carbon dioxide
hydrochloric
acid
calcium carbonate
(marble)
Relative rate
After completing this topic you should be able to :
• Relative rate for a reaction is the reciprocal of
the time taken for the reaction.
• Calculating relative rate from raw
experimental data
• Units for relative rate are s-1
Effect of concentration
• The higher the concentration, the more
particles in a given space, the more
chance there is of successful collisions.
Relative Rate
Definition: Relative rate for a reaction is the reciprocal of the time taken for the
reaction. Units for relative rate are s-1
Mathematics
Effect of concentration on Magnesium and
hydrochloric acid
Aim: To investigate the effect of varying the concentration of hydrochloric acid
concentration on the rate of reaction between hydrochloric acid and magnesium
Mg(s) + 2HCL (aq)  MgCl2 + H2
Method: Devising
a standard set of conditions
You might like to begin with the reaction between magnesium ribbon
and hydrochloric acid. The relative rate of this reaction can be
measured in terms of the time taken for a piece of ribbon to react
completely. You will have to find out how reliable this method is. Is
the length of ribbon critical for accurate measurements? Do you
have to clean it first? If the acid is in a conical flask will you stir it/
shake it, gently /vigorously, intermittently or just leave it alone? Does
the volume of acid affect your results? What happens to the
temperature of the acid during the reaction? How reproducible are
your results using the same conditions? Are you able to devise a set
of conditions that will allow you to make meaningful comparisons
when you change other variables?
Results:
Concentration of HCl (mol l-1)
Time (s)
Relative rate
(1/t) s-1
1
2
3
4
Graph: Plot a graph of concentration of HCl (x-axis) and
relative rate (s-1) (y-axis)
Conclusion: Describe the effect of HCl concentration on its
rate with magnesium ribbon
Iodine clock reaction – introduction to
chemical kinetics
Effect of concentration –the chemical clock
challenge
2I- (aq) + H2O2
(aq)
I2 (aq) + 2S2O32-
+ 2H+
(aa)
(aq)
 2H2O
(l)
+ I2
 2I- (aq) + S4O62-
(aq)
(ag)
The reaction mixture stays colourless as the iodine molecules are
converted back to iodide molecules by the thiosulphate ions.
Once all the thiosulphate ions have been used, a blue black colour appears
suddenly as iodine reacts with starch.
Relative Rate =
1
t
Units s-1
t being a measure of how long it takes for the blue/black
colour to form. (when excess I2 forms)
Effect of concentration –the chemical clock
challenge
1) Using syringes measure out
10cm3 sulphuric acid 0.1moll-1
10cm3 sodium thiosulphate 0.005moll-1
1cm3 starch solution
25cm3 potassium iodide solution 0.1mol l-1
Into a dry 100cm3 beaker
2) Measure out 5cm3 of hydrogen peroxide 0.1moll-1
into a syringe. Add it to the mixture as quickly as possible and start the timer.
3) Stop the clock when the mixture suddenly turns dark blue.
4) Repeat, using 20 cm3 of potassium iodide solution and 5cm3 of water with, then
using repeated dilutions
Effect of concentration –the chemical clock
challenge
Volume of water (cm3)
Volume of 0.1 mol l-1
KI (aq) (cm3)
O.0
25.0
5.0
20.00
10.0
15.0
15.0
10.0
20.0
5.0
Time (s)
Rate (1/t)
Effect of concentration –the chemical clock
challenge
RESULTS - Plot a graph showing the volume of potassium iodide
x axis and the rate of reaction on the y axis.
Effect of concentration –the chemical clock
challenge
Your challenge is to create a series of solutions that will
change colour in time to music. Be creative and if you can’t
think of anything you can take up one of these three
challenges.
Element Challenge
Roxanne challenge
That moment when
your jam comes on
Effect of concentration –the chemical clock
challenge
Listen to the song and identify points where you want to
have a colour change come in
Time them accurately. Allocate times to each member of
your group.
Look at your results and check that these are times you
can achieve
Calculate the rate that each time requires (rate = 1/t)
Read off the required concentration from your graph
Effect of concentration –the chemical clock
challenge
You will carry out the reaction using a series
of dilutions of the iodide solution. This will
be diluted by replacing some of the volume
with water.
Effect of concentration –the chemical clock
challenge
Use the relative concentration to help
you work out the volume of water and
KI(aq) needed to make up 100 cm3 of
the required concentration.
Good luck!