1. The effect of concentration on reaction rate
Unit 1
PPA 1
The effect of concentration on reaction rate

2. The effect of concentration on reaction rate. (Unit 1 PPA 1)
The aim of this experiment is to find the effect of varying the concentration of iodide ions on the rate of reaction between hydrogen peroxide and an acidified solution of potassium iodide:
H2O2 (aq) H+(aq) + 2I-(aq) 2H2O(l) + I2 (aq)
The course of this reaction can be followed by carrying it out in the presence of small quantities of starch and sodium thiosulphate solutions. As the iodine molecules are produced they immediately react with the thiosulphate ions and are converted back into iodide ions:
I2 (aq) S2O32-(aq) I-(aq) S4O22-(aq)
The reaction uses solutions of hydrogen peroxide, sulphuric acid, potassium iodide, sodium thiosulphate and starch.
(1) The concentration of the potassium iodide is altered in the reaction.
Describe how this is carried out.
(1) The volume of potassium iodide solution is decreased 5cm3 and the volume of water added is increased by 5cm3 each time the experiment is carried out.
(2) Once all of the sodium thiosulphate solution has been reacted the iodine
formed by reaction of hydrogen peroxide and iodide ions will react with
the starch solution. What colour change will be seen?
(2) The iodine reacts with the starch to give a blue black colour.
(3) The time taken for the blue-black colour to appear is measured each
time the experiment is carried out.
(a) How is the time turned into a rate for the reaction?
(b) What unit is used to measure the rate?
(3) (a) Rate = 1/time.
(b) The unit of rate is s-1.

3.
volume of potassium iodide (cm3)
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
Rate (s-1)
(4) The straight line passing
through the origin shows that
the reaction rate is directly
proportional to the concentration
of the potassium iodide solution.
(4) What can be concluded from the graph of reaction rate against volume of
potassium iodide solution shown above?
(5) Use the information in the graph to give the time taken for the blue-black
colour to appear when 10 cm3 of potassium iodide solution was used.
(5) The rate = s-1
So the time = 1/0.020
= 50 seconds