RP7-a-Measuring the initial rate of a reaction by an initial rate method

Question 1

Iodine Clock Required practical 7a

Question 2

Describe what happens in this reaction?

Answer 2

Hydrogen peroxide reacts with iodide ions to form iodine and the thiosulfate ion immediately reacts with iodine:
H2O2(aq) + 2H+(aq) + 2I-(aq) → I2(aq) +2H2O(l)
2S2O32-(aq) + I2(aq) → 2I-(aq) + S4O62-(aq)

Question 3

Describe how this practical would be carried out?

Answer 3

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.
2. Use the burette to add 5.0 cm3 of potassium iodide solution to the mixture in the 250 cm3 beaker.
3. Finally, add 5.0 cm3 of sodium thiosulfate solution from a burette to the mixture in the 250 cm3 beaker.
4. 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.
5. Stop the timer when the mixture in the 250 cm3 beaker turns blue-black. Record the time.
6. Repeat steps in four further experiments changing the concentration of
   potassium iodide.
7. Plot a graph of initial rate (y) versus concentration (x) to determine the order.

Question 4

how can we improve accuracy of this reaction?

Answer 4

use a colorimeter to minimise human errors

Question 5

From these results what must need to be calculated in order to determine the order?

Answer 5

• calculate the moles of potassium iodine in each reaction
• Then calculate the final concentration of potassium iodide in the solution (Concentration = Moles/ total volume of all reactants)
• Then we can calculate the rate of reaction by Rate = 1/t
• We can then plot our results on a graph and determine the order

Question 6

Finished.