RP 7

Question 1

What is the initial rate method?

Answer 1

Iodine Clock experiment

Question 2

How does the iodine clock experiment work?

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)
● S2O32- ions are used to remove iodine as it forms.

Question 3

Method iodine clock

Answer 3

Fill the 50 cm 3 burette with potassium iodide solution.

Transfer 10.0 cm 3 of hydrogen peroxide solution from a burette to a 100 cm 3 beaker

Use a 50 cm 3 measuring cylinder to add 25 cm 3 of sulfuric acid to a 250 cm 3 beaker.

Use a 25 cm 3 measuring cylinder to add 20 cm 3 of distilled (deionised) water into the 250 cm 3 beaker.

Use a plastic dropping pipette to add about 1 cm 3 of starch solution to this beaker.

Use the burette to add 5.0 cm 3 of potassium iodide solution to the mixture in the 250 cm 3 beaker.

Finally, add 5.0 cm 3 of sodium thiosulfate solution from a burette to the mixture in the 250 cm 3 beaker.

Stir the mixture in the 250 cm 3 beaker. Pour the hydrogen peroxide solution from the 100 cm 3 beaker into the 250 cm 3 beaker and immediately start the time

Stop the timer when the mixture in the 250 cm 3 beaker turns blue-black. Record the time.

Rinse the 250 cm 3 beaker with distilled (deionised) water and dry it with a paper towel.

Repeat steps in four further experiments changing the concentration of potassium iodide.

Plot a graph of initial rate (y) versus concentration (x) to determine the order.

Question 4

Improvements

Answer 4

● Use a colorimeter to minimise human error in timing.

Question 5

Continuous method of monitoring

Answer 5

1. Add 50 cm 3 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.
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).