practical 7

• In the equation H2O2(aq) + 2H+(aq) + 2I-(aq) -> I2(aq) + 2H2O(l) why would a large excess of H2O2 and I- mean the rate of reaction at a fixed temperature depends only on the concentration of H+(aq)?

  Concentrations of H2O2 and I- are effectively constant so are both zero order
• In the reaction H2O2(aq) + 2H+(aq) + 2I-(aq) -> I2(aq) + 2H2O(l) Samples of the mixture are removed and titrated with alkali to determine the [H+]. State and explain what must be done before it can be titrated.

  The reaction must be stopped by adding a reagent that will react with H2O2/I-
• Why do you need to stir the mixture after starting the clock?

  to ensure all the reactants come in to contact and react (reaction will go to completion).
• What does the thiosulfate do and why is it added last?

  The thiosulfate reacts with the iodine produced in step 1, converting it back to iodide ions, preventing a colour change until the reaction is complete. It must be added last or it would react with sulfuric acid.
• In the iodine clock reaction, why is starch added to the reaction mixture?

  The starch reacts with excess iodine once all the thiosulfate is used up, turning the solution blue-black. This observation shows the end of the reaction.
• How would you change the reaction to determine the order with respect to iodide ions?

  Change the concentration of iodide ions (changing the ratio of water to I-) whilst keeping the concentrations and volumes of the other reactants the same. The total volume should remain constant and it should be carried out at the same temperature.
• How could you determine the order of reactant (A) from a graph of rate against [A]?

  look at the shape of the graph: zero = horizontal line, first = directionally proportional, second = exponential OR use numbers from the graph to compare rates at different concentrations: zero = concentration does not affect the rate, first = concentration is proportional to rate, second = concentration increases rate by concentration squared
• what is the most suitable method to record the volume of gas in the continuous rate method?

  gas syringe ( however water displacement can also be used)
• why does the bung need to be firmly placed into the top of the flask?

  to prevent gas from escaping to ensure the data collected is reliable
• why is the gradient of each tangent used to deduce the rate?

  change in volume / change in time, which is a suitable measure of rate
• describe a method to determine order with respect to I- ions in the iodine clock reaction

  1. rinse pipette with sulfuric acid and then transfer 25cm3 of sulfuric acid to a 250cm3 beaker
  2. add 20cm3 of water
  3. add 1cm3 of starch solution using a dropping pipette
  4. use a burette to add 5cm3 of potassium iodide
  5. add 5cm3 of sodium thiosulfate
  6. add hydrogen peroxide and immediately start the timer and stir
  7. stop the timer when solution turns blue black
  8. repeat with different concentrations of iodide ions
  9. plot graph of concentration against rate (1/t)
• describe a method to determine the rate of a reaction by continuous monitoring method

  1. measure 50cm3 of HCl and add to conical flask
  2. set up gas syringe in stand (or displacement method)
  3. weigh and add 0.2g of magnesium ribbon
  4. place the bung on the beaker firmly and start the timer
  5. record volume of gas produced every 15 seconds for 3 minutes
  6. plot a graph of volume of gas against time
  7. draw tangents and calculate the rate
  8. changing concentration of HCl, allows you to determine order by comparing rates obtained by tangents at t=0