7 - initial rate method

Equations for iodine clock experiment

H2O2 (aq) + 2H+ (aq) + 2I- (aq) → I2 (aq) + 2H2O (l)

2S2O3 2- (aq) + I2 (aq) → 2I- (aq) + S4O6 2- (aq)

Explain what is happening in the iodine clock reaction

• Hydrogen peroxide reacts with iodide ions to form iodine

• Thiosulfate ions immediately react with iodine

• When iodine has reacted with all the limited amount of thiosulfate ions, excess iodine remains in solution

• Excess iodine reacts with starch which forms a blue-black colour

Describe method for iodine clock reaction

1. Put the required volumes of potassium iodide, sodium thiosulfate, starch and water into a conical flask (all concentrations remain constant except conc of potassium iodide)

2. Pour hydrogen peroxide into conical flask and start timer

3. Stir

4. Time until blue black colour appears

5. Repeat with different concentrations of potassium iodide (by varying the volumes of potassium iodide and water)

6. Work out order of reaction with respect to I-

Issues with iodine clock experiment

• Some low I- concentrations may take too long to react

• Delayed stopwatch reactions

• Concentrations may not be exact due to measuring apparatus

Method for measuring the rate of reaction by a continuous monitoring method

1. Measure 50cm3 of 1.0moldm-3 HCl and add to conical flask

2. Set up gas syringe

3. Weigh 0.20g magnesium

4. Add the magnesium ribbon to the conical flask, place the bung firmly into the top of the flask and start the timer

5. Record volume of hydrogen gas produced every 15 secs for 3 mins

6. Repeat for different HCl concentrations

Issues with continuous monitoring method

• Some gas may escape before bung is added

• Mg strips may be of different mass or surface area