PAG 10 - Rates of Reaction - Initial Rates Method

Question 1

What is a ‘clock reaction’?

Answer 1

A reaction in which the concentration of a
reagent quickly changes after a certain
time period, causing a colour change

Question 2

Briefly describe a method that could be
followed to find the order of reaction with
respect to iodide ions in the 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 the order of a reactant be

deduced from the rate of reaction?

Answer 3

The rate of reaction is proportional to
1/time. Use changes in the rate of
reaction and concentrations to deduce
the order with respect to each reagent.

Question 4

Write the word and chemical equations for the reaction between potassium iodide and potassium peroxodisulphate (K2S2O8)

Answer 4

Potassium + potassium → potassium + iodine
peroxodisulphate iodide sulphate

K2S2O8 + 2KI → 2K2SO4 + I2

Question 5

What equation links moles, volume and

concentration?

Answer 5

Moles = volume x concentration
n = V x C

Question 6

How can the initial rate be calculated for

an iodine clock reaction?

Answer 6

Initial rate =
Moles of iodine (mol)
———————————-
Time taken for colour change (s)

Question 7

Give 2 possible sources of error in an

iodine clock experiment

Answer 7

● 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 8

Suggest some control variables for this

experiment

Answer 8

• 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 9

Write the rate equation for the reaction between iodide ions and peroxodisulphate ions

Answer 9

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

Question 10

What is the overall order of the reaction
between iodide ions and
peroxodisulphate ions?

Answer 10

2