PAG 10 - Rates Of Reaction - Intial Rates

Question 1

What’s a clock reaction

Answer 1

Reaction in which conc of reagent quickly changes after certain time period, causing colour change

Question 2

Describe method to find order of reaction with respect to I- ions in iodine clock reaction

Answer 2

1. Mix potassium iodide, sodium thiosulfate and starch solutions in a conical flask
2. Add potassium peroxodisulphate and start the timer
3. Record the time taken for the solution to turn blue-black
4. Repeat steps 1-3 with varying concentrations of KI

Question 3

How can order of a reactant be deduced from reaction rate

Answer 3

Reaction rate is proportional to 1/ time

Use changes in reaction rate and concs to deduce order with respect to each reagent

Question 4

Write equations for reaction of potassium iodide and potassium peroxodisulphate

Answer 4

Potassium peroxdisulphate + potassium iodide
—> potassium sulphate + iodine

K2S2O8 + 2KI —> 2K2SO4 + I2

Question 5

How can initial rate be calculated for iodine clock reaction

Answer 5

Mol of iodine (mol)
———————————————
Time taken for colour change (s)

Question 6

Give 2 sources of error in I2 clock reaction

Answer 6

• Inaccurate timing of the appearance of blue colour:
→ Could have two students timing the experiment and calculate an average value.

• Adding starch slightly increases the volume which affects the concentrations of the reactants and thus the amount they change over time.

Question 7

Suggest control variables for this experiment

Answer 7

• Total volume of reactants

• Volume of starch and thiosulfate used

• The concentrations of all the reagents (except potassium iodide solution)

• Temperature (as this can also affect the rate of reaction)

Question 8

Write rate equation for reaction of iodide ion and peroxdisulphate ions

Answer 8

Rate = k[I-][S2O8^2-]

Question 9

What’s overall order of reaction between iodide ions and peroxodisulphate ions

Answer 9

2