1. Experiment 1 Factors Governing the Speed of Chemical Reactions

2. Experiment 1: Factors Affecting Reaction Rates
Part A
Effect of Concentration on Reaction Rate
CHEM 3310

3. Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
( clear solution ) ( pale yellow solution )
Reactants:
Ammonium persulfate
(NH4)2S2O8
Potassium iodide KI
CHEM 3310

4. Observe a COLOUR CHANGE!
How do we do this?
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
( clear solution ) ( pale yellow solution )
Observe a COLOUR CHANGE!
Problem: The pale yellow solution is visually very difficult to judge.
CHEM 3310

5. Iodine clock Reaction S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
( clear solution ) ( pale yellow solution )
Introduce two substances to help us observe the colour change
more accurately:
Starch indicator
Sodium thiosulfate, Na2S2O3, of a fixed quantity
CHEM 3310

6. S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) (1)
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq) (1)
( clear solution )
I2 (aq) + 2 S2O32-(aq) S4O62- (aq) + 2 I- (aq) (2)
( clear solution )
thiosulfate ion
I2 (aq) + starch I2 – starch complex (3)
( coloured solution )
As the I2 is formed (1), it reacts with the fixed amount of sodium thiosulfate that has
been added to the reaction mixture (2). When the sodium thiosulfate is used up,
the next quantity of I2 that is formed reacts with the starch indicator to form a colour
complex (3), and turns the clear solution into a coloured solution.
CHEM 3310

7. S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
Prepare 14 solutions in 14 Erlenmeyer flasks with
different concentrations of S2O82- and I- solutions!
Label them:
A1, A2, A3, A4, A5, A6, A7.
B1, B2, B3, B4, B5, B6, B7.
Make solutions of different concentrations.
SEPARATE the reactants until we are ready to mix them!
A solutions contains:
potassium iodide (I-)
sodium thiosulfate (S2O32- )
starch indicator
B solutions contains:
ammonium persulfate (S2O82-)
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8. Total volume of ‘A’ solutions =
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
Total volume of ‘A’ solutions =
30.00 mL.
CHEM 3310

9. Total volume of ‘B’ solutions =
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
Total volume of ‘B’ solutions =
20.00 mL.
CHEM 3310

10. Total volume of the combined solution =
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
MIX and TIME:
A1 + B1 A2 + B A3 + B A4 + B A5 + B A6 + B A7 + B7
Total volume of the combined solution =
50.00 mL.
CHEM 3310

11. When solutions A1 and B1 are combined, what is the concentration of
(NH4)2S2O8 in moles/L?
( L) x ( moles/L) = moles (NH4)2S2O8
moles (NH4)2S2O8 = M (NH4)2S2O L
in the combined
A1 + B1 solution
Total volume of the combined solution =
50.00 mL.
CHEM 3310

12. What is the reaction’s rate law?
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
What is the reaction’s rate law?
The rate law for a chemical reaction is an experimentally
determined mathematical equation that describes the progress
of the reaction.
For the Iodine clock reaction, the reaction rate of formation of I2
is proportional to the product of the concentrations each
reactant each raised to some power, m and n.
Rate  [I-]m [S2O82-]n
CHEM 3310

13. What is the reaction’s rate law?
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
What is the reaction’s rate law?
Rate  [I-]m [S2O82-]n
We can remove the proportional symbol and introduce a proportionality constant, k.
Rate = k [I-]m [S2O82-]n
The proportionality constant, k, is known as the rate constant.
We will experimentally determine the values of m and n graphically.
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14. Rate = k [I-]m [S2O82-]n Rate = k’ [S2O82-]n
Since the concentration of I- is kept constant for A1 to A4, the reaction rates for A1/B1, A2/B2, A3/B3, and A4/B4 will only depend on the concentration of S2O82- .
Rate = k’ [S2O82-]n
where k’ = k [I-]m
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15. Log (Rate) = n log [S2O82-] + log k’
This will enable us to find n graphically. Rate = k’[S2O82-]n
Take log of both sides,
Log (Rate) = n log [S2O82-] + log k’
Since Rate  time-1,
so Rate = a(time-1) where a is a constant Log (time-1) = n log [S2O82-] + log (k’/a)
Y = slope  X b
Graph of “Log (time-1) Versus log [S2O82-]” should yield a
straight line with a slope of n.
Graph #1.
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16. Rate = k [I-]m [S2O82-]n Rate = k’’ [I-]m
Since the concentration of S2O82- is kept constant for B4 to B7, the reaction rates for A4/B4, A5/B5, A6/B6, and A7/B7 will only depend on the concentration of I- .
Rate = k’’ [I-]m
where k’’ = k [S2O82-]n
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17. Similarly, this will enable us to find m graphically. Rate = k’’ [I-]m
Take log of both sides,
Log (Rate) = m log [I-] + log k’’
Since Rate  time-1, and Rate = a(time-1)
Log (time-1) = m log [I-] + log (k’’/a)
Y = slope  X + b
Graph of “Log (time-1) Versus log [I-]” should yield a
straight line with a slope of m.
Graph #2.
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18. S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
Rate = k [I-]m [S2O82-]n
Values of m and n should be very close to an integer. If not,
round m and n to the nearest integer.
m is the order with respect to I-
n is the order with respect to S2O82-
The overall reaction order is m+n.
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19. Experiment 1: Factors Affecting Reaction Rates
Part B
Effect of Temperature on Reaction Rate
CHEM 3310

20. S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
Iodine clock Reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
How do we do this?
Keep the reactant concentrations constant, and let the
reaction react at different temperatures:
0oC, 20oC, 30oC, 40oC (Part A)
Use A4, B4 solutions for all the temperatures!!
CHEM 3310

21. Verify Arrhenius’ Equation
A is the pre-exponential or frequency factor, a constant related
to the collision frequency
R is the gas constant (8.314 J / K mole)
T is the absolute temperature (K)
k is the rate constant at temperature T
Eact is the activation energy, the energy required by the reacting
species for their collisions to be effective (ie - those that lead
to the formation of products)
CHEM 3310

22. Rewrite Arrhenius’ Equation
y = slope  x + b or
should yield a straight line!!
Recall, to convert between ln and log,
ln y =  log y
CHEM 3310

23. Recall, how to use the Linest function in Excel to determine the statistical error in a set of data.
CHEM 3310