction with respect to iodide ions. pt2

  > Stop the timer when the blue/black colour appears and record the time in a table

  > Repeat the experiment using the same volume and concentration of all the reactants apart from the potassium iodide. To change the concentration of potassium iodide, water is added to keep the total volume constant.
• Outline a method to determine the order of reaction with respect to iodide ions. pt3

  > For each experiment, calculate rate. Rate = 1/t

  > Calculate log(volume of KI) and log(1/t)

  > Plot a graph of log(1/t) on the y axis against log (volume of KI) on the x axis

  > Calculate the gradient of the graph which will give the order with respect to iodide ions