Required Practical 7

• describe the initial rate method - iodine clock

  1. Fill the 50 cm3 burette with potassium iodide solution.

  2. Transfer 10.0 cm3 of hydrogen peroxide solution from a burette to a 100 cm3 beaker 3. Use a 50 cm3 measuring cylinder to add 25 cm3 of sulfuric acid to a 250 cm3 beaker.

  4. Use a 25 cm3 measuring cylinder to add 20 cm3 of distilled (deionised) water into the 250 cm3 beaker.

  5. Use a plastic dropping pipette to add about 1 cm3 of starch solution to this beaker.

  6. Use the burette to add 5.0 cm3 of potassium iodide solution to the mixture in the 250 cm3 beaker.

  7. Finally, add 5.0 cm3 of sodium thiosulfate solution from a burette to the mixture in the 250 cm3 beaker.

  8. Stir the mixture in the 250 cm3 beaker. Pour the hydrogen peroxide solution from the 100 cm3 beaker into the 250 cm3 beaker and immediately start the timer.

  9. Stop the timer when the mixture in the 250 cm3 beaker turns blue-black. Record the time. 10. Rinse the 250 cm3 beaker with distilled (deionised) water and dry it with a paper towel.

  11. Repeat steps in four further experiments changing the concentration of potassium iodide. This will allow the order of reaction to be determined.

  12. Plot a graph of initial rate (y) versus concentration (x) to determine the order.
• describe a continuous monitoring method

  1. Add 50 cm3 of 0.8 moldm-3 hydrochloric acid to a conical flask.

  2. Set up the gas syringe or alternative gas collection equipment.

  3. Add a 6 cm strip of magnesium ribbon to the conical flask, place the bung firmly into the top of the flask and start the timer. Swirl the flask every few seconds.

  4. Record the volume of hydrogen gas collected every 15 seconds for 2.5 minutes.

  5. Alter the concentration of HCl and repeat steps (1) to (4).
• what should you consider in this experiment?

  ● A typical gas syringe only measures 100 cm3 of gas so you don't want a reaction to produce more than this volume. Quantities of reactants need to be calculated carefully.

  ● Measuring initial rate is preferential as the concentrations is known at the start of the reaction.

  ● In reactions where there are several reactants, if the concentration of one of the reactant is kept in a large excess then that reactant will appear not to affect rate and will be essentially zero order . This is because its concentration stays virtually constant and does not affect rate.
• how do you analyse the results?

  ● Plot a graph of volume of hydrogen produced on the y-axis against time in seconds for each hydrochloric acid concentration. Draw a line of best fit.

  ● Draw a tangent to each line of best fit at time, t = 0 s.

  ● Calculate the gradient of each tangent in order to deduce the initial rate of each reaction at each concentration.

  ● Compare the rate values obtained.
• What are some issues with an initial rates method?

  - some low i- concentrations may take too long to react

  - delayed stopwatch reactions

  - concentrations may not be exact due to measuring apparatus
• What are some issues with a continuous rate method?

  - some gas may escape before the bung is added

  - the magnesium strips may be off different mass and surface area etc. which will affect the rate of reaction
• How do you prevent gas escaping?

  Place the solid reactant upright inside a sample tube in the conical flask, tipping the tube over by moving the conical flask around to start the reaction.